Re-evaluating microbiological testing and isolation measures for suspected MRSA carriers: insights from a retrospective study in a Dutch tertiary care center

Abstract

Background

The Netherlands has one of the lowest prevalences of methicillin-resistant Staphylococcus aureus (MRSA) in Europe due to its stringent Search and Destroy policy, including isolation of suspected carriers. Unexpected MRSA detection in non-isolated patients triggers contact tracing and screening of exposed patients and healthcare workers (HCWs). Given evolving epidemiology and healthcare practices, periodic re-evaluation of control measures is essential to sustain low prevalence while minimizing burden.

Methods

This retrospective study at a Dutch tertiary hospital (2022–2024) assessed (1) diagnostic accuracy and utility of rapid MRSA PCR using Copan eSwab and Xpert MRSA NxG (Cepheid, GeneXpert); (2) proportion of unexpected MRSA among newly detected carriers; (3) transmission risk after unprotected MRSA exposure; (4) implications for optimizing screening and isolation. Data were extracted from electronic patient and laboratory records. MRSA culture and PCR results (throat, nose, rectum combined) were compared and stratified by screening indication.

Results

Among 1939 MRSA screenings, 89 (4.6%) were positive. Major screening indications were ‘foreign healthcare exposure’ (42.8%; 1.6% positive), ‘residing in an asylum seekers’ centre’ (24.9%; 7.8% positive), or ‘livestock contact’ (14.5%; 8.6% positive). MRSA-positive household contacts had the highest MRSA test positivity (16/87; 18.4%). Compared with culture, PCR (n = 830) showed 63.9% sensitivity and 99.2% specificity, 21% of positive PCRs were false positives (7/32). Unexpected MRSA findings triggered 78 contact investigations involving 143 patients and 1550 HCWs. Nosocomial transmission occurred in 0.3% of HCWs, none in patients. Incidental MRSA was found in 0.4% of HCWs (6/1550). 30% of patients with “unexpected MRSA” lacked known risk factors; parental origin abroad was observed in 38% (exploratory analysis). “Suspected MRSA” accounted for 23% of strict isolation days.

Conclusions

Current MRSA screening remains effective but could be optimized. Given the low transmission risk, limited PCR sensitivity, and low MRSA prevalence in high-risk groups, contact isolation instead of strict isolation may suffice while awaiting culture results in suspected carriers. A revised, risk-based approach could improve efficiency, reduce unnecessary isolation, and lower costs without compromising safety. As 30% of unexpected MRSA cases lacked established risk factors, updating screening criteria to include emerging risk factors may further strengthen detection and containment.

Background

Methicillin-resistant Staphylococcus aureus (MRSA) infections remain a global challenge for healthcare institutions with considerable impact on patient outcomes, healthcare costs and infection control policies [1, 2]. Compared to infections with methicillin-susceptible S. aureus, MRSA infections are associated with higher mortality rates [3]. In 2024, methicillin resistance was detected in only 3% of S. aureus isolates from Dutch hospitals [4], positioning the Netherlands among the countries with the lowest MRSA prevalence in Europe [5, 6]. This achievement is largely attributed to the stringent national MRSA Search and Destroy (S&D) policy, introduced in the late 1980s.

The Dutch MRSA S&D policy involves a set of coordinated interventions aimed at the early detection and elimination of MRSA carriage and transmission in healthcare settings [7]. Patients are routinely screened for the presence of MRSA “risk factors” using a standardized questionnaire (Table 1) upon hospital admission and regular intervals during outpatient visits. Patients with a MRSA screening indication are tested for MRSA, and upon admission they are placed under pre-emptive strict isolation pending diagnostic results. Those with confirmed MRSA colonization or infection remain under strict isolation precautions until successful decolonization is achieved. Isolation preferably involves a single-bed room with an anteroom, controlled pressure gradient and private sanitary facilities, or alternatively a single-bed room with a closed door and en-suite private sanitary facilities, always combined with full personal protective equipment (PPE) for health care workers (HCWs), defined as a surgical mask, long-sleeved gown with cuffs and gloves.

Table 1.

MRSA screening questionnaire as part of the Dutch Search and Destroy policy (2022–2024)

Screening questions
1. Have you ever tested positive for MRSA?
2. Do you have contact with commercially kept live pigs, veal calves, or broiler chickens, or do you live on a farm where these animals are kept, now or in the past 10 years?
3. Have you been a patient in a foreign hospital or healthcare facility in the past year?
4. Have you been a patient in a Dutch hospital or healthcare facility with a known MRSA outbreak in the past year?
5. Do you have a partner or household member with MRSA, or do you care for someone with MRSA?
6. Are you an adopted child and were you adopted less than one year ago?
7. Have you lived or stayed in a (temporary) asylum seeker center, large-scale refugee shelter in the past 2 months, or are you a refugee from Ukraine? *

Although the S&D strategy has proven effective and cost-efficient at the national level, its implementation places a significant burden on hospitals [2, 7]. Strict isolation is logistically demanding and may limit patient mobility, reduce interpersonal interactions, and delay clinical care [8]. In response, rapid diagnostic technologies, such as PCR (providing results within hours) and chromogenic agar (within 24 h), offer faster MRSA identification compared to traditional culture methods, which require three to four days. According to current Dutch guidelines, high-risk patients with negative rapid test results within 24 h may be managed in a single-bed room without anteroom or pressure gradient, and HCWs may follow less stringent PPE protocols while awaiting final culture results. If the PCR is positive, isolation measures are immediately escalated.

While these advances in diagnostic methods have undoubtedly accelerated MRSA detection or exclusion in identified high-risk patients, they do not address the challenge posed by individuals who fall outside predefined risk categories. In such cases, the issue is not diagnostic speed, but the lack of initial suspicion. Consequently, patients with MRSA but without established risk factor, here referred to as unexpected MRSA cases, are not identified during admission screening and therefore are not promptly isolated, increasing the risk of nosocomial transmission. To prevent secondary transmission and mitigate potential outbreaks, contact tracing is initiated for all individuals with unprotected exposure, including both HCWs and patients. In accordance with national policy, these contacts undergo microbiological screening and are placed under pre-emptive isolation pending culture results. HCWs may continue to work during this period.

While the Dutch S&D policy has been instrumental in maintaining a low national prevalence, emerging challenges raise questions about whether the current approach remains proportionate and sustainable. Rapid diagnostic MRSA tests are now widely available and endorsed by national guidelines to guide isolation measures. However, their diagnostic performance in routine clinical practice has not been comprehensively evaluated. In particular, in a low-prevalence setting like the Netherlands, the clinical and economic value of these tests depends heavily on their sensitivity and negative predictive value. If sensitivity is suboptimal, and the number needed to screen (NNS) to detect a single MRSA carrier is high, large-scale screening may provide limited benefit. Under these conditions, expanding isolation measures based on positive rapid diagnostic MRSA tests will still miss a substantial number of cases—thereby compromising both cost-effectiveness and infection control objectives.

Simultaneously, a considerable proportion of MRSA cases continue to be detected in patients without established risk factors. These unexpected cases bypass initial screening and are not pre-emptively isolated, raising concerns regarding nosocomial transmission. The actual risk of MRSA transmission in such scenarios, reflected in the yield of contact tracing investigations, warrants thorough evaluation. Furthermore, the relative contribution of unexpected MRSA to the overall incidence may indicate whether current screening criteria remain adequate or require revision to better encompass relevant risk groups—thereby aiding in the prevention of MRSA transmission and reducing the labor- and resource-intensive burden associated with contact tracing investigations.

Taken together, these issues underscore the need for a data-driven re-evaluation of current MRSA policy. This study therefore aims to assess (1) the diagnostic accuracy and practical utility of rapid MRSA tests in clinical practice, (2) the proportion of unexpected MRSA cases among all newly detected carriers, (3) the transmission risk associated with unprotected exposure to unexpected MRSA patients, and (4) how these findings can guide optimization of MRSA screening and isolation strategies toward a more effective and targeted approach.

Methods

Study population

This retrospective study was conducted using data from electronic patient records and the medical microbiological laboratory system collected between 2022 and 2024 from patients and HCWs. The study was conducted at a large academic tertiary care hospital providing highly complex and specialized care across a broad range of disciplines. In 2024, the hospital recorded over 22,000 admissions (average length of stay 5.9 days) and more than 30,000 surgical procedures, with a total patient population exceeding 215,000 encouters annually [9].

Research objectives

The overall objective of this study was to evaluate possibilities for refining MRSA screening and isolation policies with the aim of improving effectiveness by reducing unnecessary isolation and resource use while maintaining robust infection control.

The study addresses the following main research questions:

• Is it justified to manage high-risk patients under contact isolation (single-bed room with private sanitary facilities, without anteroom or pressure gradient, using less stringent PPE with gloves and an apron-style gown for direct contact with the patient or their environment) instead of strict isolation precautions while awaiting diagnostic results? What is the added value of PCR testing in routine clinical practice in this context?

• What proportion of newly detected MRSA carriers are unexpected cases (patients without established risk factors)? What are the implications of these unexpected cases for the workload and resource demands associated with contact tracing investigations and isolation procedures?

• What is the transmission risk associated with unprotected exposure to MRSA-positive patients, assessed by the outcomes of contact tracing investigations?

Data collection

Results of MRSA screening (cultures and PCR) per risk factor

All nasal, throat and rectal swabs for MRSA culture were identified using the laboratory information management system (GLIMS). Swabs originating from HCWs, incomplete swab sets, and follow-up cultures from known MRSA-positive patients (including those classified as ‘unexpected MRSA’) were excluded from analysis. Corresponding PCR results from nasal, throat and rectal swabs were also extracted.

At our institution, MRSA-PCR testing is routinely performed for patients admitted with a reported high risk for MRSA carriage, in accordance with the current Dutch MRSA guidelines [7]. These risk groups include patients who:

1. Have a history of MRSA colonization or infection;

2. Have had direct contact with commercially kept live pigs, veal calves, or broiler chickens, or have lived on farms where these animals are kept within the past 10 years;

3. Were admitted to a hospital or healthcare facility outside the Netherlands in the past year.

4. Were admitted to a Dutch hospital or healthcare facility with a known MRSA outbreak in the past year;

5. Share a household with or provide care for someone known to be MRSA-positive;

6. Were adopted within the past year;

7. Have lived or stayed in a (temporary) asylum seeker center within the past 2 months or are refugees from Ukraine.

MRSA screening (cultures and PCR) per risk factor

At our hospital, all patients identified as high-risk for MRSA carriage have an indication for screening. Ideally, screening is performed in the outpatient clinic using culture only; if not done in advance, patients are screened upon admission using both MRSA-PCR and microbiological culture. Unlike the national Dutch guidelines, which use PCR primarily to escalate isolation measures when positive, we use PCR to de-escalate isolation. While awaiting PCR results, patients are placed under strict isolation precautions. If the PCR result is negative, isolation measures are either discontinued or downgraded, depending on the presence of other indications for isolation, while awaiting final MRSA culture results.

In our medical microbiological laboratory, swabs (eSwab, Copan Diagnostics, Murrietta, CA, USA) for culture are inoculated in Staphylococcus enrichment broth MB + 2% NaCl (Mediaproducts, Groningen, The Netherlands) and incubated under aerobic conditions at 34 °C for 18 to 20 h. Subsequently, the broth is subcultured onto MRSA-ID agar plates (bioMérieux, Marcy l’Etoile, France), which are incubated at 36 °C for 48 h. Plates are examined for suspected colonies after 24 and 48 h of incubation, followed by further microbiological characterization. MRSA-PCR tests are performed directly from the swab medium (eSwab, Copan Diagnostics, Murrietta, CA, USA) using the Xpert MRSA NxG assay (Cepheid, Sunnyvale, CA, USA). A modified protocol was used, with 100 µL Amies medium instead of the manufacturer-recommended 300 µL.

Upon each hospital admission, patients are required to complete a standardized questionnaire assessing risk factors for MRSA and/or multidrug resistant microorganisms (MDRO) carriage, either by self-report or with assistance from a HCW. For patients who underwent at least one complete set of nasal, throat and rectal swabs, questionnaire data were obtained from the data warehouse of the electronic health record system (Epic^®) and linked to the corresponding microbiological MRSA test results. PCR results were included only when MRSA PCR testing had been performed on the nasal swab as well as on at least one additional anatomical site, in accordance with the Dutch MRSA guideline. According to this guideline, if PCR results from both the nose and an additional anatomical site are available within 24 h, patients may be nursed in contact isolation (instead of strict isolation) while awaiting MRSA culture results.

To identify and exclude follow-up cultures from MRSA-positive patients, MRSA status was obtained from the web-based infection prevention and control (IPC) application Micore (Softmedex) and cross-referenced with the study database. Missing data or inconsistencies were manually verified using both Epic^® and Micore, including confirmation of the rationale for screening when all questionnaire responses were negative.

Unexpectedly MRSA positive patients

Patients who test positive for MRSA without an established risk factor according to the Dutch MRSA guidelines, are classified as ‘unexpected MRSA findings’. Since these patients are not placed under protective isolation upon admission, there is a potential risk of MRSA transmission to other patients and HCWs. Consequently, the occurrence of transmission is evaluated through contact tracing investigations.

Patients with ‘unexpected MRSA’ were identified based on these contact tracing investigations initiated by the Infection Prevention and Control (IPC) unit. For each unexpected MRSA carrier, demographic characteristics—including, sex, age and hospital department—were recorded. Additionally, data were collected on the detection method (screening culture versus diagnostic culture) and specimen location (e.g. blood, urine, wound), MRSA characteristics (livestock-associated (LA) or non-LA), presence of Panton-Valentine leukocidin (PVL), and whether retrospective assessment revealed a risk factor according to the MRSA screening questionnaire. This enabled evaluation of whether the current screening method could have detected the MRSA in retrospect.

Unexpected MRSA cases were subsequently analyzed in relation to the total number of newly detected MRSA cases, identified through questionnaire-based risk assessments combined with routine MRSA screening.

Contact tracing investigations following unprotected exposure to MRSA

To assess the size and outcomes of MRSA contact tracing investigations, all post-exposure screening results of individuals who had insufficiently protected healthcare contact with a MRSA-positive patient between 2022 and 2024 were extracted from the IPC database Micore. For each contact tracing investigation, the number of screened patients and HCWs, as well as the outcomes of these tests, were recorded. For MRSA-positive contacts (patients or HCWs), we determined whether this represented transmission from the index patient or an unrelated, incidental finding. This was determined by comparing the multiple-locus variable number tandem repeat analysis (MLVA) types between the index and the MRSA positive contact.

In certain cases, a single contact investigation involved multiple index patients. For example, if a mother and her newborn twins were all identified as MRSA-positive. The subsequent contact tracing encompassed both the maternity ward (where the mother was admitted) and the neonatal intensive care unit (NICU), with specific attention to the mother’s risk factors. Follow-up investigations initiated due to a MRSA-positive contact, were considered part of the original contact tracing, unless no evidence of transmission between the index case and the MRSA positive contact was found. Household contacts (e.g. partners or parents from an index case) were excluded from analysis.

Hospital admission and isolation days

To assess the impact of our MRSA policy on isolation practices, we extracted an annual overview of strict and contact isolation days from the IPC database Micore. These data were stratified by indication, distinguishing between suspected MRSA cases and other reasons for isolation. Particular attention was given to suspected MRSA cases currently managed under strict isolation, to evaluate the potential effect of adopting contact isolation instead. Additionally, the total annual number of hospital admission days was obtained from the hospitals’ annual reports to contextualize isolation burden relative to overall hospital activity [9].

Statistical analysis

Descriptive statistics were performed using Microsoft^® Excel^® for Microsoft 365 MSO (Version 2408 Build 16.0.17928.20654). Counts and percentages were calculated per swab set (nose, throat and rectum) and per unique patient. To assess the diagnostic accuracy of the MRSA-PCR for detecting MRSA carriage, PCR results were compared with microbiological culture results, which was considered the reference standard. The comparison was performed at swab set level, in accordance with clinical practice. For instance, if the MRSA culture was positive from the nasal swab and the PCR was positive from the throat swab within the same set, the PCR result for that set was considered a true positive. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were calculated accordingly, considering the culture result as reference standard.

Demographic characteristics of unexpectedly MRSA-positive index patients were collected from the IPC database Micore. To put the number of unexpectedly MRSA-positive patients in perspective, these findings were expressed as percentage of the total number of newly identified MRSA carriers, as well as to the annual number of isolation days. The prevalence of MRSA among healthcare workers was calculated by dividing the number of incidental MRSA findings in HCWs as identified during the contact tracing investigations by the total number of HCWs screened as a part of these investigations.

Results

Results of MRSA screening (cultures and PCR) per risk factor

In 2022, 2023, and 2024, a total of 1939 sets of nasal, throat and rectal swabs for MRSA culture were analyzed in our medical microbiological laboratory, originating from 1469 unique patients. Of these swab sets, 4.6% (89/1,939) tested positive for MRSA, corresponding to 6.1% (89/1469) of all patients screened during this period. Multiple risk factors for screening were identified in 3.7% of cases (n = 71). Most common MRSA screening indications were ‘recent admission to a hospital or healthcare facility abroad within the past year’ (n = 861; 42.8%), ‘residence in an asylum seeker center’ or ‘refugee status from Ukraine’ (n = 500; 24.9%), or a ‘livestock-associated exposure’ (n = 292; 14.5%). The highest MRSA positivity percentage was observed among individuals with a MRSA-positive household member (18.4% 16/87); the positivity percentage for the other MRSA risk groups ranged from 0.0 to 8.6% (Fig. 1).

Fig. 1.

Indications for obtaining nasal, throat and rectal swab sets for MRSA screening, including the number of positive MRSA cultures per risk factor. Percentages above the bars represent the proportion of culture sets collected for this risk factor (% of total cultures). Percentages below the bars represent the proportion of positive cultures among the total number of cultures collected for this risk factor. Y-axis: number of cultures. Because some patients presented with multiple risk factors, 3.6% of swab sets were counted more than once

For 830 swab sets (42.8%), derived from 774 patients (52.7%) an MRSA-PCR was performed in addition to MRSA culture. Of these PCR-tested sets, 3.5% (29/830) were MRSA positive, corresponding to 3.7% of the tested patients. The diagnostic performance of the MRSA-PCR compared with culture as the reference method, is shown in Table 2. The sensitivity of PCR compared to culture was 63.9% (23/36) and specificity 99.2% (788/794). Figure 2 shows the diagnostic performance of the PCR categorized by risk factor. MRSA carriage was relatively often missed by PCR in patients residing in asylum seeker facilities or with refugee status from Ukraine (7/13; 53.8%). This discrepancy was not attributable to specific MLVA types.

Table 2.

Diagnostic performance of MRSA-PCR in relation to MRSA culture based on sets of three swabs per patient

	MRSA culture positive	MRSA culture negative	Total
MRSA-PCR positive	23	6	29
MRSA-PCR negative	13	788	801
Total	36	794	830
Sensitivity		63.9% (95%CI 46.2–79.2%)
Specificity		99.2% (95%CI 98.4% − 99.7%)
Positive predictive value		79.3% (95%CI 62.5% − 89.2%)
Negative predictive value		98.4% (95%CI 97.5% − 98.9%)

95%CI = 95% confidence interval

Fig. 2.

Diagnostic performance of MRSA-PCR compared to culture. Nasal, throat and rectal swab sets were tested for MRSA using both culture and PCR. The figure shows the total number of positive results for culture and PCR categorized by risk factor. Bars above the bold horizontal line represent incorrect PCR results defined as any result that was discordant with culture, which served as the reference standard. Because some patients presented with multiple risk factors 3.6% of swab sets were counted more than once

Unexpectedly MRSA positive patients

Between 2022 and 2024, unexpected MRSA detection occurred in 78 patients. Characteristics of these patients, including MRSA MLVA types (LA or non-LA) and the departments involved, are summarized in Table 3. The majority of patients were male (51/78; 65%). A substantial proportion were infants (14/78; 18%) or children (16/78; 21%). Overall, 27 contact tracing investigations (27/78; 34.6%) involved pediatric or neonatal departments.

Table 3.

Characteristics of unexpected MRSA-positive patients (N = 78)

Characteristic	Number	%
Sex
Male	51	65.4
Female	27	34.6
Age group (years)
0–1	14	18.0
1–18	16	20.5
18–25	6	7.7
25–40	16	20.5
40–60	10	12.8
60–75	7	9.0
75+	9	11.5
Type of culture
Screening	42	53.8
Diagnostic	36	46.2
PVL presence
Yes	7	9.0
No	63	80.8
Unknown	8	10.3
MRSA strain type
Non-LA MRSA	58	74.4
LA-MRSA	16	22.5
Unknown	4	5.1
Department involved in contact tracing investigation*
Pediatric Intensive Care unit / High Care / Medium Care	7	7.6
Neonatal Intensive Care unit (NICU)	11	12.0
Adult Intensive Care unit	9	9.8
Surgery	17	18.6
Internal Medicine	5	5.4
Pediatrics	9	9.8
Emergency room	4	4.3
Gynecology and obstetrics	9	9.8
Short stay unit/Outpatient department	9	9.8
Other	12	13.0

*Of the 92 contact tracing investigations, 13 (14.1%) involved multiple departments

Of the 78 unexpected MRSA cases, 28 (36%) had an established MRSA risk factor and could therefore have been identified using the existing MRSA screening questionnaire (Table 1). In half of these cases (14/28), the protocol had been correctly followed. Notably, in three patients recent negative cultures had led to the downscaling of stringent isolation measures as per protocol, however, subsequent microbiological cultures tested positive for MRSA. In seven patients, the MRSA-PCR results were negative, but cultures became positive several days later, resulting in a contact investigation. In the remaining 14 patients (14/28), the MRSA screening protocol had not been followed. Reasons included acute clinical situations (2/28; 7.1%), incomplete information transfer from another hospital (2/28; 7.1%), or deviation from the hospital’s MRSA isolation protocol (1/28; 3.6%). In most of these cases (9/28; 32%), the reason for not identifying the MRSA risk based on the screening questionnaire remained unknown.

In 50 of the 78 unexpected MRSA cases (64%), no established MRSA risk factor was present. Of these, 34 (34/50; 68%) were identified by means of diagnostic cultures. The remaining 16 cases were identified through screening cultures (16/50; 32%), primarily performed as part of routine protocol-based surveillance in intensive care units (11/16; 68.8%) or in the context of MRSA screening for other reasons (5/16; 31.3%). Among these five, maternal screening was initiated in three cases following a positive MRSA result in their newborn during routine ICU surveillance. One patient had previously tested positive at another hospital (unknown at admission), and in another case, MRSA was identified through contact tracing. In total, 78 of the 167 newly identified MRSA-positive cases (46.7%) in 2022, 2023, and 2024 were unexpected, of which 50 cases (29.9%) had no established MRSA risk factor.

Exploratory analysis of potential new risk factors in unexpectedly MRSA positive patients

Given the unforeseen high number of MRSA-positive cases without established risk factor (29.9% of all newly identified MRSA-positive cases), we conducted an exploratory review of hospital electronic health records to identify potential commonalities that might suggest previously unrecognized risk factors for MRSA carriage. Based on literature [10, 11] and insights from a previous pilot study on unexpected MRSA findings [12] we considered the following potential risk factors: a previous negative MRSA screening as part of a contact tracing; receipt of outpatient home care; residence in a nursing home or other long-term care facility (as a proxy for care dependency); having at least one parent born abroad; antibiotic use in the past 12 months; ongoing or recent oncological treatment. In addition, based on clinical considerations, the following potential risk factors were explored: pet ownership, occupational contact with animals, working in healthcare; foreign travel within the past 12 months; residence abroad.

Table 4 provides an overview of all potential risk factors identified in patients with unexpected MRSA carriage that are not currently included in the national screening questionnaire. Notably, in 19 of these patients (38%), at least one parent was born abroad, suggesting a possible association that warrants further investigation.

Table 4.

Explored non-defined MRSA risk factors

Risk factor	Yes n (%)	No n (%)	Unknown n (%)	Comments
International travel < 1 year	6 (12)	11 (22)	33 (66)
Europe	2 (33)			Poland, Portugal
Asia	4 (67)			Syria, Thailand, China, Turkey
Employment in healthcare	1 (2.0)	40 (80)	9 (18)	Nursing home staff
Receiving outpatient care	4 (8.0)	41 (82)	5 (10)	All cases involved home care
Commercial animal contact	1 (2.0)	11 (22)	38 (76)	Goat farm
Resident in a nursing home or other care facility	1 (2.0)	49 (97)	0 (0.0)
Living abroad	5 (10)	42 (84)	3 (6.0)	Germany (n = 3), Romania (n = 1), Poland (n = 1)
Pets	7 (14)	7 (14)	36 (72)	Dog (n = 5), Cat (n = 3), Rabbit (n = 1)
Active oncological treatment	4 (8.0)	46 (92)	0 (0.0)
Parents born abroad	19 (38)	4 (8)	27 (54)
Europe	4 (21)			Georgia, Romania, Poland, Ukrain
Asia	10 (52)			Syria (n = 4), Syria and Iraq (n = 1) China (n = 1), Iran (n = 2), Turkey (n = 1), Vietnam (n = 1)
Africa	5 (26)			Egypt (n = 1), Guinea (n = 1), Morocco (n = 2), Somalia (n = 1)
Previous involvement in MRSA contact tracing	0 (0.0)	7 (14)	43 (86)
Total	48 (9.6)	258 (52)	194 (39)

Contact tracing investigations following unprotected exposure to MRSA

Between 2022 and 2024, a total of 78 contact tracing investigations were started following the identification of an unexpected MRSA: 17 in 2022, 27 in 2023, and 34 in 2024. On average, 1.8 patients (range: 0–21) and 19.9 HCWs (range: 1–222) were screened per contact investigation. Overall, 143 patients and 1550 HCWs were screened for MRSA. One of these patients tested positive for MRSA (1/143; 0.7%); however, as the MLVA-type of the contact patient and the index patient were different, this finding did not indicate transmission (patient- to-patient MRSA transmission: 0.0%, 95% confidence interval 0.0–2.1%). MRSA was detected in 11 healthcare workers (11/1550; 0.71%). Based on the MLVA-types, transmission was considered likely in five cases (0.3%; 95% confidence interval 0.10–0.75%). However, in six HCWs, the MRSA MLVA-type was different to that of the index case, indicating that the ‘baseline’ MRSA prevalence in HCWs was 0.4% (6/1550; 95% confidence interval 0.14–0.84%). The number needed to screen to detect one case of transmission was 339 for patients and HCWs combined (not calculable for patients alone due to no transmissions, and 310 for healthcare workers alone).

Hospital admission and isolation days

Table 5 shows the number of ‘strict isolation’ days for ‘high MRSA risk’, relative to the total number of hospital admission days, total number of ‘strict isolation’ days, and total number of ‘contact isolation’ days. Overall, 19% of all strict isolation days were attributable to patients with high MRSA risk (on average 277 of 1452 strict isolation days per year), compared with 126.522 inpatient days annually.

Table 5.

Patient and isolation days for clinical admissions at our academic hospital, 2022–2024

Year	Hospital admission days	Total strict isolation days	Strict isolation days due to high MRSA risk *	Total contact isolation days
2022	126.190	1444	225	7227
2023	127.518	1147	277	8461
2024	125.858	1765	328	8669

*Derived from the number of MRSA-PCR tests performed annually in patients classified as high risk upon admission

Discussion

This study evaluated the MRSA screening policy and the incidence of MRSA at a Dutch tertiary care center over the years 2022, 2023 and 2024. Our analysis focused on two key questions. First, whether the use of ‘contact isolation’ - instead of ‘strict isolation’—would be considered justified while awaiting diagnostic results, and whether this necessarily requires rapid diagnostic testing by PCR. Secondly, we aimed to determine the proportion of newly unexpectedly identified MRSA carriers and evaluate the implications of these findings for contact tracing efforts and nosocomial MRSA transmission within our healthcare settings.

MRSA carriage was detected in 6.1% of patients who were screened for MRSA according to national guidelines, indicating that even among ‘high risk groups’ the prevalence of MRSA in the Netherlands remains low. Notably, detection rates varied considerably across different risk factors. The highest detection rate was observed in patients who had household contact to a known MRSA carrier. This finding is plausible, as these individuals are directly exposed to MRSA through close and sustained contact, rather than merely residing in environments with a high prevalence of MRSA. The next highest detection rates were observed among patient with occupational exposure to commercially kept live pigs, veal calves or broiler chickens, and residents of asylum seeker facilities or refugees from Ukraine. In contrast, 42.8% of screening cultures were performed due to a history of hospitalization abroad, yet the detection rate in this group was only 1.6%. In this risk category no differentiation was made between patients undergoing dialysis just across the German border and those hospitalized in countries with a high MRSA prevalence. This lack of stratification may partially explain the low yield and limits the interpretation of the efficiency of this screenings question.

Although this study focuses on MRSA, it is important to note that several risk categories, such as patients previously admitted to healthcare facilities abroad and individuals residing in asylum seeker facilities, are included in the screening protocol for broader reasons. These categories are not only screened due to the risk of MRSA, but also because of an increased likelihood of colonization with other multidrug-resistant organisms (MDROs), such as Candida auris, carbapenemase-producing Enterobacterales (CPE), and vancomycin-resistant Enterococci (VRE). This broader infection control rationale should be taken into account when evaluating the utility of the screening questionnaire and IPC measures.

Our evaluation showed that the current screening questionnaire was able to identify only 117 out of 167 new MRSA cases (70.1%) over the past three years. This includes 89 newly identified MRSA carriers detected through the screening questionnaire and 28 unexpected MRSA cases in which an established risk factor was not identified in a timely manner. In practice, 24% (28/117) of cases with identifiable risk factors were missed suggesting suboptimal adherence to the screening process. Additionally, 30% of new MRSA cases (50/167) had no established risk factor according to national MRSA screening guidelines. These patients were therefore not identified and put in preemptive isolation timely, highlighting a possible gap in the current national risk-based MRSA screening approach. An exploratory analysis of additional risk factors revealed that 38% of unexpectedly MRSA-positive patients without a recognized risk factor had at least one parent born outside the Netherlands. This finding aligns with a previous Dutch study that identified parental country of birth as a risk factor for MRSA carriage [10]. Other potential risk factors warranting further investigation include receipt of ambulatory care, current oncological treatment, and recent international travel, all of which have been suggested in earlier studies [9, 10]. However, although these risk factors were common among the ‘unexpected MRSA’ patients, this does not necessarily mean they are suitable for screening. The overall prevalence of these factors in the general population must be considered to determine their potential applicability as screening criteria.

The impact of unexpected MRSA-positive cases on hospital practice is substantial. Contact tracing is labor-intensive and often results in the screening of large numbers of HCW and patients, whereby the yield is limited. MRSA transmission was confirmed in only a small proportion of HCWs (0.3%) and in none of the exposed patients. This is in line with findings from another Dutch study reporting 0.14% transmission among staff and no transmission among patients [11]. It underscores the need to reconsider current MRSA contact tracing strategies, as a more targeted approach could improve efficiency without increasing the risk of transmission. However, such reconsiderations do rely on a strong IPC program in hospitals, and a very high level of general basic IPC measures that have to be in place at all times.

The minimal MRSA transmission observed, together with the low prevalence among patients classified as high risk according to national MRSA guidelines, raises questions about the proportionality of the current practice of placing a relatively large number of these patients in preemptive isolation while ultimately detecting only a limited number of MRSA-positive cases.

According to the national MRSA guidelines, contact isolation in a single-bed room with private sanitary facilities and PPE (according to ‘contact isolation’) is sufficient for high-risk patients if rapid MRSA diagnostics are available within 24 h. Although MRSA-PCR is suitable for safely and promptly de-escalating isolation due to its high negative predictive value, the Dutch guidelines primarily promotes its use to escalate precautions in case of a positive PCR result. Our data indicate, however, that the PCR test yielded false-negative results in approximately one-third of patients with MRSA-positive cultures, and 21% of positive PCR results were false positives. The false-positive results led to unnecessary prolongation of isolation measures, additional diagnostics, and potential concerns among patients and staff. These findings underscore the need to reassess the role of MRSA-PCR in (de-)escalation protocols, taking into account both diagnostic performance and cost-effectiveness.

It should be noted that the PCR protocol used in our hospital deviated from the manufacturer’s instructions: a reduced input volume of 100 µL Amies medium was used instead of the recommended 300 µL due to the use of 1 mL Copan eSwabs. This change increased the limit of detection (2436 Colony Forming Units (CFU) instead of 812 CFU) and may have contributed to the lower sensitivity observed, particularly for samples with low bacterial loads. We hypothesize that low-level MRSA carriage may correspond to a lower transmission risk, whereas high-load carriers—more consistently detected by PCR—represent a more relevant source of transmission. In this context, rapid PCR may still support early risk stratification and guide timely escalation of infection control measures, despite suboptimal sensitivity.

Importantly, the sensitivity of PCR relative to culture is likely overestimated in our analysis, as we defined a positive PCR as a positive result from either the nasal swab or one additional anatomical site, in accordance with Dutch guidelines and routine clinical practice. The performance of PCR on a single anatomical site, however, is considerably lower, with false-negative rates of 40.9% for nasal swabs (9/22), 43.5% for throat swabs (10/23), and 72.7% for rectal swabs (8/11) in our cohort. Notably, in all cases with a negative swab PCR but positive culture, subsequent PCR performed on the cultured isolate itself was positive, indicating that the discrepancy was due to sampling rather than assay performance. Multiple MLVA types were involved, suggesting that these were not part of a single cluster.

Given the low prevalence of MRSA in the Netherlands—even among patients classified as high-risk based on the screening questionnaire—the limited sensitivity of MRSA-PCR in our modified protocol, and the low yield of contact tracing, multiple strategies for managing these patients are conceivable. Routine PCR remains one option, particularly for identifying high-load carriers who are more likely to contribute to transmission. However, the number needed to screen (n = 36) to detect a single MRSA carrier, together with the limited diagnostic yield, may not be proportional to the overall benefit. Based on our findings, admitting patients with high MRSA risk in ‘contact isolation’, while awaiting MRSA culture results - which are generally available within 3 to 4 days—appears justified. Complete omission of routine PCR could result in substantial resource savings, including an estimated annual cost saving of €78.850 based on a MRSA-PCR cost of €95 per test, with sampling at three anatomical sites (throat, nose, and rectum) and a total of 830 patients screened over a three-year period. This approach could also reduce strict isolation days by 254 per year, representing a reduction of over 20% of the ‘strict isolation’ days in our hospital. Alternatively, a risk-based approach could be adopted, in which only patients with the highest likelihood of MRSA carriage—such as those with known MRSA-positive household contact or residency in asylum seeker facilities—remain under pre-emptive strict isolation until diagnostic results are available, while lower-risk patients remain in standard contact isolation. This approach retains the benefits of PCR for detecting high-load carriers while minimizing unnecessary isolation and testing in lower-risk individuals.

An important limitation of this study is its single-center design, making it unclear to what extent the results can be generalized to all Dutch hospitals. Another major limitation is the absence of a comparison between MRSA-positive patients with potential novel risk factors and the MRSA-negative individuals meeting the same criteria. Such a comparison is essential to assess the predictive value of the proposed risk factors. Additionally, the retrospective design of the study restricted data collection to information recorded in the electronic health records, potentially leading to underreporting of risk factors. However, the assessment of novel risk factors was not the primary objective of our study. Instead, this analysis was conducted exploratively in response to the unexpectedly high number of patients identified as unexpected MRSA carriers. Therefore, prospective studies with comprehensive assessment of risk factors are needed to optimize screening and isolation policies in accordance with the current epidemiological landscape. Additionally, this study only covers a period of three years; a longer timeframe might have provided more robust data. The period of the COVID-19 pandemic was intentionally excluded due to its potential disruptive impact on routine healthcare processes. Furthermore, from 2022 onwards, contact tracing has been systematically documented in the registration system (Micore) of the IPC unit.

Conclusion

This study supports the continued relevance of the Dutch Search and Destroy policy for MRSA control, albeit within a reassessed framework. Our findings support the use of ‘contact isolation’—rather than ‘strict isolation’—while awaiting MRSA culture results in patients at high risk of MRSA carriage, under the prerequisite of a high level of compliance to basic infection prevention and control measures. Such an approach could reduce strict isolation days by over 20% (≈ 254 days per year). Although MRSA-PCR failed to detect one in three MRSA-positive patients, it may provide additional information on patients with higher bacterial loads, who could potentially pose a greater risk of transmission. Given the number needed to screen to detect one MRSA carrier (n = 36) and the limited diagnostic yield with our modified protocol, routine PCR for all high-risk patients may not be proportional. Complete omission of routine PCR could save approximately €78.850 annually in laboratory costs.

At the same time, contact tracing after unexpected MRSA detection required screening of large numbers of patients and healthcare workers, yet yielded transmission in only 0.3% of HCWs and in no patients. This disproportionate burden underscores the need for a more targeted, risk-based approach. Finally, the high proportion of unexpectedly MRSA-positive cases suggests novel risk factors not currently included in the screening tool. Having parents born abroad emerged as a potential risk factor that warrants further investigation. Continuous evaluation and revision of the screening questionnaire, including the integration of such emerging risk factors, are essential to ensure alignment with the evolving epidemiology of MRSA. These findings support a shift toward a more proportionate, risk-based MRSA policy that minimizes unnecessary burden while maintaining patient safety.

Abbreviations

CMO

Medical Ethics Review Committee

CI

Confidence interval

CPE

Carbapenemase-producing Enterobacterales

HCW

Healthcare worker

ICU

Intensive care unit

IPC

Infection Prevention and Control

LA

Livestock-associated

MDRO

Multidrug-resistant organism

MRSA

Methicillin-resistant Staphylococcus aureus

NNS

Number needed to screen

NICU

Neonatal intensive care unit

PCR

Polymerase chain reaction

PPE

Personal protective equipment

PVL

Panton-Valentine leukocidin

S&D

Search and Destroy

VRE

Vancomycin-resistant Enterococci

Author contributions

ST was involved in the study design, acquisition, analysis, and interpretation of patient data regarding MRSA culture and PCR results per risk factor and was a major contributor to writing the manuscript. LM participated in the study design, acquisition, collection, analysis and interpretation of data on unexpected MRSA cases and contact investigation outcomes and contributed to drafting the manuscript. JR, EB, SP and AT contributed to the conception and design of the study and substantively revised the manuscript. CM was involved in the study design, acquisition of data regarding isolation days, and substantively revised the manuscript. All authors read and approved the final manuscript.

Funding

Nothing to declare.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

The IPC unit, Dept. Medical Microbiology has a general approval from the institutional Medical Ethics Review Committee (CMO) to conduct medical-scientific research on laboratory and patient data for policy evaluation purposes (CMO number 2015 − 1764).

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.