Comparison of Automated Processing of Flocked Swabs with Manual Processing of Fiber Swabs for Detection of Nasal Carriage of Staphylococcus aureus^,

Abstract

The sensitivity of automated culture of Staphylococcus aureus from flocked swabs versus that of manual culture of fiber swabs was prospectively compared using nasal swabs from 867 patients. Automated culture from flocked swabs significantly increased the detection rate, by 13.1% for direct culture and 10.2% for enrichment culture.

TEXT

Screening is a key component of strategies to reduce methicillin-resistant Staphylococcus aureus (MRSA) (2). In 2008, the United Kingdom Department of Health mandated screening all elective hospital admissions by March 2009 and hospital admissions from the emergency department by March 2011 (3). There is also increasing evidence that screening for methicillin-sensitive S. aureus (MSSA) reduces the rate of surgical site infections (SSIs) (1). PCR detection of MRSA remains significantly more expensive than culture methods, and consequently screening specimens are often cultured manually using fiber-based swabs. A deficiency of this method is that fiber swabs release only a small proportion of the organisms sampled onto solid media, which may reduce sensitivity (5).

In 2005, Copan Italia introduced the ESwab (Elution Swab), a flocked nylon swab transported in 1 ml of liquid Amies transport medium. The increased efficiency of this system has been demonstrated in vitro (5) and in vivo on volunteers (7). Another advantage of the liquid medium is the potential for automation (6). In 2008, Copan Italia launched a new generation of specimen processors with the Walk-Away Specimen Processor (WASP). This system automatically decaps and recaps sample containers and inoculates a variety of culture plates with minimal manual intervention.

A prospective trial was undertaken to compare the sensitivity of detection of S. aureus, including MRSA, from nose swabs using the automated WASP and the ESwab against the laboratory's current manual method that uses a regular fiber swab. The study population attended the Medical Assessment Unit of Derriford Hospital, Plymouth, between January and September 2009.

An ESwab (Copan Italia 480CE) was used to sample one of the nostrils, with the other nostril being swabbed with an M40 charcoal-based Transystem swab (Copan Italia 408CST). Each swab was prewetted with 50 μl sterile saline. Aliquots (30 μl) of the ESwab nose samples were quad-streaked by the WASP onto two plates, a Staphylococcus selective medium plate (Columbia blood agar with aztreonam and polymxyin [CAP]; Oxoid PB0122A) and an MRSA selective agar plate (ChromID MRSA agar; bioMérieux 43451). M40 nose samples were inoculated and quad-streaked manually onto the same two media (4). All plates were incubated at 37°C for 48 h. The use of a 30-μl loop for streaking ESwab liquid Amies medium was based on previous experimentation that showed this volume to produce visually comparable cultures with in-house manual inoculations (6).

The remaining 800 μl of the ESwab sample and the swab tip plus 3 cm of the shaft of the M40 sample were inoculated into separate tubes of 7% salt broth (Oxoid EB1040E) for enrichment. Broths were incubated at 30°C for 24 h, and 10 μl was subcultured automatically or manually as appropriate. The resultant cultures were screened for the presence of S. aureus (MSSA and MRSA) using standard methods. Positive nasal S. aureus carriage was defined as a positive culture result obtained from one or more of the methods under investigation. The sensitivity and negative predictive value were calculated for each method. The matched-pair sign test was used to analyze the data.

From the 867 patients enrolled, 41% were found to be positive for S. aureus. A total of 237 M40 swabs were culture positive for S. aureus, compared to 268 positives for the ESwabs, an increase of 13.1% (P < 0.001).

Enrichment culture was more sensitive than direct plating for both types of swab (Table 1); increases of 24.5% were observed with the M40 samples and 21.6% with the ESwab samples. The most sensitive single method was enrichment culture of ESwabs (91.6%).

Table 1.

Method sensitivity and negative predictive value (NPV) for detecting positive patients

Methoda	No. of S. aureus-positive patients	Sensitivity of method (%)	NPVb (%)
ESwab direct	268	75.5	85.5
ESwab enrich.	325	91.6	94.5
M40 direct	237	66.8	81.3
M40 enrich.	295	83.1	89.5
All methods	355

^aDirect, direct culture; enrich., enrichment culture.

^bNPV, negative predictive value.

Previous studies used to validate flocked swabs and automated plating have been laboratory based using bacterial cultures or human volunteers. These may not always reflect clinical practice, where variables such as the presence of mucus and pus may influence swab performance. This study examines the performance of different swabs and plating methods in the hospital environment using routine clinical samples collected from patients.

The results demonstrate that the use of ESwabs increases S. aureus recovery. The ability to automate processing enables laboratories to potentially improve efficiency. A comparison of timings showed that automated processing of a batch of 72 samples took 2.7 min hands-on time, compared with 57 min for manual processing. This represents a reduction in “hands-on” time of 95%. It would be possible to improve this further by using alternative streaking protocols.

MRSA enrichment culture is used for screening Derriford Hospital's elective preoperative patients and is performed 10 to 14 days in advance of surgery. This study has shown that this method improves the detection of S. aureus and justifies its use in this setting, where an early result is less important. A short turnaround time is more important when screening emergency admissions, and in this setting direct culture using the most sensitive swab/transport system available is recommended.

During the course of this trial, the WASP produced 50,000 culture plates from ESwab samples. It consistently provided isolated colonies; there was also no evidence of cross-contamination when 50 ESwab samples spiked with 10⁷ CFU of S. aureus were interspersed with 50 blank ESwab samples. The equipment was generally reliable and user-friendly during the study, with only a couple of plate carousel and label printer faults being encountered. These were resolved remotely by Copan Italia within 1 h, causing minimal delay to processing.

In conclusion, this study has shown ESwabs to be more sensitive than fiber swabs for the detection of S. aureus colonization. It has also shown that sensitivity can be increased further by enrichment culture. Processing ESwabs using the WASP was much less labor-intensive, making the combination ideally suited for screening for MSSA and MRSA.