Equal Performance of Self-Collected and Health Care Worker-Collected Pharyngeal Swabs for Group A Streptococcus Testing by PCR

Abstract

A process employing patient- or parent-collected pharyngeal swabs for group A Streptococcus (GAS) testing would expedite diagnosis and treatment, reduce patient exposure to the health care setting, and decrease health care costs. Our aim was to determine the concordance between patient- or parent-collected (self-collected) and health care worker (HCW)-collected pharyngeal swabs for detection of GAS by PCR. From 9 October 2012 to 21 March 2013, patients presenting with a sore throat meeting criteria for GAS testing and not meeting criteria for severe disease were offered the opportunity to collect their own pharyngeal swab. The HCW also collected a swab. Paired swabs were tested by GAS real-time PCR, allowing semiquantitative comparisons between positive results. Of the 402 participants, 206 had a swab collected by the patient and 196 a swab collected by the parent. The percent positivity results were 33.3% for HCW-collected swabs and 34.3% for self-collected swabs (P = 0.41). The overall concordance between the two collection strategies was 94.0% (95% confidence interval [CI], 91.3 to 96.0). Twenty-four of the paired swabs had discordant results, with 10 and 14 positives detected only with the HCW- and self-collected swabs, respectively (P = 0.41). The person collecting the swab in the self-collected arm, the order of collection, and prior swab collection training did not influence results. Among the 124 specimens that were positive by both collection methods, the amount of GAS DNA was higher in the self-collected versus the HCW-collected swabs (P = 0.008). Self-collected pharyngeal swabs provide a reliable alternative to HCW collection for detection of GAS and offer a strategy for improved health care delivery.

INTRODUCTION

Acute pharyngitis is a common illness, responsible for an estimated 15 million ambulatory care visits in the United States in 2006 (1). Between 30% and 60% of acute pharyngitis cases are thought to be of viral origin (2) followed in number by bacterial causes. Group A Streptococcus (GAS) is the most common bacterial agent of acute pharyngitis, accounting for 20% to 30% of cases in children and 5% to 15% of cases in adults (3, 4).

Group A streptococcal pharyngitis is typically self-limiting but can be associated with significant, preventable morbidity, especially in young children and adolescents (5). Among the most serious complications are mastoiditis, peritonsillar abscess, pneumonia, bacteremia, and meningitis. Primary infection can also trigger rheumatic fever and poststreptococcal glomerulonephritis (2). For this reason, GAS pharyngitis is commonly treated with antibiotics to limit disease transmission, ameliorate symptoms, and minimize complications (2, 3).

Transmission of GAS pharyngitis usually occurs through mucosal contact with respiratory droplets from infected individuals. High transmission rates have been reported in settings with close person-to-person contact, such as homes and schools, and may also occur in health care settings. In one study, over 40% of families with a confirmed case of GAS pharyngitis had one or more secondary cases (6).

Given the significant overlap in clinical presentation of viral and bacterial pharyngitis, streptococcal pharyngitis cannot reliably be diagnosed by clinical presentation alone (7, 8). Instead, the American Academy of Pediatrics (AAP) and the Infectious Diseases Society of America (IDSA) advocate confirmatory laboratory testing for select patients in order to differentiate bacterial from viral etiologies of pharyngitis (9, 10). Certain signs and symptoms have been associated with an increased likelihood of GAS pharyngitis and are used to identify patients who should have pharyngeal specimens collected for GAS testing (3, 11). Empirical antibiotics are not recommended for treatment of pharyngitis; they should be given only in the setting of a positive test to limit potential for medication-based adverse reactions and selection of antimicrobial resistance.

The large number of patients seeking care for evaluation of pharyngitis adds significant cost to the health care system, including costs for office visits, laboratory testing, and medications (12, 13). Other costs are more difficult to assess and include missed work or school, child care exclusion, transportation, and costs associated with antibiotic resistance selected due to antibiotic misuse.

One approach to reduce costs and expedite diagnosis and treatment is to offer select patients or their caregivers the opportunity to collect a pharyngeal swab for testing without requiring an office visit. Preliminary work from the 1970s and 1980s suggests that parents are able to obtain a pharyngeal swab for GAS testing, but those studies did not directly compare results using the same testing methodology, did not use a method allowing quantitative comparisons between results, or did not simulate conditions that would allow independent (e.g., at-home) swab collection (14, 15). In addition, these studies used older technologies such as GAS antigen testing, which suffers from low sensitivity, and culture, which requires up to 48 h for detection of a positive result (16). Real-time PCR is a highly sensitive and specific method for detection of GAS; furthermore, it allows semiquantitation of the DNA present through analysis of PCR crossing points (CPs) (16).

Here, we directly compared GAS detection rates from pharyngeal swabs collected by either a patient or a parent (here referred to as self-collected) and from pharyngeal swabs collected by a health care worker (HCW) (here referred to as HCW-collected) using real-time PCR. We also compared the relative amounts of DNA detected by the two collection methods as an indicator of collection quality by comparison of PCR CPs.

(Preliminary results were presented in part at the Mayo Clinic 2013 Advanced Nursing Conference, Rochester, MN [18 October 2013], and the 14th Annual Mayo Clinic Family Medicine Forum, Rochester, MN [28 October 2013].)

MATERIALS AND METHODS

The study population comprised a community-based cohort of adults and children, aged 3 years or older, presenting to an acute care walk-in ambulatory care clinic from 9 October 2012 to 21 March 2013 with a chief complaint of sore throat. The study was approved by the Mayo Clinic Institution Review Board; written consent (for patients who were >12 years of age) or assent (for patients who were 8 to 12 years of age) was obtained from all participants. Parents provided consent for patients who were 3 to 7 years of age. If the patient met institutional criteria for a GAS test and no exclusion criteria (sore throat lasting longer than 7 days, temperature of 39.4°C or greater, inability to swallow secretions or to keep fluids down, sore throat or head/neck symptoms for more than 4 weeks, GAS-positive test results 4 times within the past 12 months or treatment 2 or more times in the antecedent month, immunocompromised state, ear pain without swallowing, or pregnancy) were present, the patient and/or parent was offered the opportunity to participate in the study and consent or assent was obtained.

A target sample size of 400 subjects (200 independent patients and 200 patients with parents) was planned in order to estimate the concordance between self- and HCW-collected swabs to within ±3.5 percentage points overall and ±5 percentage points within each group with a 95% confidence interval (CI), assuming an a priori 85% concordance rate.

Pharyngeal specimens were collecting using a rayon swab/liquid Stuart collection and transport system (BBL CultureSwab; Becton Dickinson Microbiology Systems, Franklin Lakes, NJ). Participants were given written instructions for pharyngeal swabbing, along with a visual aid for self-collection (Fig. 1) and a mirror. The parent collected the swab for patients 11 years of age and younger. Adolescent patients, 12 years of age or older, were given the option to collect their own swab. No additional guidance was provided. The HCW (one of 21 nurses, nurse practitioners, or physician's assistants with training in pharyngeal swab collection) obtained a second pharyngeal swab at the visit, for a total of two swabs collected per participant. Swabs were transported to the clinical laboratory within 4 h of collection.

FIG 1.

Instructions for collecting a pharyngeal swab. Instructions provided to patients and parents at the time of study participation are presented.

The order of swab collection was randomized so that the patient or parent and the HCW each collected the first swab 50% of the time. Collection order was varied to account for potential sampling bias (i.e., the possibility that the first swab would have a higher yield of GAS DNA than the second) and instructional bias (i.e., the possibility that the HCW would inadvertently provide an example of swab collection when collecting the first swab).

Following swab collection, participants were asked to complete a questionnaire to subjectively rate their perceived degree of difficulty (very difficult, difficult, easy, or very easy) in obtaining the swab and to ascertain whether they had prior training in pharyngeal swab collection. Demographic data, including age, gender, race, and occupation, were also obtained.

Laboratory testing.

Swabs were tested within 24 h of receipt in the laboratory. The real-time PCR assay studied has been previously described (16). Self- and HCW-collected swabs were processed by different technologists in a blind manner with respect to the PCR result of the opposite arm. All positive results, regardless of the collector category, were reported to the health care provider to allow appropriate patient management.

Statistical analysis.

Patient characteristics were summarized with frequencies and percentages or with means, standard deviations (SD), and ranges, as appropriate. The percentages of concordant results between the methods (self-collection and HCW collection of swabs) were estimated along with 95% score confidence intervals (CI). The percentages of results positive for GAS were compared between the methods using McNemar's test for paired categorical data (each pair representing a swab collected by the HCW and a self-collected swab). Overall analyses were performed as well as analyses stratified by who collected the swab, the order of collection (HCW collection versus self-collection), and whether or not the patient or parent performing the self-collection had previously received training for collecting a pharyngeal swab. To assess whether the differences in positivity rates between the methods differed by these factors, we used logistic regression with generalized estimating equations, including an interaction between the method and the factor (i.e., the method and the patient group). Additionally, the average crossing-point values of the positive tests were summarized with 95% confidence intervals and compared between the methods with paired t tests; the average differences between the methods were compared across the factor levels with two-sample t tests. All analyses were performed using SAS version 9 (SAS Institute Inc., Cary, NC). P values of less than 0.05 were considered statistically significant.

RESULTS

Paired specimens were obtained from 402 unique patients (Table 1). In the self-collection group, those who collected their own swab were 72.3% females and aged 12.8 to 72.2 (mean, 33.4) years, while those for whom the parent collected the swab were 53.1% females and aged 4.2 to 23.7 (mean, 10.7) years. The percent positivity values were 33.3% for HCW-collected swabs and 34.3% for self-collected swabs (P = 0.41) (Table 2). The overall concordance between self- and HCW-collected swabs was 94.0% (95% CI, 91.3 to 96.0). Twenty-four of the paired swabs had discordant results, with 10 and 14 positives detected only with the HCW- and self-collected swabs, respectively (P = 0.41). The person collecting the swab in the self-collected arm, the order of collection, and prior swab collection training did not influence results. There was a small but significant difference in the mean CP values between the two collection methods among the 124 specimens that were positive by both methods (HCW-collected swab results were 0.78 cycles higher on average than the self-collected swab results [95% CI, 0.21 to 1.35; P = 0.008]), indicating that more GAS DNA was present in the self-collected swabs (Fig. 2 and Table 3).

TABLE 1.

Demographic and clinic variables of participants in the self-collected group

Variable	Values for participants by study group
	Parent (n = 196)	Patient (n = 206)	Total (n = 402)
Sex
No. (%) female	104 (53.1)	149 (72.3)	253 (62.9)
No. (%) male	92 (46.9)	57 (27.7)	149 (37.1)
Age (yrs)
Mean (SD)	10.7 (3.5)	33.4 (11.9)	22.3 (14.4)
Range	(4.2–23.7)	(12.8–72.2)	(4.2–72.2)
Race
No. (%) Caucasian	182 (92.9)	198 (96.1)	380 (94.5)
No. (%) non-Caucasian	14 (7.1)	8 (3.9)	22 (5.5)
First swab collection method
Self [no. (%)]	98 (50.0)	105 (51.0)	203 (50.5)
HCW [no. (%)]	98 (50.0)	101 (49.0)	199 (49.5)
Prior swab collection training
No [no. (%)]	179 (91.3)	184 (89.3)	363 (90.3)
Yes [no. (%)]	17 (8.7)	22 (10.7)	39 (9.7)

TABLE 2.

Group A Streptococcus real-time PCR results

Collection group	No. of swab pairs	% positive			% concordant (95% CI)	No. of HCW-/self-collection results
						Concordant		Discordant
		HCW	Self	Pa		−/−	+/+b	+/−	−/+
Overall	402	33.3	34.3	0.41	94.0 (91.3, 96.0)	254	124	10	14
First swab: self	203	35.0	36.0	0.53	95.1 (91.2, 97.3)	126	67	4	6
First swab: HCW	199	31.7	32.7	0.59	93.0 (88.5, 95.8)	128	57	6	8
No prior swab collection training	363	34.2	35.0	0.53	93.7 (90.7, 95.7)	226	114	10	13
Prior swab collection training	39	25.6	28.2	0.32	97.4 (86.8, 99.5)	28	10	0	1
Self-collection individual
Parent	196	39.3	38.8	0.78	93.4 (89.0, 96.1)	113	70	7	6
First swab: non-HCW (all)	98	40.8	40.8	1.0	93.9 (87.3, 97.2)	55	37	3	3
First swab: non-HCW (no prior swab collection training)	86	40.7	40.7	1.0	93.0 (85.6, 96.8)	48	32	3	3
First swab: HCW	98	37.8	36.7	0.71	92.9 (86.0, 96.5)	58	33	4	3
Self	206	27.7	30.1	0.13	94.7 (90.7, 97.0)	141	54	3	8
First swab: non-HCW (all)	105	29.5	31.4	0.32	96.2 (90.6, 98.5)	71	30	1	3
First swab: non-HCW (no prior swab collection training)	93	30.1	32.3	0.32	95.7 (89.5, 98.3)	62	27	1	3
First swab: HCW	101	25.7	28.7	0.26	93.1 (86.4, 96.6)	70	24	2	5

^aP values from McNemar's test comparing the percentages of positive results between HCW-collected and self-collected specimens. Of note, the positivity rates of the levels for the factors in the table did not differ significantly (data not shown).

^bCrossing-point values among pairs which were positive via both methods are summarized in Table 3.

FIG 2.

Real-time PCR crossing-point values from HCW-collected versus self-collected swabs for the 124 specimens that were positive by both collection methods. The crossing point (CP), which represents the number of real-time PCR cycles at which a positive PCR is first detected, can be used as a semiquantitative measure of the amount of DNA collected by each swab. The value is inversely proportional to the amount of DNA in the specimen; lower values correlate with larger amounts of DNA. The red line indicates where the data would fall were the CPs exactly the same by the two methods.

TABLE 3.

Summary of differences in crossing points where swab specimens were positive by both HCW-collected and self-collected swab tests^a

Collection group	Parent-collected swab result					Patient-collected swab result
	No of pairs	Difference [mean (SD)]	95% CI	Pb	Pc	No of pairs	Difference [mean (SD)]	95% CI	Pb	Pc
Overall	70	0.86 (3.14)	(0.11, 1.61)	0.02	NA	54	0.67 (3.28)	(−0.22, 1.57)	0.14	NA
First swab: patient or parent	37	1.46 (3.41)	(0.32, 2.60)	0.01	0.09	30	0.88 (3.01)	(−0.24, 2.00)	0.12	0.62
First swab: HCW	33	0.19 (2.70)	(−0.77, 1.15)	0.69		24	0.41 (3.64)	(−1.13, 1.95)	0.58
No prior swab collection training	64	0.86 (3.24)	(0.05, 1.67)	0.04	0.98	50	0.63 (3.25)	(−0.30, 1.56)	0.17	0.82
Prior swab collection training	6	0.88 (1.94)	(−1.15, 2.92)	0.32		4	1.15 (4.18)	(−5.51, 7.81)	0.62
First swab: patient or parent with no prior swab collection training	32	1.51 (3.60)	(0.21, 2.81)	0.02	NA	27	0.74 (2.89)	(−0.40, 1.88)	0.19	NA

^aDifferences were calculated as the HCW-collected swab crossing-point value minus the self-collected swab crossing-point value. NA, not applicable.

^bAssessing whether the HCW-collected result was significantly different from the self-collected result (paired t test).

^cAssessing whether the values for the average HCW-collected swab crossing point versus self-collected swab crossing point differences were significantly different between groups (two-sample t test).

Ease of collection ranking.

Of the 206 patients who collected their own swab (including the 22 patients who had prior swab collection training), 122 and 77 graded the swab collection method as very easy and easy, respectively, while 4 and 0 graded it as difficult and very difficult, respectively, and 3 did not provide a response. Of the 196 parents who collected a swab (including the 17 parents with prior swab collection training), 96 and 92 graded the swab collection as very easy and easy, respectively, while 6 and 1 graded it as difficult and very difficult, respectively, and 1 did not provide a response.

Reasons for declining participation.

Approximately 85% of eligible patients and parents accepted participation in the study. Reasons for declining participation were recorded during recruitment of the final 200 study participants and included concern about the young age of the patient, aversion to subjecting themselves or their child to two pharyngeal swabs, discomfort with the process of obtaining a swab, history of a strong gag reflex, and lack of time.

DISCUSSION

This was the first study to demonstrate that patients or their parents can collect their own or their child's throat swab for GAS detection by PCR with testing results similar to those obtained from swabs collected by trained HCWs. Overall detection rates did not differ between the HCW- and self-collected swabs, even when stratified by person collecting the swab (patient versus parent), order of collection (HCW versus self), and presence or absence of prior training in throat swab collection. The use of real-time PCR with comparisons of CP data allowed us to show that slightly more GAS was detected by self-collected swabs than by HCW-collected swabs.

Thoughtfully developed programs that allow appropriately selected patients or parents and other caregivers to collect throat swabs without requiring a health care visit can limit patient exposure to other sick individuals, improve health care access, increase convenience, and decrease reliance on more costly health care options such as evening urgent care clinics or emergency departments, while still restricting antibiotic use to patients with a positive GAS test result. Our findings are in keeping with those of other studies that have demonstrated success with self-swab collection options, such as influenza testing from midnasal turbinate specimens (17), sexually transmitted infection testing from vaginal swabs (18, 19), and molecular testing of samples from buccal swabs (20, 21), and provide the foundation for implementation of GAS self-swab programs.

Elements of a successful program will need to include appropriate triage (to prevent overuse), provisions for supplies, instructions for collection, designated drop-off sites, and well-defined protocols for notification of results and appropriate treatment when a test result is positive. Specimen stability testing should be performed to ensure that GAS nucleic acid remains stable over the anticipated transportation time from collection to testing. Monitoring usage of the self-swabbing program and its impact on general office visits should also be assessed after the program has been implemented.

At our institution, we envision a self-swab program that is directed by the health care team, starting with an initial nursing phone line to screen and triage patients using clinical (e.g., Centor) criteria. Patients with signs of severe illness will be directed to a health care professional for full evaluation, while eligible patients will be offered the opportunity to collect their own pharyngeal swab. Individuals who do not wish to collect their own swab will still have the option of an office visit with a HCW. A swab kit would be obtained from one of several convenient locations for on-site or home collection and would include simple instructions in several languages, if applicable, with consideration for those who are illiterate. After collection, the swab would be dropped off at a central location to allow rapid delivery to the laboratory. Results would be obtained using our automated phone line. For patients with positive GAS results, an option currently exists at our institution for the laboratory to fax a precompleted antibiotic prescription to the patient's pharmacy of choice as soon as the positive results are detected; this system would be incorporated into a self-swab program.

A self-swab program may offer significant cost savings to the health care system and patient. At our institution, an Emergency Department visit for a chief complaint of sore throat costs the health care system approximately 1.8 times more than an outpatient office visit, while an office visit costs approximately 4 times more than an express care clinic visit. In comparison, the salary and indirect costs incurred during a 10-min telephone call with a registered nurse are approximately 5 times lower than those incurred during an express care clinic visit, indicating that use of a nurse-triage/self-swab model would result in a significant decrease in cost to the health care system. One would expect that the patient charge would also be decreased.

The strengths of this study include its large sample size of both patients and parents, spanning a range of patient ages from 4 to 72 years, as well as prospective data collection and a randomized order of swab collection. The data were subjected to overall analysis as well as to subgroup analysis to assess variables that could have impacted the results. Self-swab collection was obtained using written instructions alone, simulating conditions for at-home collection. Finally, pharyngeal swabs were collected by 21 different HCWs, making it unlikely that the results were biased by the performance of a single individual. The use of real-time PCR in this study enabled quantitative comparisons between the two collection strategies. Results generated using real-time PCR may generalize to detection of other microorganisms in throat swabs by PCR.

The main limitation of the study was that the study population consisted primarily of residents of one city, Rochester, MN, a relatively healthy and mostly Caucasian population which is highly “medically influenced,” given that the largest regional employer is Mayo Clinic. Another potential limitation to the study is that swabbing occurred in a study environment in which patients knew they were participating in research and understood that a second swab would be obtained by a HCW. This may have influenced the amount of effort that the patient or parent devoted to swab collection. It is likely that the actual setting in which a swab would be collected by a patient or parent would be different from a busy clinic practice, which might impact the quality of the collection. Finally, there was a noteworthy (73%) female predominance in the patient self-collection population compared to only a slight (53.1%) female predominance in the parent collect arm; it is unknown if this influenced results. Future studies with greater numbers of male participants might be indicated.

This report provides the first evidence that self-collected swabs provide a reliable alternative to HCW-collected swabs for detection of GAS pharyngitis by PCR. A well-designed swab self-collection program would provide clinicians with a new tool for providing cost-effective and appropriate medical care while empowering patients to take an active role in their health care.