Comparison of three methods for detection of group A streptococci in throat swabs

Rapid antigen detection test for group A streptococcus in children with pharyngitis

BACKGROUND

Group A streptococcus (GAS) accounts for 20% to 40% of cases of pharyngitis in children; the remaining cases are caused by viruses. Compared with throat culture, rapid antigen detection tests (RADTs) offer diagnosis at the point of care (within five to 10 minutes).

OBJECTIVES

To determine the diagnostic accuracy of RADTs for diagnosing GAS in children with pharyngitis. To assess the relative diagnostic accuracy of the two major types of RADTs (enzyme immunoassays (EIA) and optical immunoassays (OIA)) by indirect and direct comparison.

SEARCH METHODS

We searched CENTRAL, MEDLINE, EMBASE, Web of Science, CDSR, DARE, MEDION and TRIP (January 1980 to July 2015). We also conducted related citations tracking via PubMed, handsearched reference lists of included studies and relevant review articles, and screened all articles citing included studies via Google Scholar.

SELECTION CRITERIA

We included studies that compared RADT for GAS pharyngitis with throat culture on a blood agar plate in a microbiology laboratory in children seen in ambulatory care.

DATA COLLECTION AND ANALYSIS

Two review authors independently screened titles and abstracts for relevance, assessed full texts for inclusion, and carried out data extraction and quality assessment using the QUADAS-2 tool. We used bivariate meta-analysis to estimate summary sensitivity and specificity, and to investigate heterogeneity across studies. We compared the accuracy of EIA and OIA tests using indirect and direct evidence.

MAIN RESULTS

We included 98 unique studies in the review (116 test evaluations; 101,121 participants). The overall methodological quality of included studies was poor, mainly because many studies were at high risk of bias regarding patient selection and the reference standard used (in 73% and 43% of test evaluations, respectively). In studies in which all participants underwent both RADT and throat culture (105 test evaluations; 58,244 participants; median prevalence of participants with GAS was 29.5%), RADT had a summary sensitivity of 85.6%; 95% confidence interval (CI) 83.3 to 87.6 and a summary specificity of 95.4%; 95% CI 94.5 to 96.2. There was substantial heterogeneity in sensitivity across studies; specificity was more stable. There was no evidence of a trade-off between sensitivity and specificity. Heterogeneity in accuracy was not explained by study-level characteristics such as whether an enrichment broth was used before plating, mean age and clinical severity of participants, and GAS prevalence. The sensitivity of EIA and OIA tests was comparable (summary sensitivity 85.4% versus 86.2%). Sensitivity analyses showed that summary estimates of sensitivity and specificity were stable in low risk of bias studies.

AUTHORS' CONCLUSIONS

In a population of 1000 children with a GAS prevalence of 30%, 43 patients with GAS will be missed. Whether or not RADT can be used as a stand-alone test to rule out GAS will depend mainly on the epidemiological context. The sensitivity of EIA and OIA tests seems comparable. RADT specificity is sufficiently high to ensure against unnecessary use of antibiotics. Based on these results, we would expect that amongst 100 children with strep throat, 86 would be correctly detected with the rapid test while 14 would be missed and not receive antibiotic treatment.

Diagnosing streptococcal pharyngitis in the emergency department: Is a sore throat score approach better than rapid streptococcal antigen testing?

Background:

Reducing the number of unnecessary antibiotic prescriptions given for common respiratory infections has been recommended as a way to limit bacterial resistance. This study assessed the validity of a clinical sore throat score in 2 community emergency departments (EDs) and its impact on antibiotic prescribing. We also attempted to improve on this approach by using a rapid streptococcal antigen test.

Methods:

A total of 126 patients with new upper respiratory tract infections accompanied by sore throat were assessed by a physician. Pharyngeal swabs were obtained for a rapid test and throat culture, and information was gathered to determine the sore throat score. The sensitivity and specificity of the score approach were compared with usual physician care based on the rapid test results.

Results:

Of the 126 cases of new upper respiratory infections with sore throat, physicians who followed their usual care routine, guided by the rapid test results, prescribed antibiotics for 46 patients. Of the 46 prescriptions, 18 were given to patients with culture-negative results for group A streptococcal (GAS) pharyngitis. Use of the sore throat score would not have reduced the number of prescriptions but would have missed only 1 patient with a positive culture result (p< 0.05). The rapid test was not as sensitive as throat culture.

Conclusions:

An explicit clinical score approach to the management of GAS pharyngitis is valid in a community ED setting and could improve the pattern of antibiotic prescribing. While the addition of a rapid streptococcal antigen test significantly decreased the sensitivity of detecting GAS infections, a combined approach consisting of the clinical score and throat culture for patients with negative results on the rapid test would decrease antibiotic prescribing and telephone follow-up without decreasing the sensitivity of detecting GAS infection.

PRImary care Streptococcal Management (PRISM) study: in vitro study, diagnostic cohorts and a pragmatic adaptive randomised controlled trial with nested qualitative study and cost-effectiveness study

BACKGROUND

Antibiotics are still prescribed to most patients attending primary care with acute sore throat, despite evidence that there is modest benefit overall from antibiotics. Targeting antibiotics using either clinical scoring methods or rapid antigen detection tests (RADTs) could help. However, there is debate about which groups of streptococci are important (particularly Lancefield groups C and G), and uncertainty about the variables that most clearly predict the presence of streptococci.

OBJECTIVE

This study aimed to compare clinical scores or RADTs with delayed antibiotic prescribing.

DESIGN

The study comprised a RADT in vitro study; two diagnostic cohorts to develop streptococcal scores (score 1; score 2); and, finally, an open pragmatic randomised controlled trial with nested qualitative and cost-effectiveness studies.

SETTING

The setting was UK primary care general practices.

PARTICIPANTS

Participants were patients aged ≥ 3 years with acute sore throat.

INTERVENTIONS

An internet program randomised patients to targeted antibiotic use according to (1) delayed antibiotics (control group), (2) clinical score or (3) RADT used according to clinical score.

MAIN OUTCOME MEASURES

The main outcome measures were self-reported antibiotic use and symptom duration and severity on seven-point Likert scales (primary outcome: mean sore throat/difficulty swallowing score in the first 2-4 days).

RESULTS

The IMI TestPack Plus Strep A (Inverness Medical, Bedford, UK) was sensitive, specific and easy to use. Lancefield group A/C/G streptococci were found in 40% of cohort 2 and 34% of cohort 1. A five-point score predicting the presence of A/C/G streptococci [FeverPAIN: Fever; Purulence; Attend rapidly (≤ 3 days); severe Inflammation; and No cough or coryza] had moderate predictive value (bootstrapped estimates of area under receiver operating characteristic curve: 0.73 cohort 1, 0.71 cohort 2) and identified a substantial number of participants at low risk of streptococcal infection. In total, 38% of cohort 1 and 36% of cohort 2 scored ≤ 1 for FeverPAIN, associated with streptococcal percentages of 13% and 18%, respectively. In an adaptive trial design, the preliminary score (score 1; n = 1129) was replaced by FeverPAIN (n = 631). For score 1, there were no significant differences between groups. For FeverPAIN, symptom severity was documented in 80% of patients, and was lower in the clinical score group than in the delayed prescribing group (-0.33; 95% confidence interval -0.64 to -0.02; p = 0.039; equivalent to one in three rating sore throat a slight rather than moderately bad problem), and a similar reduction was observed for the RADT group (-0.30; -0.61 to 0.00; p = 0.053). Moderately bad or worse symptoms resolved significantly faster (30%) in the clinical score group (hazard ratio 1.30; 1.03 to 1.63) but not the RADT group (1.11; 0.88 to 1.40). In the delayed group, 75/164 (46%) used antibiotics, and 29% fewer used antibiotics in the clinical score group (risk ratio 0.71; 0.50 to 0.95; p = 0.018) and 27% fewer in the RADT group (0.73; 0.52 to 0.98; p = 0.033). No significant differences in complications or reconsultations were found. The clinical score group dominated both other groups for both the cost/quality-adjusted life-years and cost/change in symptom severity analyses, being both less costly and more effective, and cost-effectiveness acceptability curves indicated the clinical score to be the most likely to be cost-effective from an NHS perspective. Patients were positive about RADTs. Health professionals' concerns about test validity, the time the test took and medicalising self-limiting illness lessened after using the tests. For both RADTs and clinical scores, there were tensions with established clinical experience.

CONCLUSIONS

Targeting antibiotics using a clinical score (FeverPAIN) efficiently improves symptoms and reduces antibiotic use. RADTs used in combination with FeverPAIN provide no clear advantages over FeverPAIN alone, and RADTs are unlikely to be incorporated into practice until health professionals' concerns are met and they have experience of using them. Clinical scores also face barriers related to clinicians' perceptions of their utility in the face of experience. This study has demonstrated the limitation of using one data set to develop a clinical score. FeverPAIN, derived from two data sets, appears to be valid and its use improves outcomes, but diagnostic studies to confirm the validity of FeverPAIN in other data sets and settings are needed. Experienced clinicians need to identify barriers to the use of clinical scoring methods. Implementation studies that address perceived barriers in the use of FeverPAIN are needed.

TRIAL REGISTRATION

Current Controlled Trials ISRCTN32027234.

SOURCE OF FUNDING

This project was funded by the NIHR Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 18, No. 6. See the NIHR Journals Library website for further project information.

A comparison between the Strep A Rapid Test Device and conventional culture for the diagnosis of streptococcal pharyngitis

BACKGROUND

Rapid antigen detection tests are frequently used to diagnose pharyngitis due to Streptococcus pyogenes. Because a large number of kits are available commercially, performance characteristics may vary considerably. The present study evaluated one such kit currently in use in Canadian laboratories for which published evaluations are not available.

OBJECTIVE

To evaluate the performance characteristics of the Strep A Rapid Test Device (SARTD) (Nova Century Scientific Inc, Canada).

METHODS

Pharyngeal swabs from 818 patients with suspected streptococcal pharyngitis were tested. Swabs were initially inoculated onto the surface of a blood agar plate and then used to perform the rapid antigen test. The test was performed in accordance with the product monograph. Beta-hemolytic colonies were identified as S pyogenes using conventional means.

RESULTS

Four hundred ninety specimens were obtained from children and 328 from adults. S pyogenes was recovered from 171 (21%) patients. The SARTD detected S pyogenes antigens in 123 of 171 specimens from which S pyogenes was isolated on culture; the screen was negative in 610 of 647 specimens from which cultures were negative. The positive and negative predictive values of the SARTD were 76.9% and 92.7%, respectively.

CONCLUSIONS

The SARTD was much less sensitive (72%) than was suggested in the product monograph (90%). Laboratories should vigorously evaluate such products in-house, optimize specimen collection and transport, and choose more sensitive kits for use.

In vitro evaluation of five rapid antigen detection tests for group A beta-haemolytic streptococcal sore throat infections

BACKGROUND

Using accurate and easy to use rapid antigen detection tests (RADTs) to identify group A beta-haemolytic Streptococci (GABHS) sore throat infections could reduce unnecessary antibiotic prescribing and antimicrobial resistance. Although there is no international consensus on the use of RADTs, these kits have been widely adopted in Finland, France and the USA. Yet in the UK, the Clinical Knowledge Summaries, that provide the main online guidance for GPs, discourage RADTs use, citing their poor sensitivity and inability to impact on prescribing decisions in acute sore throat infections.

OBJECTIVE

The purpose of this study was to evaluate the ease of use and in vitro accuracy (sensitivity and specificity) of the five most commonly used RADTs in Europe (OSOM Ultra, Quickvue Dipstick, Streptatest, Clearview Exact Strep A and IMI Test Pack).

METHODS

To ensure the RADTs were evaluated objectively, a standardized in vitro method using known concentrations of GABHS was used to remove the inherent biases associated with clinical studies.

RESULTS

The IMI Test Pack was the easiest RADT to use overall. The ability to detect all positive GABHS (sensitivity) varied considerably between kits from 95% [95% confidence interval (CI): 88-98%], for the IMI Test Pack and OSOM, to 62% (95% CI: 51-72%) for Clearview, at the highest GABHS concentration. None of the RADTs gave any false-positive results with commensal flora-they were 100% specific.

CONCLUSIONS

The IMI Test Pack is most suitable for use in primary care, as it had high sensitivity, high specificity and was easy to use.

Recovering streptococci from the throat, a practical alternative to direct plating in remote tropical communities

Throat swabs are regarded as the "gold standard" for diagnosing streptococcal pharyngitis and for surveillance research. Culturing throats in remote tropical settings is logistically difficult, and these settings are commonly burdened by high rates of streptococcal disease. The survival of streptococci on swabs may depend on whether they are of "throat" or "skin" type, as determined by emm pattern typing. The aims of this study were to compare the recovery rates of beta-hemolytic streptococci (BHS) using three different transport methods and to determine whether the recovery rates correlated with the emm pattern type. Monthly duplicate throat swabs were taken from occupants of selected households in three remote Aboriginal communities. Paired swabs were separated and handled in one of three ways: (i) direct inoculation onto culture media with cold-box transport (plated), (ii) sealed in a bag with a silica gel desiccant and cold-box transport (desiccant), and (iii) transport at ambient temperature and humidity (ambient). emm pattern typing was done by standard methods. Over 23 months, 4,842 throat swabs were taken, and 4,122 were paired. BHS were recovered on 11.5% of the 4,842 occasions (group A, 4.5%; group C, 1.7%; group G, 5.4%). Results from paired swabs showed the plated method was superior to desiccant and desiccant was better than ambient. Pooled data indicated that plated and desiccant were equivalent, and both were significantly better than ambient. There was no correlation between the emm pattern type and recovery of group A streptococci by different methods. In tropical and remote settings, cold-box transport with desiccant and subsequent inoculation of culture plates in the laboratory is a practical alternative to direct plating.

Comparison of LightCycler PCR, rapid antigen immunoassay, and culture for detection of group A streptococci from throat swabs

We compared the performance characteristics of a real-time PCR method, the LightCycler Strep-A assay (Roche Applied Science, Indianapolis, Ind.), to those of a rapid antigen immunoassay, the Directigen 1-2-3 Group A Strep Test kit (BD Diagnostic Systems, Sparks, Md.), and a standard culture method for detection of group A streptococci (GAS) from 384 throat swabs. The LightCycler PCR produced more positive results (n = 58) than either culture (n = 55) or the Directigen immunoassay (n = 31). The results of the LightCycler PCR and the Directigen method were independently compared to the results of the accepted "gold standard," bacterial culture. The sensitivities, specificities, and positive and negative predictive values for this comparison were as follows: for the Directigen method, 55, 99, 97, and 93%, respectively; for the LightCycler PCR, 93, 98, 88, and 99%, respectively. In no case was a throat swab positive by both the LightCycler PCR and the Directigen method but negative by culture. The medical histories of patients whose throat swabs were negative by culture but positive by either the LightCycler PCR (n = 7) or the Directigen method (n = 1) were reviewed. All of these patients had signs or symptoms compatible with GAS disease, and therefore, all of these discordant positive results (along with positive results by either the Directigen method or the LightCycler PCR that agreed with the culture results) were counted as true positives for statistical analysis. For this analysis, the LightCycler PCR detected more true-positive results than the culture method (58 versus 55 swabs); however, this difference was not statistically significant (P = 0.5465). In contrast, statistically significantly more true-positive results occurred by culture than by the Directigen method (55 versus 31 swabs; P < 0.0001) and by the LightCycler PCR than by the Directigen method (58 versus 31 swabs; P < 0.0001). The LightCycler PCR is a suitable stand-alone method for the detection of GAS from throat swabs. Additionally, this method requires less than half the personnel time and the procedure can be completed in considerably less time ( approximately 1 h) than our standard approach (up to 2 days) for detection of GAS in throat swabs (i.e., testing by the Directigen method with negative results verified by culture).

Management of group A streptococcus pharyngitis with a second-generation rapid strep screen: Strep A OIA

Empirical therapy of presumed group A streptococcus (GAS) pharyngitis leads to unnecessary and inappropriate use of antibiotics and pressure for emergence of resistant strains of common bacteria. Immunoassay technology has produced several rapid tests for GAS. The wide variations in diagnosis and treatment of acute pharyngitis are reviewed including the impact of rapid strep tests. Published studies comparing Strep A OIA to routine agar culture and other rapid strep screens are presented to assess the reliability and applicability of this novel assay. These data suggest that Strep A OIA may be superior to the "gold standard" throat culture and other rapid streptococcal antigen detection assays. Judicious use of Strep A OIA can result in rapid, precise diagnoses and eliminate unnecessary and inappropriate use of antibiotics. Careful antibiotic selection can reduce the pressure for emergence of resistant bacteria. This innovative, effective assay can contribute to a lessening of this serious clinical challenge.

Importance of inoculum size and sampling effect in rapid antigen detection for diagnosis of Streptococcus pyogenes pharyngitis

Current recommendations suggest that negative rapid Streptococcus pyogenes antigen tests be backed up with a culture, reflecting evidence that culture may have a higher sensitivity and also that testing of a second swab may yield a different (i.e., a positive) result because of variation in sample size or distribution. If the latter is common, the sensitivities of current antigen detection tests might be improved by simply increasing the amount of sample tested. The present study assessed the effect of antigen testing of two swabs extracted together compared to independent testing of each swab extracted separately for children with clinical pharyngitis. S. pyogenes grew from one or both swabs for 198 (37%) of 537 children. The combined culture was significantly (P < 0.05) more sensitive than culture of either swab alone. Compared to combined culture, antigen testing of two swabs extracted and tested together was significantly more sensitive than two single swab extractions (94.1 versus 80%; P = 0.03); however, the specificity was decreased (81.5 versus 89.8 to 92.7%; P < 0.05). This study suggests that sample size and/or uneven sample distribution may have influenced the apparent sensitivities of prior studies that compared antigen tests to a single plate culture. A strategy, such as the one used in the present study, that increases the sample size available for antigen testing (i.e., extraction of samples from both swabs) may improve detection rates to a level that will better approximate true disease status and obviate the need for backup cultures if specificity can be improved.

Evaluation of the patient with sore throat, earache, and sinusitis: an evidence based approach

Sore throat, earache, and sinusitis are common presenting complaints in the emergency department, and all fall within the top ten in the United States. These complaints usually have a benign course but rarely can be a symptom of a serious disease process. This article provides an evidence-based review of the literature regarding the diagnosis of pharyngitis, otitis media, and sinusitis.

A comparison of the BioStar Strep A OIA rapid antigen assay, group A Selective Strep Agar (ssA), and Todd-Hewitt broth cultures for the detection of group A Streptococcus in an outpatient family practice setting

In some studies the BioStar Strep A OIA (optical immunoassay) has yielded inconsistent results, although originally it was reported to be more sensitive than conventional culture for the detection of group A Streptococcus (GAS). The Group A Selective Strep Agar with 5% sheep blood (ssA) incubated anaerobically has been reported to be more sensitive than conventional culture in the detection of GAS. We compared the BioStar Strep A OIA GAS rapid antigen detection kit to anaerobic culture on ssA with and without preincubation in Todd-Hewitt broth (THB) for the detection of GAS. From September 1995 through January 1996, throat swabs were collected in duplicate from 75 children (< or = 18 years) and 188 adults (> 18 years) who presented with pharyngitis in the outpatient University of New Mexico Family Practice Clinic. Thirty-one (12%) of the 263 cases were positive for GAS by culture and/or broth. Compared with anaerobic culture on the ssA, with and without preincubation in THB, the sensitivity, specificity, positive predictive value, and negative predictive value of the BioStar Strep A OIA were 77, 62, 22, and 95%, respectively. Compared with enrichment in THB followed by subculture on ssA and anaerobic incubation, the sensitivity, specificity, positive predictive value, and negative predictive value of direct culture on ssA and anaerobic incubation were 79, 99, 98, and 96%, respectively. All isolates were serologically grouped. The BioStar Strep A OIA is as sensitive as direct culture on ssA incubated anaerobically, but the low specificity and low positive predictive value when the OIA is used in low prevalence populations could lead to unnecessary antibiotic treatment.

Rapid antigen testing for group A Streptococcus by DNA probe

The Gen-probe group A Streptococcus direct test (GASD), a nucleic acid probe assay for detecting GAS from throat swabs, has recently been developed. The test uses an acridium ester-labeled DNA probe which is complementary to the rRNA of Streptococcus pyogenes. In this study, 318 single culturette throat swabs were tested by this method using culture as a "gold standard." After plating onto trypticase soy agar plates with 5% sheep blood, swabs were stored at 4 degrees C for no more than 72 h before the probe assay was performed. Our patient population consisted of symptomatic outpatients seen in the Memorial Hospital Emergency Department and in the Family Care Center. After discrepancy testing, sensitivity, specificity, and positive and negative predictive values were 91.4%, 97%, 91.4%, and 97%. The GASD is a rapid, easy-to-perform method for batch screening for streptococcal pharyngitis.

Relative value of selective group A streptococcal agar incubated under different atmospheres

A commercially available selective group A streptococcal agar (ssA) was evaluated for the recovery of group A streptococci (GAS) in comparison with recovery from simultaneous cultures on conventional sheep blood agar (SBA). Both sets of plates were incubated in air, 5% CO2, and anaerobically for 48 h, with a first reading taken at 24 h. A total of 402 (67.0%) GAS were isolated from the 600 specimens that were submitted. Recovery of GAS was significantly greater after 48 h of incubation than after 24 h of incubation for each medium-atmosphere combination. After 48 h of incubation, the sensitivities of GAS detection obtained by each culture technique were as follows: ssA-anaerobic atmosphere, 98.5%; SBA-anaerobic atmosphere, 89.5%; ssA-CO2 atmosphere, 88.0%; SBA-air, 86.5%; SBA-CO2 atmosphere, 82.0%; and ssA-air, 74.6%. There were no cultures positive in air or CO2 which were not positive anaerobically on either medium. The increased sensitivity of detecting positive GAS cultures when incubation was done in an ssA-anaerobic atmosphere for 48 h uncovered patients truly infected with the organisms.

Comparison of two rapid streptococcal antigen detection assays with culture for diagnosis of streptococcal pharyngitis

In this study, 801 pharyngeal specimens were cultured for group A streptococci and tested with the Biostar Strep A Optical Immunoassay (Strep A OIA). The respective sensitivities and specificities were as follows: culture, 97.1 and 100%; Strep A OIA, 91.5 and 94.8%. Of the 801 specimens, 597 were also tested with the Abbott TestPack Strep A Assay (TP-ST). For those specimens tested by all three methods, the respective sensitivities and specificities were as follows: culture, 98.1 and 100%; Strep A OIA, 92.3 and 95.4%; and TP-ST, 79.4 and 100%. The Strep A OIA is significantly more sensitive than TP-ST and compares favorably with culture.

Superiority of conventional culture technique over rapid detection of group A Streptococcus by optical immunoassay

An optical immunoassay (OIA) has been reported to be more sensitive than conventional culture for the detection of Group A Streptococcus, eliminating the need for culture. We attempted to confirm the sensitivity and specificity through a laboratory quantitation study and a clinical trial. OIA did not detect Group A Streptococcus below 10(5) colony forming units (CFU). Culture detected Streptococcus to 10(2) CFU from the inoculated swab. In the clinical study, throat swabs were obtained from 77 patients in an outpatient clinic. Compared with culture, the sensitivity of OIA was 78% and the specificity was 90%. These results demonstrate that OIA was less sensitive than culture in seeded experiments and missed 22% of positives in clinical practice. Our study, contrary to previous reports, suggests that OIA is not sensitive enough to be used as the sole assay for Group A Streptococcus pharyngitis.

Evaluation of a new immunologic test kit for rapid detection of group A streptococci, the Abbott Testpack Strep A plus

We compared the Testpack Strep A plus (TPSAP) enzyme-linked immunosorbent assay test for group A beta-hemolytic streptococcal (GAS) antigen rapid detection with blood agar culture in 454 pediatric patients with clinical pharyngitis. Of the 454 patients, 118 (25.9%) had positive oropharyngeal cultures for GAS. TPSAP sensitivity was 89.9% (106 of 118) and specificity was 95.8% (322 of 336). We conclude that the TPSAP is specific enough to indicate treatment for a patient with a positive test but that a negative test should be confirmed by culture.

Evaluation of two rapid antigen assays, BioStar Strep A OIA and Pacific Biotech CARDS O.S., and culture for detection of group A streptococci in throat swabs

Two rapid methods, BioStar Strep A OIA (OIA; BioStar, Inc., Boulder, Colo.), an optical immunoassay, and CARDS O.S. (O.S.; Pacific Biotech, Inc., San Diego, Calif.), a color immunochromographic assay, and two culture methods, one with 5% sheep blood agar (SBA) and one with Todd-Hewitt broth (TH; Remel, Lenexa, Kans.), were evaluated for use in the detection of Streptococcus pyogenes from pharnygeal swabs. Seven hundred forty-six double swabs (Culturette II) were processed, with OIA and SBA culture performed on one swab and O.S. and SBA culture performed on the other swab. The pledget from the Culturette II was incubated overnight in TH and was subcultured onto SBA for an additional 48 h in ambient air. All beta-hemolytic streptococci from culture were tested by a direct fluorescent-antibody test (Difco Laboratories, Detroit, Mich.). Specimens with discordant fluorescent-antibody test and rapid test results were also tested by using the Streptex latex agglutination reagent (Murex Diagnostics Limited, Dartford, England). The results obtained by all testing methods were compared with a combined test result ("gold standard"), which was defined as any positive culture detected by the SBA or TH culture methods and confirmed by Streptex latex agglutination or, in the case of negative results by both culture methods, a concomitant positive result by OIA and O.S. antigen testing. Sensitivity and specificity results for each of the methods were as follows, respectively: OIA, 81.0 and 97.5%; O.S., 74.4 and 99.0%; SBA culture, 92.3 and 98.3%; and TH culture, 86.4 and 100%. Both OIA and O.S. are suitable screening methods for detecting S. pyogenes directly from throat swabs but are of insufficient sensitivity to eliminate the need for backup cultures for specimens with negative OIA and O.S. results.

Comparison of Gen-Probe Group A streptococcus Direct Test with culture for diagnosing streptococcal pharyngitis

The Group A Streptococcus Direct Test (GP-ST test; Gen-Probe, Inc., San Diego, Calif.) was compared with culture for the detection of Streptococcus pyogenes from throat swabs of 767 patients with pharyngitis. Swabs were tested by the GP-ST test after inoculating a 5% sheep blood agar (SBA) plate. SBA plates were incubated at 35 degrees C in room air for 72 h. SBA plates with no evidence of beta-hemolytic colonies after 18 to 24 h of incubation were subcultured by taking a swipe across the primary inoculum from the SBA plate to an agar selective for Streptococcus spp. In a low-prevalence (11.9%) population and in comparison with the number of positive cultures detected by the 72-h single-culture method (SBA plate method), the GP-ST test had a sensitivity of 88.6%, a specificity of 97.8%, a positive predictive value of 83.9%, and a negative predictive value of 98.5%. In comparison with the growth of any colonies of S. pyogenes on the 72-h SBA plates plus a subculture onto selective blood agar, the sensitivities and specificities were as follows: 72-h SBA plate method, 96.7 and 100%, respectively; GP-ST test, 85.7 and 97.8%, respectively. The GP-ST test is an easy-to-perform, reliable test for batch screening of throat swabs for S. pyogenes.

Comparison of the Gen-Probe Group A streptococcus Direct Test with culture and a rapid streptococcal antigen detection assay for diagnosis of streptococcal pharyngitis

The Gen-Probe Group A Streptococcus Direct Test (GP-ST) is a new assay which utilizes a nucleic acid probe to detect group A streptococci directly from pharyngeal swabs. In this study, 1,103 specimens were cultured and tested by GP-ST. The sensitivities and specificities were as follows: culture, 98.8 and 100%; GP-ST, 92.4 and 99.6%. Of the 1,103 specimens, 808 were also tested with the TestPack Strep A assay. For the specimens tested by all three methods, the sensitivities and specificities were as follows: culture, 99.5 and 100%; TestPack Strep A assay, 76.3 and 99.7%; GP-ST, 93.5 and 99.7%. The GP-ST is a very user-friendly assay which has the potential to replace culture for the diagnosis of streptococcal pharyngitis.

Comparison of direct specimen testing utilizing TestPack strep A with testing of specimens following a two-hour broth enrichment

In a previous study, we demonstrated that a 2-h enrichment of throat swabs in broth followed by a direct fluorescent antibody test was significantly more sensitive than the Abbott TestPack Strep A for the detection of group-A streptococci. In this study, in an effort to simplify the technique, we utilized broth enrichment with an enzyme immunoassay detection method. A total of 1017 pharyngeal specimens were tested with TestPack Strep A (DTP), culture on a selective streptococcal agar, and a 2-h broth enrichment in Todd-Hewitt broth (THB) followed by TestPack Strep A (ETP). Subculture of the THB was used to arbitrate discordant test results. The sensitivities and specificities respectively were as follows: DTP (69% and 99%), ETP (90% and 98%), and SSA (97% and 100%). This enrichment method offers significantly greater sensitivity than direct testing for antigen yet still allows the potential for finalized same-day reporting of results.