Evaluation of an ELISA for group A streptococcal antigen for diagnosis of pharyngitis

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.

Rapid antigen detection testing in diagnosing group A β-hemolytic streptococcal pharyngitis

Group A beta-hemolytic streptococcus (GABHS) is the most common bacterial cause of acute pharyngitis. Clinical criteria alone are not reliable enough to diagnose GABHS pharyngitis. Microbiological-testing is required for correct diagnosis. Although a throat swab culture remains the gold standard for documenting the presence of GABHS, a significant disadvantage of the culture is the delayed time of 1-2 days to obtain results. Most rapid antigen detection tests can provide results in less than 15 min. Rapid identification and treatment of patients with GABHS pharyngitis can reduce the risk of the spread of disease, may shorten the duration of symptoms, decrease the incidence of suppurative complications, decrease the amount of time lost from school/work, decrease the inappropriate use of antibiotics, reduce patient/parent dissatisfaction and alleviate the need for costly follow-up visits. All rapid antigen detection tests involve extraction of the group-specific carbohydrate antigen from the GABHS cell wall and identification of the antigen by an immunological reaction. There are numerous rapid antigen detection testing methods, namely latex agglutination, enzyme immunoassay, optical immunoassay, chemiluminescent DNA probes and PCR methods. Most of the rapid antigen detection tests that are currently in use have an excellent specificity of greater than 95% and a sensitivity of greater than or equal to 90%. Owing to the high specificity of the rapid antigen detection tests, a positive rapid antigen detection test is accepted as adequate for the diagnosis of GABHS pharyngitis. Conversely, confirmation of a negative antigen detection test with a throat culture result is necessary, unless the physician has ascertained in his/her practice that the sensitivity of the rapid antigen test used is comparable with that of a throat culture.

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.

Diagnosis and treatment of group a streptococcal pharyngitis

Pharyngitis caused by the group A streptococcus requires accurate diagnosis and timely treatment to prevent acute rheumatic fever. Clinical signs and symptoms often do not distinguish pharyngitis caused by group A streptococci from pharyngitis caused by other microorganisms. Rapid antigen detection or throat culture are recommended for diagnosis except when viral signs and symptoms are prominent. Therapy with penicillin, the drug of choice, is associated with prevention of rheumatic fever, more rapid clinical improvement, and prompt loss of contagiousness. Bacteriologic treatment failure occurs despite universal sensitivity of group A streptococci to penicillin. The cause of treatment failure (and of chronic carriage) remain to be determined. Newer, more expensive antibiotics do not substantially enhance treatment success and need not be prescribed for most patients.

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.

Verification bias in pediatric studies evaluating diagnostic tests

Improperly designed evaluations of diagnostic tests may lead to inaccurate conclusions about a test's accuracy. One problem, verification bias, occurs if subjects are not equally likely to have the diagnosis verified by a gold-standard evaluation and if selection for further evaluation is dependent on the diagnostic test result. To determine whether verification bias is a problem in pediatric studies of diagnostic tests, we conducted a critical appraisal of all studies evaluating diagnostic tests published in three pediatric journals during a 3-year period. Thirty-six percent were subject to verification bias. The most prevalent cause was restriction of the patient sample to those whose diagnosis had been verified by a gold standard evaluation, when the decision to obtain the gold standard was influenced by the diagnostic test result. Verification bias may have serious effects on the estimated sensitivity and specificity of a test. Improved awareness of the potential for verification bias may help physicians improve their selection and interpretation of diagnostic tests and thereby improve the quality and efficiency of patient care.

Rapid antibody capture assay for detection of group-A streptococci using monoclonal antibody and colloidal gold-monospecific polyvalent antibody conjugate

A rapid one step, sensitive and specific antibody capture assay for detection of group-A streptococci from the throat swabs of children is described. Monoclonal antibody either MA-106 or MA-107 specific for group-A streptococci polysaccharide (APS) was used as the capture antibody on nitrocellulose paper and rabbit monospecific polyvalent antibody conjugated with colloidal gold to detect the presence of antigen. The lower detection limit of this assay is 15.6ng APS/ml. The assay is specific for APS and failed to recognize polysaccharides obtained from group-B,-C,-G streptococci as well as Staphylococcus aureus. Antigen extracted from throat swabs of children who were positive for beta-hemolytic plaques (other than group-A streptococci) as seen on blood agar culture gave negative readings, thereby confirming the specificity of the assay for APS.

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

Group A streptococci are generally detected in throat swabs by (i) rapid antigen tests, (ii) conventional culture, or (iii) combinations of both. Direct fluorescent-antibody testing of a 2-h enrichment broth (FA/EN) was an accepted method for same-day results before the advent of rapid antigen tests. We compared FA/EN in Todd-Hewitt Broth (THB) with conventional culture and a rapid antigen test, TestPack Strep A (TPS). Nine hundred seventy specimens were evaluated in this study. Cultures were performed for 48 h on sheep blood agar (SBA) incubated aerobically and on a selective agar for group A streptococci (SSA) incubated in 5 to 10% CO2. Following a 2-h incubation, the fluorescent-antibody test was performed. A subculture of the centrifuged sediment from the THB enrichment was also done. In comparison with a positive culture on SBA or SSA or subculture of the THB pellet, the sensitivities and specificities of the different methods were as follows: SBA, 92 and 100%; SSA, 92 and 100%; TPS, 68 and 99%; FA/EN, 88 and 98%. The FA/EN method offers the potential for definitive finalized reports on the same day as specimen collection with greater sensitivity than TPS. This study included sequential plating and rapid antigen testing of a single swab. In a separate set of experiments to validate this study design, it was shown that recovery of streptococci from swabs plated sequentially on five plates did not vary with the order of plating and the actual proportion of organisms recovered from a swab on a single plate was only 1%.

Comparison of TestPack Plus Strep A with selective and nonselective culture media for detection of group-A streptococci

The TestPack Plus Strep A (TPPSA), an immunoassay method, was compared with conventional culture methods including nonselective trypticase soy agar with 5% sheep blood and selective SXT blood agar for detection of group-A streptococci (GAS). A total of 452 throat swabs was evaluated, of which 261 (57.7%) and 191 (42.3%) were compared with culture using nonselective and selective media, respectively. Of 261 specimens evaluated on nonselective media, 71 (27.1%) were culture positive for GAS. TPPSA demonstrated a sensitivity of 91.6% and a specificity of 94.2% with positive and negative predictive values of 85.5% and 96.8%, respectively. Of 191 specimens evaluated on selective media, 44 (23.0%) were culture positive for GAS. TPPSA demonstrated a sensitivity of 93.2% and a specificity of 98.0% with positive and negative predictive values of 93.2% and 98.0%, respectively. The performance of TPPSA when compared with nonselective and selective media demonstrated a similar sensitivity, but a higher specificity was seen when compared to selective media. Overall, TPPSA was extremely easy to perform, had built-in procedural controls, required minimal technologist time, and was easy to interpret. With an accuracy of 93.5% when compared with nonselective media and 96.9% when compared with selective media, TPPSA could be recommended as a reliable method for detection of GAS.

Evaluation of the modified Visuwell Strep-A enzyme immunoassay for detection of group-A Streptococcus from throat swabs

The modified Visuwell Strep-A enzyme immunoassay (EIA) was compared with culture for detection of group-A Streptococcus from throat swabs. Throat swabs in modified Stuarts medium obtained after culture at two institutions were tested in Visuwell. Cumulative results were n = 417, sensitivity 87.8%, specificity 89.9% predictive value positive (PVP) 67.9%, predictive value negative (PVN) 96.8%, and accuracy 89.5%. At another site, swabs were delivered to the laboratory without transport medium, cultured, and subsequently tested by Visuwell (n = 202, sensitivity 79.6%, specificity 84.5%, PVP 65.2%, PVN 91.9%, accuracy 83.2%). When 1+ culture-positive specimens were considered negative, the sensitivity and PVN increased from 79.6% to 90.2% and 91.9% to 97.1% respectively. Overall performance of the modified Visuwell was comparable with that of the initial assay for throat swabs transported with or without modified Stuarts medium. Cross reaction with organisms other than group-A Streptococcus normally found in the oropharynx was negligible in Visuwell and the limit of detection of group-A Streptococcus was 5 x 10(4) colony-forming units.

Liposome immunoassay for rapid identification of group A streptococci directly from throat swabs

The Q Test Strep (Becton Dickinson and Co., Franklin Lakes, N.J.) is a new solid-phase liposome immunoassay for the rapid diagnosis of group A beta-hemolytic streptococcal pharyngitis. Compared with blood agar plate cultures, the Q Test Strep had a sensitivity of 91%, a specificity of 83%, a positive predictive value of 88%, and a negative predictive value of 87%. Liposome technology can be used to facilitate the rapid diagnosis of group A beta-hemolytic streptococcal pharyngitis.

Detection of group A streptococcal antigen from throat swabs with five diagnostic kits in general practice

During a 5-month period, 248 general practitioners from 164 general practice offices obtained duplicate throat swabs from 2469 patients with acute pharyngotonsillitis. At the general practice office, one of the swabs was assayed for the presence of group A streptococcal antigen, using one of five commercially available diagnostic kits. (The participants did not receive instruction in the use of the kits). The other swab was cultured in our laboratory for the presence of group A streptococci. The diagnostic parameters for the five kits were as follows: Abbott TestPack Strep A--sensitivity, 79% (110/139) and specificity, 98% (490/501); Direct Strep A EIA (Roche)--sensitivity, 79% (104/132) and specificity, 63% (178/281); Respirastick--sensitivity, 55% (46/84) and specificity, 96% (298/309); Reveal Colour Strep A--sensitivity, 82% (114/139) and specificity, 83% (223/268); Tandem ICON Strep A--sensitivity, 78% (111/143) and specificity, 98% (362/368). The antigen detection test results were heavily influenced by the clinical prediction of a streptococcal or viral etiology. It is concluded that in the absence of a standard throat culture, the Abbott TestPack Strep A kit and the Tandem ICON Strep A kit both offer a more safe diagnostic alternative than clinical assessment. Depending on the current prevalence of group A streptococci, the additional use of a throat swab culture should be considered in case of negative antigen detection test results.

Evaluation of two ELISA tests for the rapid detection of group A streptococci

We examined 579 oropharyngeal swab specimens from children presenting with pharyngitis in order to compare two rapid diagnostic ELISA tests for group A streptococcal antigen: the TestPack Strep A (TPSA), and the Direct Strep A EIA (DSAE), with an anaerobically incubated sheep blood agar culture. The sensitivities of the methods were respectively 60.8% (TPSA) and 64.4% (DSAE), the specificities 98.3% (TPSA) and 93.2% (DSAE), the positive predictive values 88.6% (TPSA) and 70.3% (DSAE) and the negative predictive values 92.0% (TPSA) and 91.2% (DSAE). We conclude that the TPSA is easy to interpret and is sufficiently specific to initiate treatment in patients with a positive test. It is not sufficiently sensitive to ensure negative or intermediate results. The performance of the DSAE test is similar but the specificity is slightly lower and more false positive results occur.

Evaluation of the pacific biotech CARDS STREP A test for detecting group A streptococci from cases of pharyngitis

The Pacific Biotech CARD STREP A test for rapid screening of throat specimens for group A streptococci was evaluated. Relative to conventional culture, this assay had a sensitivity of 76.1% (108 of 142) and a specificity of 98.7% (392 of 397). The relatively low sensitivity of the test makes it of questionable value as a screening procedure.

Rapid streptococcal antigen detection in experimental keratitis

We assessed the role of commercially available immunodiagnostic procedures in comparison to Gram stain and culture in experimental bacterial keratitis. Rabbit corneas were inoculated with Streptococcus pneumoniae, S. pyogenes, S. faecalis, or Haemophilus influenzae. Corneal scrapings were processed before and during antibacterial therapy using a coagglutination assay to detect pneumococcal capsular antigen (Phadebact Pneumococcus test) and an enzyme immunoassay to detect group A streptococcal cell-wall antigen (TestPack Strep A test). In untreated infected eyes, both immunoassays were highly specific and as sensitive as Gram stain for detection of the respective microorganisms. For S. pneumoniae keratitis, the sensitivity of coagglutination was 82% and Gram stain, 73%. For S. pyogenes keratitis, the sensitivity of enzyme immunoassay was 100% and Gram stain, 62%. Immunoassays and Gram stain were less sensitive than culture during antibacterial therapy. Successful clinical application of the coagglutination assay in a patient with pneumococcal keratitis permitted early use of specific cephalosporin treatment.

Streptococcal pharyngitis. What's new

Group A beta-hemolytic streptococcal pharyngitis continues to be a major problem and accounts for a large number of physician visits. The recent resurgence of acute rheumatic fever in several areas in the United States underlines the need to accurately diagnose and correctly treat streptococcal pharyngitis. Appropriate treatment with antibiotics effectively prevents rheumatic fever. Early institution of treatment also leads to prompt alleviation of symptoms. The "gold standard" for diagnosing group A streptococcal pharyngitis is the throat culture. Newer rapid diagnostic tests may be used, but the clinician must recognize that there are a substantial number of false-negative tests (low sensitivity). Therefore, patients with negative rapid tests should have standard throat cultures as well. Patients with positive rapid tests should be treated with appropriate antibiotics, as should patients with positive throat cultures. Patients with signs and symptoms that are highly suggestive of streptococcal pharyngitis can also be treated, pending throat culture results. Penicillin continues to be the drug of choice for treatment, and American Heart Association guidelines suggest the use of oral penicillin V for ten days or intramuscular benzathine penicillin G. Alternative antibiotics commonly used include erythromycin and various cephalosporins. Throat cultures need not be obtained from most patients after therapy. However, some patients may seem to be having frequent streptococcal infections or may be recognized as asymptomatic carriers. Carriers may be considered for therapy with intramuscular benzathine penicillin G plus oral rifampin.

Evaluation of TestPack Strep A for the detection of group A streptococci in throat swabs

The performance of TestPack Strep A (Abbott Laboratories), a rapid enzyme immunoassay, was compared with a culture-based method for the detection of group A streptococci in 648 throat swabs. The rapid test correctly detected 99 of the 128 positive and 511 of the 520 negative specimens, a sensitivity of 77% and a specificity of 98%. Although highly specific, TestPack Strep A is less sensitive than culture techniques for the detection of group A streptococci in throat swabs.