False positive detection of group A Streptococcus antigen resulting from cross-reacting Streptococcus intermedius (Streptococcus milleri group)

Detection of group a Streptococcus in pharyngitis by two rapid tests: comparison of the BD Veritor™ and the QuikRead go® Strep A

We compared the performance of two rapid antigen tests-QuikRead go® Strep A test (Orion Diagnostica, Espoo, Finland) and BD Veritor™ system (Becton, Dickinson and Company, Sparks, MD) with throat culture. Our aim was to evaluate each assay's performance and agreement compared to throat culture in order to choose one of the assays as a point-of-care test in the emergency room. One hundred throat samples were collected in triplicates from patients with suspected pharyngitis admitted to the emergency room. One throat swab was seeded for a throat culture. The other two throat swabs from each patient were analyzed at the emergency room by the QuikRead go® Strep A test, and by the BD Veritor™ system, according to each manufacturer's instructions. Agreement level between BD Veritor™ test and throat culture was 79%; sensitivity and specificity of this test were 80% and 78.7%, respectively. QuikRead go® Strep A test had an agreement level of 75% with throat culture; sensitivity and specificity of this test were 80% and 73.3%, respectively. Both tests have a good diagnostic performance. Other characteristics such as costs, size of instrument, and ease of implementation should be taken into consideration when choosing a point-of-care test.

Reassessment of the Role of Rapid Antigen Detection Tests in Diagnosis of Invasive Group A Streptococcal Infections

Rapid antigen detection tests (RADTs) for group A streptococci (GAS) are widely used for diagnosing acute pharyngitis, which has led to a considerable reduction in antibiotic prescriptions over the past decade. Beyond this intended use, their reassessment on invasive samples may be relevant in the management of life-threatening GAS infections. To this end, we evaluated the performances of three RADTs, culture, GAS PCR, and 16S rRNA gene PCR assays, and compared them with a composite gold standard (GAS-PCR assay and/or culture) for the diagnosis of severe GAS infection. A total of 192 specimens from deep-tissue (mostly normally sterile) sites enriched for 75 GAS-positive samples were enrolled in the study. The three evaluated RADTs showed sensitivities ranging from 88.0% to 94.7% versus 98.7% for GAS PCR, 84% for 16S rRNA gene PCR, and 77.3% for culture. The sensitivities of the ImmunoCardSTAT! Strep A test (Meridian Bioscience) and the NADAL Strep A strip (Nal Von Minden) were similar to that of GAS PCR (P= 0.25 and 0.03, respectively) and higher than that of culture (P= 0.001 and 0.006, respectively), whereas the SD Bioline Strep A test strip (Standard Diagnostics) showed a performance similar to that of culture (P= 0.02). The three RADTs detected 10 distinctemmtypes, including a predominance ofemm1 (33.3%),emm89 (10.6%), andemm12 (7.6%). No false-positive results were observed, leading to a specificity of 100% for all the evaluated RADTs. The GAS RADTs turned out to be sensitive, specific, and easy-to-use tools that may aid in the management of invasive GAS infections in 24/7 point-of-care laboratories by enabling early diagnosis and focused therapy.

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.

Rapid-antigen detection tests for group a streptococcal pharyngitis: revisiting false-positive results using polymerase chain reaction testing

We investigated mechanisms of the false-positive test results on rapid-antigen detection test (RADT) for group A Streptococcal (GAS) pharyngitis. Most RADT false-positives (76%) were associated with polymerase chain reaction-positive GAS results, suggesting that RADT specificity could be considered close to 100%. Finding that 61% of GAS culture-negative but RADT-positive cases were positive on both GAS polymerase chain reaction and Staphylococcus aureus testing, we posit bacterial inhibition as causative.

Rapid diagnosis of pharyngitis caused by group A streptococci

Although commercial rapid antigen detection tests (RADTs) are more expensive than blood agar plate (BAP) cultures, the advantage they offer is the speed with which they provide results. Rapid identification and consequent prompt treatment of patients with pharyngitis due to group A beta-hemolytic streptococci (GABHS) can reduce the risk of spread of GABHS, can allow patients to return to school or work sooner, and may reduce the acute morbidity of this illness. In most studies, RADTs have been compared with BAP cultures as the criterion standard. However, these comparisons are complicated by the fact that there is no universally accepted procedure for performing a BAP culture. The great majority of the RADTs that are currently available have a high specificity (i.e., 95% or greater) and a sensitivity of between 70 and 90% compared with BAP cultures. Few published studies have compared the performance of various RADTs to each other or examined the performance of various RADTs in the office setting. There is also relatively little published information about how physicians in practice actually use RADTs, but the available information suggests that many physicians do not follow recommended guidelines. While the development of easy-to-perform RADTs for the diagnosis of GABHS pharyngitis has altered clinical practice substantially, only limited data about cost-effectiveness are currently available.

Pseudoepidemic of streptococcal pharyngitis in a hospital pharmacy

CONTEXT

Streptococcus pyogenes has recently re-emerged as a significant pathogen causing disease ranging from pharyngitis to lethal systemic infection. Six hospital pharmacy employees were diagnosed as having streptococcal pharyngitis during 1 week, and antibiotic prophylaxis was requested to halt the outbreak.

OBJECTIVE

Outbreak investigation.

DESIGN

Review of initial cases and prospective evaluation of the remaining pharmacy employees and the antigen detection test being used.

SETTING

Pharmacy and occupational health department of a university hospital.

POPULATION

Sixteen employees of the hospital pharmacy and 19 other employees of the hospital.

RESULTS

The 6 pharmacy employees who had positive streptococcal antigen detection tests did not have symptoms suggesting streptococcal pharyngitis. Of the 10 remaining pharmacy employees, none had a positive throat culture for S pyogenes. Specificity of the antigen detection test being used was 53% (95% CI, 30%-75%) in prospective evaluation.

CONCLUSIONS

This was believed to represent a pseudoepidemic because none of the 6 cases had signs or symptoms typical of streptococcal pharyngitis, none of the remaining 10 pharmacy employees had positive throat cultures, and prospective evaluation found low specificity of the antigen detection test. Whereas use of an accurate test in such a low prevalence setting could have resulted in a higher percentage of results being false-positive, the low specificity of the antigen detection test being used also contributed to the pseudoepidemic.