What is a GAS Rapid Antigen Test and Is It a Serum Test?
A GAS (Group A Streptococcus) rapid antigen test is NOT a serum test—it is performed on a throat swab specimen obtained directly from the tonsils and posterior pharyngeal wall. 1
Specimen Collection Method
The test requires a throat swab, not blood or serum. The swab must be obtained from the surface of either tonsils (or tonsillar fossae) and the posterior pharyngeal wall to maximize accuracy. 1
Other areas of the oral pharynx and mouth are not acceptable sites for specimen collection, as they will not provide accurate results. 1
Proper swabbing technique is crucial—the healthcare provider must immobilize the patient's neck (especially in uncooperative children) to obtain an adequate and representative specimen. 1
How the Test Works
RADTs detect the group-specific cell-wall carbohydrate antigen of Group A Streptococcus directly from the throat swab specimen through immunologic reactions. 1
Results are available within minutes, which is the primary advantage over traditional throat cultures that require 18-24 hours or longer. 1
The test uses various methodologies including enzyme immunoassay techniques, chemiluminescent DNA probes, or optical immunoassay, with newer tests offering more sharply defined endpoints and increased sensitivity. 1
Test Performance Characteristics
Specificity is excellent at approximately 95%, meaning false-positive results are highly unusual and therapeutic decisions can be made confidently on the basis of a positive test result. 1
Sensitivity ranges from 70-90% compared to blood agar plate culture, which means the test can miss 10-30% of true infections. 1
In children and adolescents (3-18 years), a negative RADT requires backup throat culture due to the lower sensitivity, but in adults, backup culture after negative RADT is generally not necessary. 2
Common Pitfalls to Avoid
Do not confuse RADTs with streptococcal antibody tests—antibody tests measure past immune response in serum and are not appropriate for diagnosing acute pharyngitis. 2
Recent antibiotic use can cause false-negative results if the patient received antibiotics shortly before or at the time of specimen collection. 1
Inadequate swabbing technique (such as only swabbing the tongue or cheek) will result in inaccurate results regardless of test quality. 1