From the Guidelines
Rapid strep tests (strep swabs) are generally quite accurate, with a specificity of about 95%, meaning they rarely give false positives, but their sensitivity ranges from 80-90%, indicating they can miss some cases of strep throat, as reported in the most recent guideline by the Infectious Diseases Society of America 1. The accuracy of a strep swab depends on several factors, including proper technique when collecting the sample, which involves swabbing both tonsils and the back of the throat.
- The test works by detecting specific antigens from Group A Streptococcus bacteria.
- For adults, a negative rapid test is often considered sufficient to rule out strep throat without further testing, as the risk of complications is lower.
- However, in children, confirming strep infection is crucial to prevent complications like rheumatic fever, and a negative rapid test result might still require a follow-up throat culture. The test provides results within minutes, making it a valuable tool for quick diagnosis in clinical settings, though it's not perfect, and the clinical significance of the number of GAS colonies on the throat culture plate is problematic, as there is too much overlap between patients with true acute GAS pharyngitis and streptococcal carriers 1. Key points to consider when interpreting the results of a strep swab include:
- The manner in which the swab is obtained has an important impact on the yield of streptococci.
- False-negative results may be obtained if the patient has received an antibiotic shortly before the throat swab is obtained.
- The duration of incubation can also affect the throat culture result, and additional incubation overnight at room temperature may identify a number of additional positive throat culture results 1.
From the Research
Accuracy of Strep Swab
- The accuracy of a strep swab can be measured by its sensitivity and specificity, with sensitivity referring to the ability to correctly identify those with the disease and specificity referring to the ability to correctly identify those without the disease 2.
- A study published in the International Journal of Pediatric Otorhinolaryngology found that the sensitivity and specificity of rapid antigen detection for group A beta-hemolytic streptococci were 64.6% and 96.79%, respectively 2.
- The positive predictive value was 80.95%, whereas the negative predictive value was 92.82%, indicating that a positive result is more likely to be a true positive, while a negative result is more likely to be a true negative 2.
Factors Affecting Accuracy
- The accuracy of a strep swab can be affected by various factors, including the age of the patient, with sensitivity and positive predictive value being higher in children than in adults 2.
- The specimen collection technique and education of healthcare workers can also impact the accuracy of the test 2.
- Other studies have found that the use of clinical decision rules, such as the modified Centor score, can help improve the accuracy of diagnosis and reduce unnecessary antibiotic use 3, 4.
Comparison with Other Diagnostic Methods
- Throat culture is considered the diagnostic standard for streptococcal pharyngitis, but rapid antigen detection testing has improved significantly in terms of sensitivity and specificity 4.
- Polymerase chain reaction (PCR) has high sensitivity and specificity for detection of group A streptococcus, but its use is not routine in all clinical settings 5.
- A systematic review and meta-analysis found that antibiotics for pharyngeal group A streptococcal infection can achieve a high rate of culture conversion within 24 hours, but further research is needed to inform public health control measures 6.