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 10-20% of actual strep throat cases, as reported in the most recent study by 1. This means a positive result is reliable, but a negative result may require a follow-up throat culture for confirmation, especially if symptoms strongly suggest strep throat. The accuracy of strep swabs depends on several factors, including:
- Proper sample collection technique, with throat swab specimens obtained from the surface of either tonsils (or tonsillar fossae) and the posterior pharyngeal wall, as emphasized by 1
- The specific test brand used, with some tests being more sensitive than others, as noted by 1
- Whether the patient has recently taken antibiotics, which can interfere with results, as mentioned by 1 Healthcare providers often use clinical judgment alongside test results, considering factors like:
- Fever
- Swollen lymph nodes
- Absence of cough or cold symptoms For children and adolescents, follow-up cultures are more commonly recommended after negative rapid tests due to the importance of treating strep to prevent complications like rheumatic fever, while this is less necessary for adults, as suggested by 1. It is essential to note that the clinical significance of the number of GAS colonies on the throat culture plate is problematic, and the differentiation between acute infection and asymptomatic carriage cannot be made accurately on this basis alone, as stated by 1. Therefore, the use of strep swabs should be guided by the most recent and highest-quality evidence, and clinical judgment should be used to interpret the results and make treatment decisions, as recommended by 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.