From the Guidelines
Rapid strep swabs can have a significant rate of false negatives, with sensitivities ranging from 70% to 90% compared to blood agar plate cultures. This means that while a positive rapid test is reliable, a negative result doesn't definitively rule out strep throat 1. The false negatives occur because rapid tests detect bacterial antigens and require a substantial bacterial load to show positive, whereas cultures can detect smaller amounts of bacteria. Factors that can increase false negatives include improper swabbing technique, recent antibiotic use, or testing early in the infection when bacterial counts are lower. Some key points to consider when interpreting rapid strep test results include:
- The specificity of rapid strep tests is approximately 95%, making false-positive test results highly unusual 1
- The sensitivity of rapid strep tests is lower, ranging from 70% to 90%, which can lead to false negatives 1
- Clinical judgment remains important, and patients with classic strep symptoms (fever, tonsillar exudate, swollen lymph nodes, absence of cough) might be treated empirically with antibiotics like penicillin or amoxicillin while awaiting culture results
- A negative rapid test should be accompanied by a follow-up or back-up throat culture in children and adolescents, while this is not necessary in adults under usual circumstances 1.
From the Research
Rapid Strep Swab Accuracy
- The rapid strep swab has been shown to have a high specificity, with a study from 2 finding a specificity of 0.96, indicating a low rate of false positives.
- However, the sensitivity of the rapid strep swab has been found to be lower, with a meta-analysis from 3 estimating a sensitivity of 0.86, indicating a potential for false negatives.
- The study from 2 also found that the rapid strep test was more reliable in patients who had recent streptococcal pharyngitis, with a sensitivity of 0.91 compared to 0.70 in those without recent infection.
- Another study from 3 found that molecular-based rapid antigen diagnostic tests had the best diagnostic accuracy, with a sensitivity of 0.86 and a specificity of 0.96.
False Negative Rates
- The false negative rate of the rapid strep swab is a concern, as it may lead to undertreatment of streptococcal pharyngitis.
- A study from 4 recommends that a throat culture be performed after a negative rapid antigen test result in children and adolescents to confirm the diagnosis.
- The study from 3 found that the overall summary estimate for sensitivity of rapid antigen diagnostic tests was 0.86, indicating that approximately 14% of cases may be missed.
- The use of molecular-based rapid antigen diagnostic tests may help to reduce the false negative rate, as they have been shown to have higher sensitivity than other types of tests 3.