From the Guidelines
The likelihood of having a positive throat culture but a negative rapid strep test is approximately 10-20%. This discrepancy occurs because rapid strep tests, while highly specific (95-99%), have lower sensitivity (70-90%) compared to throat cultures, which are considered the gold standard 1. The difference in detection rates is due to the rapid test requiring a higher bacterial load to show positive results, while cultures can detect smaller numbers of bacteria through growth amplification.
Key Points to Consider
- The sensitivity of rapid strep tests can be as low as 80% compared to throat cultures, which can lead to false-negative results 1.
- Throat cultures are considered the gold standard for diagnosing Group A Streptococcal (GAS) pharyngitis, but they require a longer wait time for results (24-48 hours) 1.
- Factors that can affect test accuracy include proper specimen collection technique, recent antibiotic use, and the stage of illness when testing occurs 1.
- In clinical practice, especially for children and adolescents, a negative rapid strep test is often followed by a confirmatory throat culture when strep throat is strongly suspected based on symptoms 1.
Recommendations for Practice
- Use rapid strep tests as an initial diagnostic tool, but consider confirmatory throat cultures in cases where the rapid test is negative but clinical suspicion remains high 1.
- Ensure proper specimen collection technique to maximize the accuracy of both rapid strep tests and throat cultures 1.
- Be aware of the potential for false-negative results with rapid strep tests and consider the clinical context when interpreting test results 1.
From the Research
Likelihood of a Positive Throat Culture and Negative Rapid Strep
- The likelihood of a positive throat culture and negative rapid strep test is not directly stated in the provided studies, but it can be inferred that false negatives can occur with rapid strep tests 2, 3, 4.
- A study from 2000 found that using a high-sensitivity rapid strep test without culture confirmation of negative results did not lead to an increase in streptococcal complications 4.
- The sensitivity and specificity of rapid antigen detection testing have improved significantly, but throat culture is still considered the diagnostic standard 3.
- The modified Centor score can be used to help physicians decide which patients need no testing, throat culture/rapid antigen detection testing, or empiric antibiotic therapy 3, 5.
- A multi-criteria decision analysis found that the optimal management of adults with pharyngitis depends on both the clinical probability of a group A streptococcal infection and clinical judgments that incorporate individual patient and practice circumstances 5.
Factors Influencing the Likelihood
- The quality of the specimen can affect the results of rapid strep tests 2.
- The clinical manifestations of group A streptococcal and nonstreptococcal pharyngitis overlap, making diagnosis challenging 6.
- The population prevalence of Group A streptococcal pharyngitis can influence the optimal clinical management strategy 5.
Diagnostic Testing
- Throat culture is considered the diagnostic standard, but rapid antigen detection testing can be used to identify streptococcal pharyngitis, especially in patients with severe symptoms or special situations 2, 3.
- The use of a rapid strep test with culture confirmation of negative results is a recommended management strategy for patients with a high clinical probability of group A streptococcal infection 5.