A strep test identifies Streptococcus pyogenes (Group A Streptococcus or GAS) bacteria to diagnose strep throat. While highly effective, these tests are not perfectly accurate, and understanding their limitations is important. An accurate diagnosis ensures timely antibiotic treatment, which helps prevent rare but serious complications like rheumatic fever. Test reliability is affected by the method used, sample collection technique, and the patient’s health status.
The Two Main Types of Strep Tests
The two primary methods for strep throat testing are the Rapid Antigen Detection Test (RADT) and the throat culture. The rapid test is favored for its speed, delivering results in five to ten minutes while the patient is still in the clinic. RADTs detect specific proteins, or antigens, on the surface of GAS bacteria directly from a throat swab. The throat culture, historically the gold standard, involves placing a swab in a growth medium to allow the bacteria to multiply. This method is exceptionally reliable because it allows for the physical isolation and identification of the Streptococcus pyogenes organism, but it requires an incubation period of 24 to 48 hours.
Why a Negative Result Can Be Wrong
A false negative occurs when a person with strep throat receives a negative test result. This error is primarily a concern with the Rapid Antigen Detection Test (RADT) due to its inherent technical limitations and lower sensitivity. The sensitivity of most RADTs ranges from 80% to 90%, meaning they miss 10% to 20% of true infections compared to a traditional culture.
A significant factor contributing to a false negative is insufficient bacterial load, meaning not enough bacteria are present on the swab for the test to detect. This low count occurs if the infection is early in its course or if the swabbing technique was inadequate. Proper technique requires vigorously swabbing the tonsils and the back of the throat, avoiding the tongue. Because the RADT requires a minimum concentration of antigens, it can fail to detect low-level infections that a culture would eventually identify. Therefore, clinical guidelines often recommend following up a negative RADT with a throat culture in children and adolescents, who face a higher risk for serious complications like acute rheumatic fever.
Why a Positive Result Can Be Misleading
A positive strep test result is generally highly reliable, as the specificity of RADTs is excellent, often reaching 95% to 99%. This high specificity means that a true false positive—where the test incorrectly detects GAS antigens due to technical error or cross-reactivity—is quite rare. However, a positive result can be misleading due to the asymptomatic carrier state.
A strep carrier harbors Streptococcus pyogenes without experiencing active infection symptoms. These individuals are typically not contagious and have a very low risk of developing complications. If a carrier develops a sore throat from a viral infection and is tested, the result will be positive because the bacteria are present. The test is technically correct, but the positive result is misleading because the bacteria are not causing the current illness. Treating an asymptomatic carrier with antibiotics is generally not recommended, as it contributes to unnecessary antibiotic use and fails to address the underlying viral cause.
Confirming the Diagnosis After the Initial Test
When the initial Rapid Antigen Detection Test (RADT) is negative, but the physician has a strong clinical suspicion of strep throat, a definitive confirmation is sought through a throat culture. The culture acts as a safety net, especially for children and teenagers, where the consequences of missing a true strep infection are more significant. The swab taken for the RADT is transferred to an agar plate, and the plate is placed in an incubator.
If Streptococcus pyogenes bacteria are present, they will multiply and form visible colonies on the plate, providing a near-certain confirmation of the infection. This backup strategy addresses the sensitivity limitation of the rapid test, ensuring that a low bacterial load does not result in a missed diagnosis. For adults, a negative RADT is often considered sufficient because their risk of developing rheumatic fever is extremely low, making the extra step of a culture less medically necessary. If a positive RADT result is obtained in a patient with minimal symptoms, such as a potential adult carrier, the physician relies on clinical judgment rather than blindly prescribing antibiotics. The decision to treat is a balance between the test result and the patient’s symptom profile, ensuring that only active infections requiring intervention are targeted. Newer molecular tests are also emerging, which offer the speed of a rapid test with sensitivity closer to that of a culture, potentially reducing the need for backup testing in the future.