Strep throat is a common bacterial infection causing discomfort in the throat and tonsils. When symptoms appear, a rapid strep test is often used to determine if Group A Streptococcus bacteria are present. While these tests offer quick results, a false negative is possible, meaning the test indicates no infection even when bacteria are present.
How Strep Tests Work
Healthcare providers typically use two main types of tests to identify Group A Streptococcus bacteria: rapid antigen detection tests (RADTs) and throat cultures. A rapid strep test involves swabbing the back of the throat to collect a sample. This sample is then immediately tested for specific antigens found on the surface of the bacteria. Results from a rapid strep test are usually available within minutes, often around 15 minutes.
In contrast, a throat culture also begins with a throat swab, but the collected sample is then sent to a laboratory. There, technicians place the sample on a special plate to allow bacteria to grow. This process allows for the detection of even small amounts of bacteria that might be missed by a rapid test. While throat culture results are highly accurate, they typically take 24 to 48 hours to become available because of the time required for bacterial growth.
Why False Negatives Occur
A false negative result on a rapid strep test can happen for several reasons, primarily due to the test’s sensitivity limitations. Rapid tests work by detecting bacterial antigens, and they require a sufficient amount of bacteria to be present in the sample to register a positive result. If the bacterial load is low, such as in the very early stages of an infection, the test might not detect it.
Improper swabbing technique is another factor that can lead to a false negative. If the swab does not collect enough bacteria from the tonsils and the back of the throat, the test may not accurately reflect the presence of the infection. Additionally, if antibiotics were recently used, even a small dose can reduce the bacterial count in the throat, making it harder for the test to detect the infection. Issues with the test kit itself, such as expired or faulty kits, could also contribute to an inaccurate negative result.
When to Be Concerned After a Negative Test
Even with a negative rapid strep test, certain persistent or worsening symptoms should raise concern and warrant further evaluation. A severe sore throat that comes on suddenly and makes swallowing painful, especially when accompanied by a fever, is a common indicator of strep throat. Swollen, tender lymph nodes in the neck are also frequently observed.
Visual signs like red and swollen tonsils, sometimes with white patches or streaks of pus, or tiny red spots on the roof of the mouth (petechiae), strongly suggest a strep infection. A rash, known as scarlet fever, can also develop with strep throat, often appearing first on the neck and chest. The absence of typical cold symptoms, such as a cough or runny nose, alongside these strep-like symptoms, can further suggest a bacterial cause rather than a viral one.
Taking Action After a Negative Result
If symptoms strongly suggest strep throat despite a negative rapid test, seeking follow-up with a healthcare provider is prudent. Healthcare professionals often recommend a throat culture in such cases, especially for children and adolescents. A throat culture is considered the “gold standard” for diagnosis, offering higher accuracy in detecting Group A Streptococcus bacteria even when rapid tests miss them.
Timely treatment of strep throat with antibiotics is important to prevent potential complications. Untreated strep can lead to serious conditions like rheumatic fever, which can affect the heart, joints, and nervous system, or kidney inflammation (post-streptococcal glomerulonephritis). Antibiotics can also help reduce the spread of the infection to others. If severe symptoms like difficulty breathing or swallowing develop, immediate medical attention is necessary.