Strep throat, a common bacterial infection, can cause a painful sore throat, fever, and difficulty swallowing. Diagnosing strep throat accurately is important because it is treated with antibiotics to prevent potential complications like rheumatic fever. While diagnostic tests for strep throat are generally reliable, understanding the circumstances under which their results might be inaccurate is beneficial for patients.
How Strep Tests Work
Healthcare providers use two main types of tests to diagnose strep throat: the Rapid Antigen Detection Test (RADT) and the throat culture. The RADT is a quick method that identifies specific proteins, called antigens, from Streptococcus pyogenes bacteria directly from a throat swab. Results are typically available within 10 to 20 minutes, allowing for rapid diagnosis.
The throat culture, considered the “gold standard” for strep diagnosis, involves growing bacteria from a throat swab in a laboratory setting for definitive identification. While more accurate, results take longer, typically 24 to 48 hours, as the bacteria need time to grow.
Why Results Can Be Misleading
Despite their utility, strep tests can sometimes produce misleading results, leading to either false negatives or false positives. A false negative occurs when a person has strep throat, but the test indicates no infection. This can happen if the bacterial load is too low (e.g., early infection), if the throat swab did not collect enough bacteria, or due to improper swabbing technique. Rapid strep tests are more prone to false negatives, missing 10% to 30% of actual strep cases.
Conversely, a false positive means the test indicates strep throat when no active infection is present. This is less common, especially with throat cultures, which have a high specificity (95% to 98%). Reasons include the presence of Streptococcus pyogenes bacteria in the throat without an active infection (being a “carrier”), contamination during sample collection or processing, or lingering bacterial antigens after a recent strep infection has resolved.
When to Question Your Test Result
There are specific situations where you might question the accuracy of a strep test result. If you receive a negative test result but your symptoms, such as a severe sore throat, fever above 101°F, painful swallowing, or swollen lymph nodes, persist or worsen, it may indicate a false negative. These persistent symptoms, especially without typical cold symptoms like a runny nose or cough, warrant further consideration.
You might also question a positive result if your symptoms are unusually mild or atypical for strep throat, or if you do not improve after starting antibiotic treatment. If you have been in close contact with someone diagnosed with strep throat but your test comes back negative, it could be a reason for concern. Healthcare providers also rely on their clinical judgment, considering the full picture of symptoms and local prevalence of strep, even alongside test results.
What Happens Next
If you suspect an inaccurate strep test result, discussing your concerns with a healthcare provider is important. They will evaluate your symptoms, medical history, and the specific test results to determine the appropriate next steps.
If an RADT was negative but strep throat is still strongly suspected, particularly in children and adolescents, your doctor might recommend a follow-up throat culture. Throat cultures are more accurate and can detect infections missed by rapid tests. The healthcare provider may also consider other potential causes for your symptoms, such as viral infections. Based on the clinical assessment and any retesting, your doctor may adjust treatment or investigate other conditions.