Strep throat is a bacterial infection of the throat, caused by Group A Streptococcus (GAS). Unlike viral sore throats, strep throat requires antibiotic treatment. Accurate diagnosis is important to prevent serious issues like rheumatic fever, which can cause permanent heart damage. Timely identification also reduces bacterial spread and ensures appropriate antibiotic use, combating resistance.
Types of Strep Tests
Healthcare providers use two methods to test for strep throat: Rapid Antigen Detection Tests (RADTs) and throat cultures. RADTs involve a throat swab to detect Group A Streptococcus. They identify specific bacterial antigens, providing quick results, often within 10 to 20 minutes.
Throat cultures also use a swab but require lab processing. The sample is cultured to grow bacteria. This takes longer, with results typically in 24 to 48 hours.
Understanding Test Accuracy
Test reliability is described by sensitivity and specificity. Sensitivity refers to a test’s ability to correctly identify infected individuals (true positives). High sensitivity means very few false negatives. Specificity measures a test’s ability to correctly identify uninfected individuals (true negatives). High specificity means fewer false positives.
Rapid Antigen Detection Tests (RADTs) generally have high specificity, often exceeding 95%, meaning false-positive results are uncommon. This allows confident antibiotic treatment for positive RADT results. However, RADT sensitivity is lower, typically ranging from 70% to 90%. This means that RADTs can produce false-negative results.
Throat cultures are the “gold standard” for diagnosis due to their higher sensitivity. Though slower, they can detect bacteria RADTs might miss, ensuring true infections are not overlooked.
Factors Influencing Test Outcomes
Several factors can influence strep test accuracy. The technique used for sample collection is particularly important. A healthcare provider must use a sterile cotton swab to firmly rub the back of the throat and tonsils, avoiding contact with the lips, teeth, or tongue. Improper swabbing can lead to an insufficient sample, potentially causing an inaccurate result.
The timing of the test during the course of the infection can also affect outcomes. Testing too early, before the bacterial load is high enough, might result in a false negative. Recent antibiotic use can suppress bacterial growth, making it harder for tests to detect the presence of Group A Streptococcus. Patients are generally advised to inform their doctor about any recent antibiotic use or if they have used mouthwash before the test.
Some individuals can carry the strep bacteria in their throat without showing any symptoms, a condition known as asymptomatic carriage. Some asymptomatic school-aged children can be carriers. While carriers are less likely to spread the bacteria and rarely develop complications, they can still test positive, which may complicate diagnosis if they have symptoms from a different illness.
Interpreting Results and Next Steps
Interpreting strep test results involves considering both the test outcome and the patient’s clinical symptoms. A positive Rapid Antigen Detection Test (RADT) generally indicates the presence of Group A Streptococcus, and treatment with antibiotics is typically initiated. Further testing is usually not needed when a RADT is positive.
If a RADT result is negative, but the healthcare provider still suspects strep throat based on symptoms, especially in children and adolescents, a follow-up throat culture is often performed. This is because RADTs can produce false negatives, and untreated strep throat in younger individuals carries a greater risk of complications like rheumatic fever. For adults, a follow-up culture after a negative rapid test is not always necessary, as the risk of rheumatic fever is lower in this population. A negative throat culture confirms that the sore throat is likely caused by a viral infection, which does not require antibiotics.