Sore throats are a frequent complaint, but determining whether the cause is a simple virus or Group A Streptococcus (GAS), which causes strep throat, is crucial for treatment. Prompt diagnosis is important because untreated infection can lead to complications such as rheumatic fever, a serious condition affecting the heart and joints. Fortunately, a diagnostic tool provides a near-immediate answer, allowing healthcare providers to start antibiotic treatment quickly.
The Rapid Strep Test (RADT)
The quick diagnostic tool used is the Rapid Antigen Detection Test, commonly referred to as the RADT or Rapid Strep Test. This test identifies antigens, which are unique surface proteins, belonging to the Streptococcus pyogenes bacterium. Detecting these specific markers confirms the presence of the bacteria responsible for strep throat.
The RADT is a point-of-care assay, meaning it can be performed right in a healthcare provider’s office or clinic, eliminating the need to send the sample to an outside laboratory. This streamlined process allows for fast results, typically within five to ten minutes. This rapid turnaround contrasts sharply with the traditional throat culture, which requires 24 to 48 hours for the bacteria to grow and provide a result.
The Testing Process and Immediate Interpretation
The RADT procedure begins with collecting a sample from the patient’s throat using a sterile cotton swab. The provider gently brushes the swab over the back of the throat and tonsils, targeting red areas or visible patches of exudate to ensure a good sample is collected. This collection process takes only a few seconds and is the most uncomfortable part of the test.
The swab is immediately exposed to a chemical reagent that extracts the bacterial antigens, which are then introduced to the testing device. If GAS antigens are present, the test displays a positive result, often indicated by a visible line or color change. A positive result is highly specific, meaning it is reliable evidence of an active infection, and antibiotic treatment can be started right away.
A negative result means the test did not detect the target bacterial antigens. In some cases, a second swab is collected during the initial procedure. If the rapid test is negative, the second swab can be sent for a more comprehensive laboratory test. This strategy helps ensure that an actual infection is not missed, particularly in specific patient populations.
Accuracy and Confirmatory Testing
The accuracy of the RADT is described using two measures: specificity and sensitivity. Specificity refers to the test’s ability to correctly identify people who do not have the infection. RADTs are known for high specificity, often above 95%, meaning a positive result is highly dependable, and generally no further testing is needed.
Sensitivity is the test’s ability to correctly identify people who do have the infection, and this is where RADTs have limitations. Sensitivity is often lower than specificity, estimated to be around 85.6%. This means a rapid test may miss a small number of true infections, resulting in a false-negative result that could leave the strep throat untreated.
Because of the risk of false-negative results, clinical guidelines recommend confirmatory testing for certain patients. For children and adolescents, a negative RADT is typically followed up with a traditional throat culture or a molecular test, such as Polymerase Chain Reaction (PCR). The throat culture is considered the “Gold Standard” because it is more accurate, though it takes 24 to 48 hours to get results.
Molecular tests, which detect the bacteria’s genetic material, offer high sensitivity and faster turnaround than a culture. For adults, a back-up culture is not usually required after a negative RADT. This is because the incidence of strep throat and the risk of developing serious complications like rheumatic fever are significantly lower than in younger populations.