Rapid strep test
|This article needs additional citations for verification. (May 2010)|
The Rapid Strep Test (RST), or rapid antigen detection test (RADT), is an in-office test done by a clinician in order to determine whether or not a patient has streptococcal pharyngitis (colloquially called "strep throat" in American English), which is a group A streptococcal infection of the pharynx and possibly other parts of the body. The Rapid Strep Test is one of the most common tests for streptococcal pharyngitis.
Streptococcal pharyngitis is a common infection caused by the Streptococcus pyogenes bacterium, also known as group A Streptococcus or beta-hemolytic Streptococcus. It is generally treated aggressively because of the potential serious complications of any group A streptococcal infection. In order to treat pharyngitis adequately it is helpful to identify the infection, but none of the symptoms are specific to streptococcal pharyngitis; they can also be caused by other diseases, so testing is often used.
This is a test usually performed in a doctor's office. The throat is swabbed, and results are available in 10 or 15 minutes. Kits are available for home use, but many physicians are concerned that home testing is prone to the overdiagnosis of strep infections.
However, one problem with the test is that, though it has high specificity of approximately 95 to 98%, the sensitivity is only 95%. This means that the odds of a false positive are lower than the odds of a false negative and one can be more confident about a positive result than a negative result. If the rapid test is negative, a follow-up culture (which takes 24 to 48 hours) might be performed. A negative culture could suggest a viral infection, infection with other typical bacterium such as Haemophilus influenzae, or an infection with an atypical bacterial organism like Mycoplasma.
The rapid strep test works by detecting the presence of a carbohydrate antigen unique to group A Streptococcus. This may account for some of the problems with the rapid strep test sensitivity. Gargling, eating, or other infusion of liquids into the mouth prior to the test may affect the results. If the test is performed before sufficient organisms are present in the throat, late in the infection when most of the strep germs have been cleared by the immune system, or after someone has been partially treated with antibiotics that kill the organism, then the rapid strep test is less likely to detect the organism. The downside to the rapid test is that it only looks for Group A Strep. Both Groups C and G Strep can also cause streptococcal pharyngitis. This is one of the main reasons why a negative Rapid Strep test automatically reflexes to a Beta Strep culture. The culture, and subsequent application of the latex agglutination test (PathoDex) to any Beta strep colonies will indicate if either Group C or G (or both) are present.
If the rapid strep test detects strep, the infection should be treated with appropriate antibiotics to prevent long-term damage and sequelae. Should the rapid strep test fail to detect streptococcal pharyngitis, the clinician might still treat the throat infection based on his or her own judgment. Viruses do not respond to antibiotics. Most of the other organisms are not treated aggressively because they do not generally cause the long term sequelae of group A Streptococcus and are considered to be self-limited, though some of them can still respond to antibiotics.
- Nohle, Robert. "Parents Stress over Strep".
- Robert D. Sheeler et al., "Accuracy of Rapid Strep Testing in Patients Who Have Had Recent Streptococcal Pharyngitis". Journal of the American Board of Family Medicine, 2002-09-04.
- John Mersch, "Rapid Strep Test" at MedecineNet.com.