Rapid strep test
Overview
Strep throat is a common infection caused by Group A Streptococcus. It is generally treated aggressively because of the potential for subsequent heart valve, kidney, lung and joint damage from post-streptococcal auto-immune response. In order to treat strep throat it is helpful to identify the infection, but none of the symptoms are specific to strep, so testing is often used.
One of the most common tests for strep throat is the Rapid Strep Test. This is a test performed in a doctor's office. The throat is swabbed, and results are generally available in 10 or 15 minutes. However, one problem with the test is that, though it has high specificity of approximately 95 to 98% [1], the sensitivity is only 75 to 85%[2]. 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. 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 Strep and are considered to be self-limited, though some of them can still respond to antibiotics.
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, or late in the infection when most of the strep germs have been cleared by the immune system, or if it is performed after someone has been partially treated with antibiotics that kill the organism, then the Rapid Strep Test is less likely to detect the organism.
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 strep throat, the doctor might still treat the throat infection based on his or her own judgment.