What is a Rapid Strep Test?
A rapid strep test involves swabbing the throat to check for the presence of group A Streptococcus bacteria, which can cause Strep Throat and other medical issues (including scarlet fever, pneumonia and heart problems). Within minutes, you can know if your sore throat is caused by bacteria, which requires antibiotics for treatment.
A rapid strep test is an antigen detection test, meaning the test is checking for the concentration of bacteria in the sample. This method is widely used in doctor offices, clinics, urgent cares, and emergency departments and now is available to use at home! Before the invention of the rapid strep test, a throat culture was the only way to know if your sore throat was caused by bacteria. The throat culture, however, takes 2 to 3 days to give results, whereas the rapid test gives you results in 5 minutes.
Obtaining a Sample
Obtaining a specimen is the same whether your doctor orders a throat culture or rapid strep test. A cotton swab, similar to a Q-tip, is quickly rubbed over both tonsils as well as the back wall of the mouth (see diagram below). The swab is then placed in a specialized container and five drops of special reagents are added to the container.
There are several manufactures of the rapid strep test and each has similar limitations. The test will respond only to the presence of the particular Group A Streptococcal bacteria and will not detect other bacteria, which are less much less likely to cause sore throats. Also, it will not detect viral causes of sore throat.
Sensitivity and Specificity
The sensitivity of the rapid strep test is approximately 95%. This means that if we swabbed 100 patients who we confirmed had Strep Throat (by using a throat culture), the test would result ‘positive’ in 95 of 100 of those patients. There would be 5 “false negatives”, meaning 5 of 100 patients with strep throat will be missed using a rapid strep test. Therefore, patients are always advised to follow up with their doctor, especially if their symptoms continue, so that a throat culture can be performed. A throat culture that comes back positive also requires antibiotics, just like a positive rapid strep test.
The rapid strep test has a 98% specificity. This means that 98 of 100 positive tests correctly indicate the presence specifically of Group A Streptococcus bacteria; 2 of 100 positive results are "false positives," meaning that the patient is either carrying the Strep bacteria in their tonsils but does not have an active infection from it, or the test detected a similar bacteria by accident. Although a rapid test cannot distinguish a strep throat infection from asymptomatic carriage of the organism, most authorities recommend antibiotic treatment in the event of a positive rapid test result from a person with a sore throat.
In summary, rapid strep tests are a quick and convenient tool that your doctor can use to assist in the diagnosis of Strep Throat.