Strep Test

What is a strep test?

A strep test looks for infection caused by bacteria called Group A streptococcus.

Why is this test done?

Strep tests are done to find out if strep bacteria are causing a sore throat. If your health care provider finds that you have strep throat, he or she will prescribe antibiotics. Treatment of strep throat with antibiotics helps you feel better more quickly than if you do not have treatment. It also reduces the chance of developing more serious problems that can be caused by strep, such as heart or kidney problems. Strep is the only common cause of sore throat for which antibiotics are needed.

How do I prepare for this test?

It is best not to take any antibiotics before a check for strep. Tell your health care provider if you took antibiotics during the 3 days before the test.

How is the test done?

There are two types of strep tests: throat culture and rapid antibody tests. For both tests your health care provider gets a sample by rubbing a cotton swab against a tonsil in the back of your throat. The sample is sent to a lab.

If the rapid antibody test is done, the lab checks for a chemical reaction between a protein (antigen) in the cell wall of strep bacteria and another protein (antibody) in the test kit. The lab will have the result in 1 hour or less. If the test is positive, it means that strep bacteria were in the sample from your throat.
If a throat culture is done, the lab checks for growth of strep bacteria in the sample overnight. This test may be done to check the results of a rapid strep test. Results are usually available in 24 to 48 hours.

How will I get the test result?

Ask your health care provider when and how you will get the result of your test.

What does the test result mean?

Usually, a positive strep test result means that you have strep, and a negative result means that you don't have strep throat.

Although these tests are very precise, they are not perfect. Results of the rapid tests are wrong about 10% of the time. Cultures are wrong just 1% to 2% of the time if a good sample was obtained.

The strep culture test provides more information than the rapid antibody test. In addition to showing whether you have strep throat, it may show the specific kind (strain) of strep bacteria infecting your throat. It can also help your health care provider know which antibiotic will be most effective in treating the infection. For this reason, your provider may not prescribe an antibiotic until the test results are back.

If your test result is positive, ask your health care provider:

what antibiotic he or she is prescribing
if you need additional tests
when you need to be tested again.

This content is reviewed periodically and is subject to change as new health information becomes available. The information is intended to inform and educate and is not a replacement for medical evaluation, advice, diagnosis or treatment by a healthcare professional.

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