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Tests and diagnosis
By Mayo Clinic staffDoctors usually diagnose the cause of a sore throat on the basis of a physical exam and lab tests. During the exam, your doctor looks for signs and symptoms of strep throat, such as fever and enlarged lymph nodes, and will probably use a tongue depressor to get a good look at your throat and tonsils.
Your doctor will check for redness, swelling, and white streaks or pus on the tonsils. There also may be tiny red spots on the soft or hard palate — the area at the back of the roof of the mouth. Although these signs indicate an infection, there's no way to accurately tell by just looking whether it's viral or bacterial. In fact, some viral throat infections look worse than infections caused by streptococcal bacteria.
For that reason, your doctor is likely to use one or more of the following tests to check for the presence of bacteria, including streptococcal bacteria:
- Throat culture. For this test, a sterile swab is rubbed over the back of the throat and tonsils to get a sample of the secretions. It's not a painful procedure, but it may cause brief gagging. The sample is then cultured in a laboratory for the presence of bacteria, but results may take as long as two days.
- Rapid antigen test. Because of the waiting period for a throat culture, your doctor may also order a rapid antigen test on the swab sample. This test can detect strep bacteria in minutes by looking for foreign substances (antigens) in the throat. If you or your child tests positive for strep bacteria, antibiotic treatment can begin right away. But rapid strep tests have a downside. They may miss some strep throat infections. For this reason, many doctors still use throat cultures, especially if results of the rapid test are negative.
- Rapid DNA test. Newer rapid tests use DNA technology to detect strep throat in a day or less from a throat swab. These tests are as accurate as throat cultures, and the results are available sooner.
