Strep Throat (cont.)

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How is strep throat diagnosed?

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Throat culture

During the doctor's physical examination, a throat culture might be taken by touching a soft cotton swab (similar to a Q-tip) to the throat and tonsil areas. The swab is then sent to the laboratory for evaluation of strep or other bacteria. After 24-48 hours, the rapidly-growing strep bacteria can be identified if present in the sample. A throat culture is the gold standard in diagnosing strep throat infection.

A culture for strep throat should not be done as a screening test in an individual who is experiencing no symptoms suggestive of strep throat. This is because bacteria may be present without causing an infection. A person may be a carrier of the bacteria but not infected.

Rapid strep test

Rapid strep tests (also called the Rapid Antigen Detection Test or RADT) are available that can give results in minutes. A sample from the back of the throat or tonsils is taken using a swab similar to the sample for culture. Using a RADT kit, the doctor can determine within a few minutes if strep is the likely cause of sore throat. This test is less precise than throat cultures.

Most RADT tests are approximately 95% accurate in diagnosing strep throat. This implies a "false negative" rate of 5%. In such circumstances the more definitive throat culture should be obtained. If your health care professional has a high suspicion for strep throat, both tests (RADT and throat culture) may be performed, and treatment should be started as soon as possible even if the rapid test is negative for strep while waiting for the definitive culture results. If the throat culture is negative for strep infection, then antibiotics should be stopped.

Taking any "self prescribed" antibiotic prior to seeing the doctor should be avoided because even a single dose of antibiotic can interfere with the culture results and the health care practitioner's ability to correctly diagnose and treat the infection.

Blood test for strep throat infection

Streptococcus infection can be detected by measuring the antibodies produced by the body against the bacteria. Two blood tests are most commonly used: (a) ASO titer (anti streptolysin O) and (b) anti DN-ase B titer. These antibodies may not be present in the blood for a few weeks after an infection, and therefore their measurement may not be accurate to detect an active or acute infection. ASO and anti-DN-ase B titers also may not be detectable if the person had been appropriately treated with antibiotics.

These tests may be useful in detecting prior infections and evaluating a person with complications of strep throat infection such as rheumatic fever or glomerulonephritis (see below).

Medically Reviewed by a Doctor on 10/16/2014

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