During the doctor's physical examination, a throat culture might be taken by touching a soft cotton swab to the throat and tonsil area. Germs from the swab are smeared onto a special plate and sent to the laboratory for evaluation of strep or other bacteria. After 24-48 hours, the rapidly-growing strep germs can be identified if present in the sample. Throat culture is the gold standard in diagnosing strep throat infection.
A culture for strep throat may not be done as a screening test in an individual without any 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. Therefore, if the health care practitioner 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.
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. This is called the Anti-streptolysin O or ASO titer. This antibody may not be present in the blood for a few weeks after an infection, and therefore its measurement may not be accurate to detect an active or acute infection. ASO titers also may not be detectable if the person had been appropriately treated with antibiotics.
This test may be useful in detecting prior infections and evaluating a person with complications of strep throat infection such as rheumatic fever or glomerulonephritis.
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