Strep Throat (GAS) (cont.)

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How is the diagnosis of strep throat established?

There are two common tests used to establish whether an individual has a strep throat infection. The traditional test, a throat culture, involves the doctor obtaining a swab from the back of the throat and tonsils. It takes approximately 36 to 48 hours to determine the presence or absence of the GAS bacteria. This test is considered the "gold standard" in diagnosing strep throat.

Many medical offices, clinics and hospital emergency departments use a rapid strep test (RADT - rapid antigen detection test) in an effort to provide a more timely diagnosis for their patients. The throat swab is obtained similarly to that for a traditional throat culture, but a RADT "kit" is employed to help determine the presence or absence of the strep bacteria. The turn around time is generally 10 minutes. The RADT is less precise when compared with a throat culture. Most RADT tests are about 95% accurate in determining the presence of strep bacteria. Most guidelines recommend that a negative rapid strep test be confirmed with a throat culture. Of note, antibiotic pretreatment prior to obtaining a throat swab for strep bacteria can interfere with throat culture results, and thus the ability to correctly diagnose and treat a strep throat infection.

The above tests are designed to determine whether an individual currently has a strep throat infection. Sometimes it may be necessary to determine whether a patient has had a recent history of a strep infection. In that case, two blood tests are available: (a) an ASO (anti streptolysin O) titer, and/or (b) an anti DNase titer. The antibodies that these tests are sensitive to are not commonly present until a few weeks following an acute strep infection. These tests may be ordered when evaluating a patient who may have developed delayed complications (such as rheumatic fever or glomerulonephritis) after a strep throat infection.

Medically Reviewed by a Doctor on 10/4/2013

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