November 25, 2015
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Strep Throat (GAS) (cont.)

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Is strep throat contagious?

Strep throat is moderately contagious and is most commonly spread via direct person-to-person contact. Passage of airborne droplets and/or saliva from the infected individual to another is the most likely mechanism of contracting strep throat. As such, close living quarters (home, classroom, day care centers, college dorms, etc.) provide an ideal environment for passage of GAS bacteria from one person to another. The risk of contracting a strep throat infection is approximately 40% in household environments. Most infectious disease specialists believe that a patient is no longer contagious after 24 hours of effective antibiotic therapy. Spread of strep bacteria via food borne transmission is less common than direct person-to-person exchange of droplets or saliva. The exact likelihood of developing strep throat from family pets is unknown, but most experts believe it is minimal.

How is strep throat diagnosed?

There are two common tests used to establish whether an individual with symptoms of strep throat infection has a strep throat. 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. Antibiotic treatment prior to a rapid strep test is less likely to have a false negative result since the test is designed to document the presence or absence of the GAS bacteria. Residual dead bacteria in the patient may allow a positive result.

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 4/20/2015


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