Diagnosing Strep Throat

September 07, 2008

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Diagnosing Strep Throat

by Dr. Ed Kaplan
reviewed by Lynn Cates, M.D., F.A.A.P.

You've probably heard repeatedly that it's very important for you to take your child to his healthcare provider to be evaluated whenever he has a sore throat--and this is true. Unfortunately, the diagnosis of strep throat cannot be made from just looking at your child, although there are some signs and symptoms that are strongly suggestive of strep. The only way to know for sure is to perform a laboratory test.

Why it's important to make the diagnosis of strep throat
Since strep throats are so common, you may not realize that they have the potential to cause serious long-term complications like rheumatic fever and a kidney disease called acute glomerulonephritis (AGN). These complications are unusual in developing countries today because healthcare providers have learned the importance of early diagnosis and appropriate treatment of strep throat (usually with penicillin). Even in the United States, these complications can occur when strep throats are not treated.

Why it's important to make the diagnosis of strep throat before starting antibiotic therapy
It may seem like a nuisance to have to wait for the results of a laboratory test to start antibiotics when you have a sick, miserable child. Although it is very important that strep throat be treated with antibiotics, it has been well documented that ordinarily it's safe to wait until the diagnosis has been confirmed to start treatment. The main reason healthcare providers are reluctant to give your child an antibiotic unless the diagnosis of strep is confirmed is to help prevent the development of antibiotic-resistant bacteria. These resistant bacteria have the potential to cause even more problems, including infections that may require hospitalization for intravenous (in the vein) antibiotics, or multiple antibiotic therapy.

Tests for strep throat
Two kinds of laboratory tests are acceptable for confirming the presence of the bacterium that causes strep throat (group A streptococcus) in the throat or on the tonsils. These are the throat culture and a relatively new series of tests called rapid antigen detection tests (often referred to as "rapid strep tests").

Throat culture. The throat culture is the classical way of determining the presence of the group A strep in the throat. This is done by rubbing a sterile cotton swab carefully across the back of the throat and the tonsils, and then putting it onto a culture plate that contains nutrients that allow the strep to grow and to be identified in the laboratory. It normally takes 24 or even 48 hours to be certain whether or not group A strep is present. If the culture is performed correctly (that is, if a good specimen is taken), and if it is handled properly in the laboratory, this is a very accurate test. However, if a child has received antibiotics before the culture is done, the strep may not grow or be identified.

Rapid strep tests. Because of the delay required to accurately interpret the throat culture, more rapid methods have been developed that do not require growth of the bacteria. These tests use chemical and immunologic means to determine the presence of strep.

These new tests have become quite popular because they allow healthcare providers to make the diagnosis at the time of the office visit. This significant advantage has been favorably received, but there are remaining concerns in the minds of some authorities about the relative accuracy of the rapid tests. Some are not as sensitive as they should be; processing these tests, as with any other laboratory test, requires an experienced person.

If the rapid strep test is positive, indicating the presence of the group A streptococci, the result is likely valid. There are some, but relatively few, instances that are falsely positive (i.e., they show a positive result, but it can be proven that the group A streptococci are not present by using a throat culture). When the rapid test is negative for a child who has signs and symptoms that are typical for strep throat, a regular throat culture should be done to be sure that he does not have the illness.

Rapid tests generally are more expensive than throat cultures.

Other tests for strep. While there are other laboratory tests that may be helpful in detecting group A streptococci, they are not often useful for diagnosis of routine strep throat infections. These blood tests and urine tests are more helpful when strep infections other than strep throat are being considered.


RELATED INFORMATION

			Treatment of Strep Throat
			Strep Infections

Created December 23, 2000
Reviewed February 24, 2001

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