Strep Screen/Throat Culture

A child's throat can be sore for many reasons. If due to an infection, the most common cause is a virus. In most cases, the soreness goes away as the infection does and almost never leads to further problems. But in some cases, sore throats are caused by a more serious bacterial infection from germs known as group A streptococci, or strep. These germs cause strep throat, an infection that commonly affects school-age children.

It's important to find out whether strep is the cause of a sore throat because prompt treatment can decrease symptoms and reduce the risk of complications such as rheumatic fever (which can cause heart damage). To determine this, doctors usually perform either a strep screen or throat culture.

How Is a Strep Screen or Throat Culture Done?

In a strep screen, the doctor or medical assistant wipes the back of the throat with a long cotton swab. This tickles the back of the throat, and can cause a child to gag. While a strep screen can be uncomfortable for a few seconds, it should not be painful. It may help to tell your child exactly that — the test will tickle and feel a little uncomfortable, but will be over very quickly, especially if your child stays still.

In a laboratory, the swab is placed in a test tube with a chemical mix that extracts part of the strep germ (the antigen) from the swab. This extract is then combined with antibodies, which attach to the strep antigen if it is present. A third substance is added to the tube that detects the antigen-antibody combination. If this combination is present, a color change is seen. This means that strep was present in the initial sample from the throat swab. The specimen for a throat culture is taken in the same way, but the fluid from the swab is put into a culture dish in which the bacteria from the specimen must be grown for 2 to 3 days before strep germs, if present, can be identified.

A positive strep screen or throat culture means the child has strep throat and would benefit from taking antibiotics to kill the strep germs.

How Long Does It Take to Get the Results?

A rapid strep screen can offer results in minutes, whereas a throat culture takes 2 to 3 days. Waiting for results will still leave enough time to treat the strep infection and avoid potentially serious, preventable complications. Sometimes, depending on the severity of your child's symptoms and other specific circumstances, your doctor may recommend beginning antibiotic treatment while waiting for the culture results.

If the rapid strep screen is negative, a throat culture is done as well because the rapid test can miss some cases of strep throat. Often, the doctor will take both samples at once, in case the culture is needed. That way, your child only feels uncomfortable once. If the throat culture results are negative for strep, antibiotic treatment can be avoided or stopped. This reduces the risk of your child experiencing an allergic reaction or other side effects from unnecessary antibiotic treatment. A negative throat culture means that a virus is probably the cause of your child's sore throat. Talk to your doctor if you have any other questions or concerns.

Reviewed by: Larissa Hirsch, MD
Date reviewed: August 2006

Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor.

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