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Strep Screen/Throat Culture

A child's throat can be sore for many reasons. When a sore throat is caused by 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 about 15% of 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 affects mostly school-age children.

"Strep throat is more serious than the other 85% of sore throats because, if not treated with antibiotics, it sometimes can cause more serious pus-forming infections," says Frederick Meier, MD, a pathologist. In fact, the scientific name for the strep that causes sore throats is pyogenes, or pus-maker. Group A streptococci also produce toxins that can cause circulatory collapse (shock) in streptococcal toxic shock syndrome or fever and a rash in scarlet fever.

Furthermore, strep-triggered glomerulonephritis can damage the kidneys, and rheumatic fever - another potentially serious complication of strep throat - can affect the heart, joints, and sometimes the brain. Damage from these poststreptococcal diseases can be permanent or even life-threatening.

Timely antibiotic treatment can dramatically decrease a child's chance of developing rheumatic fever and many of the other possible complications of a strep infection. To determine whether a sore throat is caused by strep, either a strep screen or throat culture can be performed.

How Is a Strep Screen or Throat Culture Done?
In a strep screen, the doctor or medical assistant wipes the back of the child's throat with a long cotton swab. In the 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 to group A strep antigen (these antibodies are protein molecules that attach to the group A strep bacteria's surface antigen). When a third substance that detects the antigen-antibody combination is added to the tube, the liquid changes color if strep germs are present.

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.

The strep screen is about 75% to 85% sensitive, meaning that it will detect the bacteria in 75 to 85 of 100 people with strep throat proven by a positive throat culture. A positive strep screen or throat culture test means your child might 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. It's important to find out whether strep is the cause of the sore throat because prompt treatment for strep throat can decrease your child's symptoms and reduce the risk of rheumatic fever. Waiting for results will still allow 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 child's doctor may recommend beginning antibiotic treatment while waiting for the culture results.

If the strep screen or throat culture results are negative for strep, antibiotic treatment can be avoided or stopped. This reduces the risk of your child having an allergic reaction or experiencing other drug side effects from unnecessary antibiotic treatment.