If you or your baby develops painful white lesions inside the mouth, see your doctor or dentist. He or she will likely diagnose oral thrush simply by looking at the lesions but may also examine a small sample under a microscope to confirm the diagnosis.

If thrush develops in older children or adolescents who have no other risk factors, seek medical care. An underlying condition such as diabetes may be the cause. In that case, your doctor will perform a thorough physical exam as well as recommend certain blood tests to help find the source of the problem.

Thrush that extends into the esophagus can be serious. To help diagnose this condition, your doctor may ask you to have one or more of the following tests:

• Throat culture. In this procedure, the back of your throat is swabbed with sterile cotton and the tissue sample cultured in a special medium. This helps determine if bacteria or fungi are causing your symptoms.

• Esophagogastroduodenoscopy (EGD). In this procedure, your doctor examines the lining of your esophagus, stomach and the upper part of your small intestine (duodenum) using a lighted, flexible tube with a camera on the tip (endoscope). The test takes between 30 and 60 minutes. You'll be given a sedative to make you more comfortable and a local anesthetic so that you don't cough or gag when the endoscope is inserted. There's a slight risk of perforation of your esophagus, stomach or duodenum and of an adverse reaction to medication you may receive.

• Barium swallow. In this test, you'll need to drink one or two barium "milkshakes" — glasses of thick, chalky liquid that may be flavored so they go down more easily. X-rays are then taken as the barium flows through your esophagus into your stomach.

Oral thrush is seldom a problem for healthy children and adults, although the infection may come back even after it's been treated. For people with compromised immune systems, however, thrush can be more serious.

Children with HIV may have especially severe symptoms in their mouth or esophagus, which can make eating painful and difficult. As a result, they don't receive adequate nutrition, just when they need it most. In addition, thrush is more likely to spread to other parts of the body in people with cancer, HIV or other conditions that weaken the immune system. In that case, the areas most likely to be affected include the digestive tract, lungs, liver and skin.