Doctors often can easily recognize ringworm by its ring-shaped rash. Your doctor will probably also look at a skin sample (KOH preparation) under a microscope to confirm that fungus is present.

Tests for ringworm of the skin include:

• A KOH (potassium hydroxide) preparation. This test can help your doctor find out whether a fungus is causing your rash. The doctor will take a sample of your skin by lightly scraping the infected area. The sample is placed on a slide with potassium hydroxide (KOH) solution and gently heated. If fungus cells are present, the doctor can then see them with a microscope.
• A fungal culture (skin culture). This test can identify the type of fungus that is causing your infection. Usually a culture is only done if the diagnosis is difficult or the skin condition is not responding to treatment. This test may take up to 4 weeks, because it takes time for the fungus to grow.