 [ Symptoms ] what exams and tests help doctors to evaluate or test people for ringworm of the skin [ Disease ] 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 webmd medical reference from healthwise this information is not intended to replace the advice of a doctor healthwise disclaims any liability for the decisions you make based on this information © 1995 2011 healthwise incorporated healthwise healthwise for every health decision and the healthwise logo are trademarks of healthwise incorporated