Ask A Skin Doctor: Fungal Skin Infection Tests

Fungal Skin Infection Tests

Dear Doctor

Rash looks like ringworm but was treated for that for a month with no luck. I am now on week 3 of doxycycline and doc says it is nummular eczema. Nothing is helping and it is only spreading.

Fungal Skin Infection Tests

Dear Patient

Fungal Skin Infection Tests

Tests that can be done to determine if a rash is caused by fungi include:

  1. KOH Skin Prep

In this painless test the affected skin is gently scraped with a blade. The skin scrapings are mixed with 20% potassium hydroxide (KOH) and examined under a microscope for the presence of fungal elements.

False negative results can be obtained if you have been applying corticosteroids which make the lesions less scaly. To prevent this from happening, stop applying corticosteroids for a few days before doing the test.


  1. Fungal Culture

Fungal cultures should also be done even if the KOH test is negative. If these are positive they also help determine the antifungals that should be used to treat the infection.

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