Fungal Testing: Understanding Your Report

Section Editor: Rodney E. Rohde PhD
June 4, 2026

Fungal infections are caused by fungi, a group of microorganisms that includes yeasts and molds. A microorganism is a living thing too small to see without a microscope. Fungal infections range from common, mild problems on the skin, nails, or mouth to serious infections deep in the body, which most often occur in people with weakened immune systems. Testing is used to find out whether a fungus is causing an infection and, often, exactly which one, so that the right treatment can be chosen.

This article explains the different ways fungal infections are tested for, including looking at a sample under the microscope, growing the fungus, examining a tissue sample, and blood tests, and what the wording on your report means, so you can better understand a report you have received.

What are fungal infections, and why test for them?

Fungi are all around us, and many live harmlessly on the skin and inside the body. Yeasts, such as Candida, are single-celled fungi, while molds grow as branching threads. Superficial infections affect the skin, nails, mouth (where a yeast infection is called thrush), or genital area. Deeper, or invasive, infections affect the lungs, blood, or other organs; these are more serious and occur mainly in people with a weakened immune system, such as those receiving chemotherapy, organ transplant recipients, or people with advanced HIV. Identifying the specific fungus matters because different fungi respond to different antifungal medicines.

Looking at the sample under the microscope

For superficial infections, a sample such as a skin scraping, a nail clipping, or a swab can be examined directly under the microscope. A common method is a KOH preparation, in which a drop of potassium hydroxide (KOH) is added to the sample to dissolve the human cells, leaving any fungus easier to see. This provides a quick, early indication of whether a fungus is present, though it does not identify the exact type.

Fungal culture

A fungal culture grows the fungus from a sample, such as skin, nail, sputum, blood, or tissue, so that it can be identified. Some fungi, especially molds, grow slowly, so a culture can take days to weeks. Where the sample came from matters: growing a fungus from a normally sterile site, such as blood, is more likely to mean a true infection, while growing a common yeast from the skin or mouth, where it often lives harmlessly, may not. In some cases, culture is also used to determine which antifungal medicines will be effective, similar to the susceptibility testing described in the article on culture and sensitivity testing.

Tests on a biopsy (tissue)

When a tissue sample (a biopsy) is taken, a pathologist examines it under the microscope and may use special stains that highlight fungi. Two common ones are the Grocott (GMS) stain, which colors fungi black, and the PAS stain, which colors them a magenta-pink. The pathologist may also see a pattern of inflammation called granulomatous inflammation, which can be the body’s response to certain fungal infections. These findings can show that a fungus is present in the tissue and sometimes suggest its type.

Blood tests: antigen and antibody

For some deeper fungal infections, blood tests can help. There are two main kinds:

Antigen tests — Detect pieces (antigens) of the fungus in the blood or other fluids, which point to an active infection. Examples include a test for the fungus Cryptococcus and a marker found in many different fungi.
Antibody tests — Detect antibodies your immune system makes against a fungus, which show that you have been exposed.

These two kinds of tests, and how they differ, are explained in the article on PCR, antigen, and antibody tests. Some laboratories also use molecular (PCR) tests, described in the same article, that detect the fungus’s genetic material.

Colonization versus infection

A key point in reading any fungal result is that many fungi, especially the yeast Candida, normally live on the skin and in the mouth and gut without causing harm. Finding such a fungus in a sample from one of these areas does not always mean an infection. A result is more likely to mean a true infection when the fungus comes from a normally sterile site, such as blood, or when it appears together with symptoms. Your doctor interprets the result alongside how you feel and where the sample came from.

How to read your result

The wording on your report depends on which test was used.

Microscope (KOH) examination: fungal elements seen or not seen — Fungus was, or was not, seen directly in the sample. Not seeing it does not always rule out an infection.
Culture: a fungus grown (and named), or no growth — The report names the fungus that grew, or states that none grew. Slow-growing fungi may take time to appear.
Special stains on a biopsy — The report may note fungus on stains such as GMS or PAS, sometimes accompanied by granulomatous inflammation.
Antigen test positive or negative — A piece of the fungus was, or was not, detected, which points to whether an infection is active.
Antibody test positive or negative — Shows whether your immune system has responded to the fungus, reflecting exposure rather than necessarily a current infection.
Molecular (PCR) test detected or not detected — The fungus’s genetic material was, or was not, found.

What happens after these tests

Fungal testing describes what was found and informs the decisions you and your healthcare team make together, rather than dictating them on its own.

Superficial fungal infections of the skin, nails, or mouth are usually treated with antifungal creams, drops, or tablets. Deeper or invasive infections require stronger antifungal medicines, sometimes administered in a hospital, and the choice depends on the type of fungus and, in some cases, on susceptibility testing. People with a weakened immune system are watched especially closely, because they are at higher risk of serious fungal infections. Throughout, your doctor interprets the results together with your symptoms and the site from which the sample came.

Questions to ask your doctor

What type of fungus, if any, did my test find?
Does this result mean I have an infection, or could the fungus be living harmlessly?
Where was my sample taken from, and does that affect what the result means?
Could a negative result still miss an infection?
If I have a fungal infection, what does treatment involve?
Will the fungus be tested to see which antifungal medicines work?
Does my immune system put me at higher risk of a serious fungal infection?
How long will treatment take, and how will we know it is working?
Could this infection come back?
Who should I contact with questions about my results or treatment?