Skin Scrapings for KOH Prep (Fungal Screen)—often called a KOH prep—is a quick, in-office or laboratory microscope test used to look for signs of a superficial fungal infection in the skin, hair, or nails. A clinician gently collects a small sample (scraping scales from a rash, plucking a few hairs, or taking debris from under a nail). The sample is placed on a glass slide with potassium hydroxide (KOH).

KOH helps by dissolving keratin (the main protein in skin, hair, and nails) so that fungal structures are easier to see under the microscope. The test is designed to detect common fungal elements such as dermatophytes (which cause “ringworm” or tinea) and yeast (including Candida).

What the results mean (in plain language):

Positive (fungal elements seen): The microscope shows fungal forms such as branching filaments (hyphae) or yeast forms, supporting a diagnosis of a fungal infection. This helps your clinician choose the most appropriate treatment (for example, topical vs. oral antifungals) and avoid unnecessary antibiotics or steroid creams that may worsen some fungal rashes.

Negative (no fungal elements seen): No fungus was seen in the sample. This can mean there is no fungal infection, but it can also be a false negative if the sample was taken from an area without enough organisms or if antifungal treatment was started before the test. If suspicion remains high, your clinician may repeat the scraping, take a sample from the most active edge of the rash, or order a fungal culture or other testing.

A KOH prep fungal screening test is commonly ordered when symptoms or exam findings suggest a fungal infection of the skin, scalp, or nails and your clinician wants a fast answer. It can help confirm that a rash is due to fungus (like tinea or candida skin infection) rather than eczema, psoriasis, dermatitis, or a bacterial infection.

Your clinician may recommend a KOH prep if you have:

Skin symptoms such as an itchy, scaly, or peeling rash; redness; cracking (especially between toes); or a ring-shaped rash with a more active, scaly edge (classic “ringworm”). It may also be used for rashes in warm, moist areas (groin, under breasts, skin folds) where yeast or dermatophytes can thrive.

Hair/scalp concerns such as patchy hair loss, scalp scaling, broken hairs, or “black dots” at the scalp, which can suggest tinea capitis (scalp ringworm), especially in children.

Nail changes such as thickened, brittle, crumbly, discolored nails or debris under the nail, which can suggest onychomycosis (fungal nail infection). Testing helps avoid long courses of oral antifungal therapy when a nail problem is due to another cause.

Risk factors that increase the chance of fungal infections, including frequent sweating, tight/occlusive footwear, locker room or shared shower exposure, close contact sports, living with someone with a fungal infection, diabetes, immune suppression, or prior use of topical steroids on a rash (which can mask and worsen tinea).

Because the KOH prep is rapid and low-cost, it is often used as an early diagnostic step before or alongside treatments for suspected tinea, ringworm, or candida skin infections.

• Tinea corporis (ringworm of the body)
• Tinea pedis (athlete’s foot)
• Tinea cruris (jock itch)
• Tinea capitis (scalp ringworm)
• Tinea manuum (fungal infection of the hands)
• Onychomycosis (fungal nail infection, often dermatophytes)
• Cutaneous candidiasis (Candida skin infection, including intertrigo in skin folds)
• Pityriasis (tinea) versicolor (Malassezia overgrowth causing light/dark scaly patches)

• Quick confirmation of a fungal rash (supporting targeted treatment and faster symptom relief)
• Appropriate antifungal use (avoiding unnecessary antibiotics or prolonged antifungal therapy)
• Preventing spread to others in households, schools, gyms, and close-contact settings
• Improving skin and scalp comfort (reducing itching, scaling, cracking, and irritation)
• Supporting nail health by verifying fungal causes before long-term treatment
• Managing recurrent infections by confirming diagnosis when symptoms keep coming back
• Guiding next-step testing (e.g., fungal culture) when symptoms persist despite treatment