KOH Test for Fungal Infections
Potassium hydroxide (KOH) direct microscopy is one of the classic bedside tests in dermatology: fast, reliable and indispensable. It is our first-line, in-clinic screening method for diagnosing superficial fungal infections (superficial mycoses) such as tinea corporis, onychomycosis, athlete's foot, pityriasis versicolor and tinea capitis.
In everyday practice, fungal skin lesions (scaly red patches, ring-shaped lesions, discolored nails) are easily mistaken for other inflammatory conditions such as eczema, psoriasis or keratotic dermatitis. Applying corticosteroid creams to fungus-infected skin not only fails to clear it but lets the infection spread (tinea incognito). That is why KOH microscopy is so important: it lets the doctor confirm whether fungi are actually present before starting targeted treatment.
At Haute-Derma, sampling and KOH microscopy follow a strict, standardized medical protocol, with results available within minutes. This saves you time and means treatment can be effective from the very first dose.
How KOH Works: The Biology and Chemistry
The test relies on how potassium hydroxide acts differently on keratin and on fungal structures:
- Selective keratolytic action: KOH solution (usually 10–20%) breaks protein bonds and dissolves corneocytes, skin scales, hair and tough nail fragments, while leaving the fungal cell wall intact.
- Preservation of the chitin cell wall: fungal cell walls are made mainly of chitin and complex polysaccharides, which resist the strong alkali. As the surrounding keratin is digested and turns transparent, fungal hyphae and spores stand out clearly under the light microscope.
- Better contrast: gentle warming or adding DMSO (dimethyl sulfoxide) speeds up keratin digestion, so microscopic structures can be seen clearly even in thick samples such as nail plate.
Common Fungal Findings on KOH Microscopy
| Organism and condition | Typical microscopic appearance | Usual clinical sites |
|---|---|---|
| Dermatophytes | Branching septate hyphae, sometimes with chains of arthrospores. | Scalp, trunk, skin folds, between fingers and toes. |
| Malassezia furfur (pityriasis versicolor) | The classic "spaghetti and meatballs" picture: clusters of round to oval yeast cells mixed with short, curved hyphae. | Oily areas: chest, back, neck, shoulders. |
| Candida spp. (yeast) | Single budding yeast cells (blastoconidia) with pseudohyphae and clusters of budding cells. | Finger and toe webs, fingernails, moist skin folds. |
| Onychomycosis | Fragmented, wavy hyphae or clusters of spores deep within or beneath the nail plate. | Discolored, crumbly, thickened fingernails or toenails. |
Our Standardized KOH Testing Procedure at Haute-Derma
KOH microscopy at Haute-Derma follows these steps under strict infection control:
- Cleaning the site: the skin or nail to be sampled is gently wiped with 70% medical alcohol to remove debris and surface bacteria.
- Taking the sample:
- Skin: scale is scraped from the edge of the lesion, where the fungus is most active, using a sterile surgical blade.
- Nails: debris is scraped from under the nail, or a piece of crumbly, discolored nail is clipped from deeper layers.
- Hair: broken hairs are plucked or scaly scalp is scraped.
- KOH preparation: the sample is placed on a glass slide with 1–2 drops of 10–20% KOH, covered with a cover slip and gently passed over a spirit lamp flame or warmed to speed keratin digestion.
- Microscopy: the doctor reads the slide personally under a light microscope, scanning at 10x and then examining hyphae and spores in detail at 40x.
Treatment Planning and Multimodal Diagnosis at Haute-Derma
The KOH result guides our doctors at Haute-Derma in choosing the right treatment:
- Antifungal treatment (topical or systemic): if superficial dermatophytes are found, we prescribe targeted topical antifungals (azoles, allylamines). For extensive nail or scalp infections, oral treatment is added, with doses chosen and liver and kidney function closely monitored.
- Avoiding misdiagnosis: if KOH is negative despite scaly lesions, the doctor sets fungal infection aside and looks instead at immune-mediated conditions such as guttate psoriasis, nummular eczema or seborrheic dermatitis, so antifungals are not misused.
