KOH Mount and Wood Lamp Examination
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Direct answer
A KOH mount and a Wood lamp examination are bedside tests that help confirm fungal infection and assess pigmentary disease. Potassium hydroxide dissolves keratin, so fungal hyphae and spores can be seen under the microscope in scale from the active border of a lesion. The Wood lamp emits long-wave ultraviolet light at about 365 nm, and certain organisms and porphyrins fluoresce in characteristic colours.
What you must remember
- KOH concentration: 10 to 20 per cent potassium hydroxide digests keratin, so the scale clears enough to reveal fungal elements.
- Specimen: scrapings from the advancing edge of a scaly lesion, or infected hairs for suspected tinea capitis.
- Wood lamp: long-wave ultraviolet light at about 365 nm, examined in a darkened room.
- Microsporum canis: bright green fluorescence on infected hairs in tinea capitis; Trichophyton tonsurans does not fluoresce.
- Erythrasma: coral red fluorescence of the intertriginous skin, caused by porphyrins from Corynebacterium minutissimum.
- Pityriasis versicolor: golden yellow fluorescence of the involved scale, associated with Malassezia.
- Pseudomonas infection: yellow-green fluorescence of wounds or nails, caused by pigments produced by the organism.
- Vitiligo: depigmented patches look chalk white or bluish-white under the lamp, which accentuates the contrast with normal skin.
A bedside sequence from scraping to interpretation
Both tests are quick and inexpensive, so they are often done in the clinic before culture is sent. Use them together, and remember that fluorescence is a clue rather than a diagnosis.
- Clean the site: remove surface dirt and oil so the scale is not contaminated.
- Collect the scale: scrape the advancing active edge onto a clean glass slide.
- Add KOH: place a drop of 10 to 20 per cent KOH on the scale and cover it with a coverslip, then warm gently if needed.
- Examine under the microscope: look for septate hyphae and arthroconidia, which indicate dermatophytes.
- Use the Wood lamp: examine the lesion in a darkened room and record the colour of any fluorescence.
- Confirm: send scale for fungal culture, since a negative lamp or a clear KOH slide does not rule out infection.
The table below gives the fluorescence colours most often tested in examinations.
| Condition | Wood lamp colour | Cause |
|---|---|---|
| Tinea capitis (M. canis) | bright green | Microsporum canis |
| Erythrasma | coral red | Corynebacterium minutissimum |
| Pityriasis versicolor | golden yellow | Malassezia |
| Pseudomonas infection | yellow-green | Pseudomonas aeruginosa |
| Vitiligo | chalk white | non-infective pigment loss |
A positive fluorescence should be interpreted alongside the clinical picture, because some products and cosmetics also fluoresce. Lamp findings guide the next test, but they do not replace culture or histology.
Where the bedside tests catch candidates
The colours are easy to memorise, but the limits of each test are where examiners set traps.
- Trap: assuming a negative Wood lamp rules out tinea capitis Correct: Trichophyton tonsurans does not fluoresce, so a negative lamp does not exclude it, and culture is required.
- Trap: calling green hair fluorescence a Malassezia finding Correct: green hair fluorescence is Microsporum, whereas Malassezia gives golden yellow fluorescence on the scale of pityriasis versicolor.
- Trap: mounting scale in water without KOH Correct: KOH digests keratin, so without it the scale stays too opaque to show hyphae.
- Trap: expecting vitiligo lesions to glow Correct: vitiligo is not a fluorescing infection; the lamp only accentuates the chalk-white depigmentation against the surrounding pigmented skin.
Frequently asked questions
What concentration of potassium hydroxide is used in a KOH mount?
A KOH mount usually uses 10 to 20 per cent potassium hydroxide. This concentration digests keratin, clearing the scale so that fungal hyphae and arthroconidia can be seen under the microscope.
Which fungus gives green fluorescence under a Wood lamp in tinea capitis?
Microsporum canis gives bright green fluorescence on infected hairs under a Wood lamp. Trichophyton tonsurans does not fluoresce, so a negative lamp examination cannot exclude tinea capitis caused by that organism.
What colour does erythrasma fluoresce under a Wood lamp?
Erythrasma fluoresces coral red under a Wood lamp, because Corynebacterium minutissimum produces porphyrins. The finding is typical of intertriginous areas such as the axillae, groins and toe webs.
Why is a KOH mount used in dermatophyte infection?
A KOH mount is used because potassium hydroxide dissolves keratin, which makes the scale transparent enough to show dermatophyte hyphae and arthroconidia. It gives a rapid answer, but fungal culture is still needed to identify the species.
What does a Wood lamp show in pityriasis versicolor?
A Wood lamp shows golden yellow fluorescence of the scale in pityriasis versicolor, which is associated with Malassezia. The fluorescence helps localise scaly patches, but it is a clue that should be confirmed by a KOH mount.