Performing In-Office KOH Prep Tests


Joe R. Monroe, PA, MPAS
Dawkins Dermatology, Oklahoma City

Potassium hydroxide (KOH) solution, being alkaline, has the ability to dissolve keratin that is scraped from the outer layer of skin, which allows the microscopic identification of organisms such as dermatophytes or scabies, helping to establish the correct diagnosis and facilitating effective treatment. Without confirmation by a KOH preparation test, patients with dermatologic conditions might suffer needlessly for years, while determined but puzzled providers unsuccessfully try one empirical treatment after another.


KOH prep testing is indicated for patients with suspected fungal infection or suspected scabies.

Possible fungal infection (dermatophytosis) cases are suggested by localized rashes with annular, scaly margins, often with pruritus; rashes that fail to respond to or worsen with the application of corticosteroids; rashes that persist for months; exposure to hot, sweaty conditions; a history of atopy; a history of exposure to animals (especially cats, but also dogs, horses, cows, and pigs); or a history of immunosuppression. Among the most common dermatophytes are Epidermophyton, Trichophyton, and Microsporum species, which commonly cause superficial epidermal infections of the feet (tinea pedis), the groin and/or perineum (tinea cruris), and, particularly in pediatric patients, the scalp (tinea capitis).


Figure 1. To collect a skin sample (here, from a patient with suspected tinea pedis) for KOH prep testing, forcefully scrape scales from the lesion’s margin with a scalpel blade onto a slide held underneath.

Possible scabies is suggested by a history of similar, unremitting itching in family members and/or other close contacts. Scabies is caused by infestation with the mite Sarcoptes scabiei var hominis. It is commonly seen in the pediatric population, and it can be sexually transmitted in sexually active adolescents. The condition is characterized by severe itching 24 hours a day that typically is worse between the fingers, on the volar wrists, and on the anterior axillary areas. Tiny blisters (vesicles) and burrows in these areas are the best locations for scraping. Scabies also can manifest on the shaft of the penis or on the scrotal wall with discrete, edematous, red, 2- to 4-mm papules. Microscopic examination seeks to confirm clinical suspicions by detecting actual organisms, parts of organisms, eggs, parts of eggs, or droppings (scybala).


For possible superficial fungal infection: Using a No. 10 scalpel blade (handheld, without a handle), forcefully scrape the scales from the active margin of the lesion, so that the scales land directly onto a new, clean, plain glass microscope slide held just underneath (Figure 1). Using a plain glass coverslip, heap the scales into an area small enough to fit under the coverslip (Figure 2). Place the coverslip over the piled-up scales, and proceed to the microscope work area.


Figure 2. Using the edge of a coverslip, push the scales into a pile in the center of the slide and place the coverslip over the pile.

Apply drops of 10% to 20% KOH solution to the slide, immediately adjacent to the edge of the coverslip (Figure 3). The solution will be drawn by capillary action into the space between the coverslip and slide, filling it (Figure 4). Pick up the slide, tilt one end up slightly, about 10 degrees, and gently tap the end of the slide with your fingernail 5 or 6 times to loosen the tiny bubbles that inevitably form under the coverslip, allowing them to escape lest they obscure the view.

Then, using the flame of a small butane lighter, gently heat the sample from underneath to just short of boiling (Figure 5). The heat accelerates the digestion of the keratin in the scales by the alkaline KOH, facilitating visualization of the target fungal elements that otherwise might remain buried out of sight in the tissue.


Figure 3. Apply KOH solution to the slide, placing a drop immediately adjacent to the edge of the coverslip.

Place the prepared slide on the microscope’s stage and examine it carefully using the low power (10×) objective (Figure 6). The most common evidence of fungal infection in a skin scraping sample is the presence of hyphae—long, slender, refractile filaments of uniform width resembling the branches of a tree or streaks of lightning (Figure 7). Usually long enough to cross multiple cell walls, these hyphae can seem hard to see at first, but with a bit of practice, they can be readily found. If time permits, saving the prep slide and reexamining it 30 to 60 minutes later might reveal hyphae even if the original examination of the slide was negative.


Figure 4. After the KOH solution wicks into the space between the coverslip and the slide, immersing the skin scrapings, gently tap the edge of the slide a few times to dislodge and disperse bubbles under the coverslip.

The only way to learn to do KOH preps is to do them, at first for obvious cases of tinea pedis or tinea cruris, looking for the characteristic hyphae. Repetition is the only way to master performance of KOH preps.

For possible scabies: The most reliable source of scabietic elements is a tiny vesicle between the fingers, on the volar wrist, on the thenar or hypothenar hand, or on the ankles. On infants, these same lesions also can be seen on the soles. In my experience, so-called “burrows” are hard to see, are not nearly as common as vesicles at these locations, and are not as reliable a source of the scabietic elements.


Figure 5. Place the flame of a butane lighter a few inches below the slide, gently heating the sample and solution from underneath to accelerate the action of the KOH.

With a No. 10 scalpel blade (again, without a handle), scrape the vesicle with gusto (that is, with more firmness than would be used for fungal KOH preps), and wipe the material onto the slide. If possible, sample 3 or 4 such lesions.

From this point forward, the procedure is the same as for KOH preps for fungal elements. However, in contrast with fungal KOHs, confirmatory scabietic elements can include several different findings, such as whole adult mites, severed parts of adult or juvenile mites, partial or whole eggs, or scybala. To prevent the loss of the specimen, some clinicians use mineral oil in place of the KOH solution, since KOH will dissolve scabietic elements within an hour or two of application.

The only way to learn to do KOH prep tests for scabies is by doing them on obviously positive cases.




Figure 6. Place the slide on the microscope’s stage; using the 10× objective (A), scan the slide carefully (B) for evidence of fungal infection.


KOHMost pediatricians and other primary care providers don’t routinely perform KOH preps, offering many reasons for not doing so. Many simply have never taken the time to learn how to do them, while others have told me that even if they were to perform a KOH prep, they wouldn’t know what they were seeing on the slide. But the most common excuse I hear is, “I don’t have time,” even though many of these same providers do have the time to perform vaginal wet preps for trichomoniasis, bacterial vaginosis clue cells, or candidiasis in their female patients, or to order extensive diagnostic laboratory and radiologic tests, for which the patient has to be roomed again to discuss the results.

Many of them tell me this: “If it looks fungal, I’m going to treat with an antifungal, which is what I’m going to do if the KOH test is positive. If the KOH test is negative, and I still think it’s fungal, I’m still going to treat with an antifungal—so what’s the point of doing a KOH prep?” In other words, they haven’t developed a differential for presentations that look fungal but aren’t. The same is true for scabies, which is vastly overdiagnosed because of the lack of a differential diagnoses in suspected cases (Table). These clinicians often are content with this approach, even though it leads to much unnecessary suffering and worry for patients and parents. They say they don’t have time to perform a KOH prep, but they find they have time for the multiple phone calls and return visits from patients whose treatment has failed and who still do not have a definitive diagnosis.


The beauty of doing KOH preps is that the procedure leads to a correct diagnosis, which in turn dictates the correct treatment. And in CLIA-approved laboratories, a charge can be generated for the performance of a KOH prep.


Figure 7. The pathognomic sign of infection with dermatophytes—in this case, infection with Trichophyton rubrum, the most common cause of tinea pedis—is the presence of fungal hyphae, which appear on a KOH prep slide as slender filaments that resemble tree branches or streaks of lightning. (Photograph courtesy of Theodore Scott, RN, MSN, FNP-C, DCNP)

There’s a world of difference between telling parents that their child might have scabies or a fungal infection versus being able to positively confirm the diagnosis microscopically, then go directly to the patient or parents with this news, start treatment, and resolve the condition. Without the diagnostic confidence offered by KOH prep confirmation, symptoms persist, treatment often is changed, parents lose confidence and take their child to another provider, and the same process starts over again. Patients and families suffer needlessly. They deserve better.



1. Diseases resulting from fungi and yeast. In: James WD, Berger TG, Elston DM. Andrews’ Diseases of the Skin: Clinical Dermatology. 11th ed. Philadelphia, PA: Saunders Elsevier; 2011:294.

2. Cutaneous fungal infections. In: Habif TP, Campbell JL Jr, Chapman MS, Dinulos JGH, Zug KA. Skin Disease: Diagnosis and Treatment. 2nd ed. Philadelphia, PA: Elsevier Mosby; 2005:240-241.

Mr Monroe is a dermatology physician assistant at Dawkins Dermatology in Oklahoma City.