How to perform a potassium hydroxide (KOH) preparation?

How to Perform a Potassium Hydroxide (KOH) Preparation

To perform a KOH preparation, collect the specimen with a cotton swab or scraping tool, place it on a glass slide, add 1-2 drops of 10% KOH solution, apply a coverslip, and examine under microscopy after allowing 10-15 minutes for tissue digestion. 1

Specimen Collection

For Vaginal Infections

• Collect vaginal discharge using a cotton swab from the vaginal pool or walls 1
• Obtain the specimen during speculum examination before other samples 1
• For optimal results, the specimen should be examined within 30 minutes to 2 hours of collection 1
• Avoid contaminating the specimen with cervical mucus, as this affects test accuracy 2

For Skin/Nail Fungal Infections

• Use a #15 scalpel blade or the edge of a glass slide to scrape the active border of skin lesions 1
• For suspected dermatophyte infections, scrape scales from the advancing edge of the lesion 1, 3
• Collect subungual debris or nail clippings for nail infections 3
• For hair infections, pluck affected hairs with roots rather than cutting them 3

Slide Preparation Technique

Basic Method

• Place the specimen directly on a clean glass microscope slide 1
• Add 1-2 drops of 10% potassium hydroxide solution to the specimen 1
• Apply a coverslip over the specimen 1
• Gently heat the slide (do not boil) to accelerate tissue digestion, or allow to sit at room temperature for 10-15 minutes 1, 4

Enhanced Visualization

• Adding black or blue-black ink (such as Parker Quink or India ink) to the KOH solution highlights fungal organisms and distinguishes them from cellular debris 4
• Some laboratories use Calcofluor-KOH stain for enhanced visualization of fungal elements under fluorescent microscopy 1

Microscopic Examination

For Vaginal Specimens

• Examine under low power (10x) first to scan for clue cells, then switch to high-dry power (40x) for detailed examination 5, 2
• Look for budding yeast and pseudohyphae in suspected candidiasis 1, 2
• The KOH dissolves epithelial cells and debris, making fungal elements more visible 1
• Perform the "whiff test" by noting if a fishy amine odor is released when KOH is added—this indicates bacterial vaginosis 1, 5

For Skin/Nail Specimens

• Scan systematically under low power (10x) for fungal elements, then confirm with high power (40x) 1, 4
• Look for septate hyphae, branching patterns, and arthrospores characteristic of dermatophytes 1, 3
• Fungal elements appear as refractile, branching structures against a clear background after KOH digests the keratin 4, 6

Critical Technical Points

Timing Considerations

• The KOH preparation must be examined promptly—within 2 hours for vaginal specimens to maintain diagnostic accuracy 1
• Allow adequate time (10-15 minutes) for KOH to digest tissue before microscopic examination 1
• If examining immediately, gentle heating accelerates the clearing process 4

Common Pitfalls to Avoid

• Do not use too much KOH solution, as this creates excessive fluid that makes focusing difficult 1
• Do not overheat the slide, as this can destroy fungal elements and create artifacts 4
• Ensure adequate specimen collection—insufficient material is a common cause of false-negative results 3, 7
• Wide variation exists in interpretation between providers; proficiency in microscopic examination is essential 2

Diagnostic Limitations

Sensitivity Issues

• KOH preparation has only 40-80% sensitivity for detecting yeast compared to culture 1, 2
• For trichomoniasis, wet mount (which can be combined with KOH) has even lower sensitivity of 40-70% compared to NAAT 1, 2
• Consider culture when KOH is negative but clinical suspicion remains high, or in recurrent infections 1, 2

When to Use Alternative Testing

• NAATs are preferred for trichomoniasis screening due to superior sensitivity, with recent data showing Trichomonas prevalence equal to or greater than chlamydia and gonorrhea in certain populations 2
• For bacterial vaginosis, Gram stain is the gold standard but is more specific than necessary for routine clinical diagnosis when Amsel criteria can be applied 5, 2
• Culture is indicated for recurrent vulvovaginal candidiasis to identify drug-resistant Candida species 1

Clinical Context

The KOH preparation remains a simple, accurate, and inexpensive diagnostic technique that provides immediate point-of-care results 2, 7, 4. However, there is evidence of declining usage in clinical practice, particularly in primary care settings, where empiric treatment is increasingly favored over diagnostic confirmation 3, 7. Despite its limitations, the KOH preparation combined with clinical findings enables rapid diagnosis without requiring more expensive laboratory testing 5, 2.