When should a potassium hydroxide (KOH) wet preparation be used to evaluate for fungal infection?

When to Use KOH Wet Preparation for Fungal Infection

Use a KOH wet preparation immediately at the point of care when you suspect superficial fungal infections of the skin, nails, hair, or mucocutaneous surfaces—it provides rapid, inexpensive diagnosis without waiting for culture results. 1, 2

Primary Indications for KOH Preparation

Vaginal/Mucocutaneous Candidiasis

• Perform KOH wet prep as first-line testing for suspected vulvovaginal candidiasis, where adding 10% KOH to vaginal discharge enhances visualization of pseudohyphae and budding yeast 3, 1, 2
• The KOH preparation serves dual purposes in vaginal specimens: it reveals fungal elements AND produces the characteristic "fishy" odor when bacterial vaginosis is present (the "whiff test") 1, 2
• Examine the specimen within 30 minutes to 2 hours of collection for optimal results 1, 2
• Blood contamination does not interfere with KOH visualization because the solution dissolves red blood cells while preserving fungal elements 1

Dermatophyte Infections (Skin, Hair, Nails)

• Use KOH preparation to confirm tinea infections before initiating antifungal therapy, scraping the active border of skin lesions with a #15 scalpel blade or glass slide edge 2, 4
• KOH is particularly valuable for hair dermatophytes, which dermatologists identify as the most common indication for this test 5
• For onychomycosis, obtain nail specimens from beneath the nail plate or from subungual debris, though culture may be needed to identify specific species for treatment selection 6

Deep Tissue and Invasive Fungal Infections

• Perform Calcofluor-KOH stain on tissue biopsies, pericardial fluid, or resected graft material when invasive aspergillosis or other deep fungal infections are suspected 3
• For suspected invasive aspergillosis, examine bronchoalveolar lavage specimens or tissue biopsies as wet mount preparations with or without 10% KOH, though microscopy sensitivity is only 50% at best 3
• Transport deep tissue specimens in sterile containers at room temperature within 2 hours 3

Technical Execution

Specimen Preparation

• Place the specimen directly on a clean glass microscope slide, add 1-2 drops of 10% KOH solution, and apply a coverslip 2
• For vaginal specimens, collect from the vaginal pool or lateral walls using a cotton swab, avoiding cervical mucus contamination 1, 2
• For skin lesions, scrape scales from the active advancing border where fungal elements are most concentrated 2, 4

Microscopic Examination

• Examine under low power (10x) first to scan the entire specimen, then switch to high-dry power (40x) for detailed identification of fungal structures 2
• Look specifically for budding yeast and pseudohyphae in candidiasis, or septate hyphae in dermatophyte infections 1, 2
• Adding black or blue-black ink to the KOH solution can highlight fungal organisms and distinguish them from cellular debris 7

Critical Limitations and When NOT to Rely on KOH Alone

Poor Sensitivity Scenarios

• Do not rely on KOH wet prep alone for trichomoniasis—sensitivity is only 40-80%, meaning more than half of infections can be missed 1, 2
• KOH preparation for vaginal candidiasis has only 40-80% sensitivity compared to culture, so negative results do not exclude infection 1, 2
• For invasive aspergillosis, microscopy sensitivity is approximately 50%, requiring additional diagnostic methods 3

When to Order Culture or NAAT Instead

• Order vaginal yeast culture when KOH is negative but symptoms persist, particularly for recurrent infections that may involve non-albicans Candida species requiring different treatment 1, 2
• For trichomoniasis, order NAAT testing immediately rather than relying on wet prep, as NAAT provides superior sensitivity and Trichomonas prevalence equals or exceeds chlamydia/gonorrhea in certain populations 1, 2
• For onychomycosis, fungal culture is required to identify the specific genus and species when treatment selection depends on the pathogen 6
• Consider multiplex NAAT panels (e.g., BD Max Vaginal Panel) for comprehensive pathogen detection with superior sensitivity and specificity compared to wet prep 1

Proficiency Requirements

• Wide variation exists in interpretation between laboratories and providers—proficiency in microscopic examination is essential 1
• The technique requires hands-on teaching from experienced clinicians, as it is a "disappearing clinical art form" in many primary care settings 4
• Consider quantitative Gram stain (Nugent criteria) as the gold standard for bacterial vaginosis when expertise with wet prep is limited 1

Common Pitfalls to Avoid

• Do not culture Gardnerella vaginalis for bacterial vaginosis diagnosis—it lacks specificity as the organism is present in normal flora 1
• Do not contaminate vaginal specimens with cervical mucus, as this affects test accuracy 1
• Do not delay examination beyond 2 hours, as motile trichomonads lose viability and fungal elements may deteriorate 1, 2
• Do not forget concurrent STI testing when evaluating vaginal symptoms, as many infections coexist 1
• For invasive fungal infections, do not rely solely on KOH when specialized techniques like Calcofluor White fluorescent staining are available, as these increase sensitivity 3