What is the likely diagnosis for a positive KOH (potassium hydroxide) prep taken from the scalp?

Positive KOH Preps from the Scalp Most Likely Indicate Tinea Capitis

A positive potassium hydroxide (KOH) preparation from scalp samples most likely indicates tinea capitis, a fungal infection of the scalp and hair caused by dermatophyte fungi. 1, 2

Diagnostic Significance of KOH Preparation

KOH preparation is a valuable diagnostic tool in dermatology that helps identify fungal elements by:

• Dissolving keratin and cellular debris while preserving fungal structures
• Allowing visualization of fungal hyphae and spores under microscopic examination
• Providing rapid preliminary diagnosis before culture results are available

Sensitivity and Specificity

• KOH preparation has high sensitivity (up to 100% in some studies) for detecting fungal elements 3
• It is particularly useful for tinea capitis diagnosis in clinical settings 2

Clinical Presentation of Tinea Capitis

Tinea capitis presents with various clinical manifestations:

• Scaly patches on the scalp with hair loss
• Broken hair shafts
• Characteristic "corkscrew hairs" (especially in Afro-Caribbean children) 1
• "Comma-shaped" hairs (more common in white children) 1
• May present with inflammation, pustules, or kerion formation in severe cases

Epidemiology and Risk Factors

• Most common in children but can affect adults (3-11% of cases) 2
• In adults, postmenopausal women are most frequently affected 2
• Higher prevalence in African American or Black individuals 2
• Risk factors include:
  • Crowded living conditions
  • Close proximity to animals
  • Immunosuppression
  • Household contact with affected individuals 2

Management Considerations

According to guidelines for infectious diseases in organized sports 4, management of tinea capitis requires:

• Minimum of 2 weeks of systemic antifungal therapy
• Patients with extensive and active lesions (confirmed by KOH prep) should be disqualified from certain activities until adequately treated
• Topical therapy alone is insufficient for treating tinea capitis 2

Diagnostic Algorithm

1. Collect appropriate specimens:

   • Hair plucking
   • Scalp scraping
   • Hair brushing

2. Perform KOH preparation:

   • Place specimen on slide
   • Add 10% KOH solution
   • Apply coverslip and examine under microscope

3. Look for characteristic findings:

   • Fungal hyphae within or around hair shafts (ectothrix or endothrix)
   • Arthroconidia (spores) arranged in chains

4. Confirm with fungal culture:

   • Identifies specific dermatophyte species
   • Takes longer (days to weeks) but provides definitive identification

Common Pitfalls and Caveats

• False negatives can occur if:

  • Insufficient sample is collected
  • Improper technique is used
  • Patient has recently used antifungal shampoo

• Other conditions that may present with scalp lesions but would not show fungal elements on KOH include:

  • Seborrheic dermatitis
  • Psoriasis
  • Bacterial folliculitis
  • Alopecia areata

• Delayed diagnosis or inadequate treatment can result in scarring alopecia 2

• Rising antifungal resistance is a concern, requiring appropriate antifungal stewardship 2

While other conditions like candidiasis can occasionally produce positive KOH preparations from skin sites, the specific location of the scalp makes tinea capitis the most likely diagnosis when fungal elements are identified in KOH preparations from this site.