What is the proper technique for collecting a toenail specimen for suspected onychomycosis?

How to Collect Toenail Specimens for Fungal Testing

For suspected toenail fungus, scrape material from the distal underside of the nail plate first, then collect all debris from the distal nail bed using a small dental scraper, submitting as much material as possible to maximize diagnostic yield. 1

Step-by-Step Collection Technique

For Distal Lateral Subungual Onychomycosis (Most Common Type)

1. Start with the distal underside of the nail plate:

   • Use nail clippers to cut back any onycholytic (separated) nail 1
   • Scrape material from the underside of the nail plate using a dental scraper or scalpel blade 1
   • This location yields positive KOH results in 56.6% and positive cultures in 71.7% of confirmed cases 2

2. Then collect from the distal nail bed (most important site):

   • Use a small dental scraper to obtain subungual debris from beneath the nail 1
   • Scrape from the most proximal part of the infection accessible, as dermatophyte onychomycosis primarily affects the nail bed rather than the nail plate 1
   • This location yields positive KOH results in 79.2% and positive cultures in 87.7% of confirmed cases 2

3. Maximize specimen volume:

   • Submit as much material as possible because fungal elements are relatively scarce within specimens 1
   • Combining samples from both locations (underside of nail plate + nail bed) increases diagnostic yield to 86.8% for KOH and 94.3% for culture 2

For Superficial White Onychomycosis

• Scrape the surface of the infected nail plate directly with a scalpel blade 1
• This type presents as white, powdery patches on the nail surface 1

Alternative Technique: Microdrill Method

• Consider using a microdrill to obtain nail plate samples when conventional scraping has repeatedly failed despite strong clinical suspicion 3
• This technique yields consistently heavier fungal growth on culture media compared to standard clipping/scraping 3

Critical Collection Principles

Why proper collection matters:

• The most common cause of treatment failure is incorrect diagnosis based on clinical grounds alone without laboratory confirmation 1
• Only 50% of nail dystrophy cases are actually fungal in origin, making laboratory confirmation essential before starting 12-month treatment courses 1
• Inadequate specimen collection from superficial nail plate rather than proximal subungual debris is a major contributor to false-negative results 4

What to submit to the laboratory:

• Collect specimens from discolored, dystrophic, or brittle parts of the nail 4
• Cut through the entire thickness of the nail and include all crumbly material 4
• The quality of the sample directly determines the success of microscopy and culture 1

Common Pitfalls to Avoid

• Don't sample only the superficial nail surface in distal lateral disease—you must reach the nail bed where dermatophytes primarily reside 1
• Don't submit insufficient material—fungal elements may be very scanty, requiring generous sampling 1
• Don't assume a single negative test rules out infection—if clinical suspicion remains high, repeat testing with enhanced techniques (calcofluor white staining or PAS staining) or consider the microdrill method 5, 3
• Don't start treatment without laboratory confirmation—the cost of diagnostic testing is always small relative to inappropriate 12-month antifungal therapy 1