Laboratory Testing for Nail Ridges
For nail ridges, laboratory testing is only indicated when fungal infection, inflammatory disease, or systemic conditions are suspected based on specific clinical features—vertical ridges from normal aging require no testing. 1
When Laboratory Testing is NOT Needed
- Vertical ridges alone in older adults are a normal aging change and require no laboratory workup 1
- Simple vertical ridging without nail thickening, discoloration, brittleness, or associated skin findings represents benign age-related changes in the nail matrix 1
When Laboratory Testing IS Indicated
For Suspected Fungal Infection (Onychomycosis)
Mycological confirmation is mandatory before initiating antifungal therapy—never treat based on clinical appearance alone, as 50% of dystrophic nails are non-fungal. 2, 3
Primary Diagnostic Tests:
- Potassium hydroxide (KOH) preparation with microscopy to visualize fungal elements—this is the first-line screening test 2, 4
- Calcofluor white staining significantly increases sensitivity compared to standard KOH by enhancing visualization of fungal elements 3, 4
- Fungal culture on Sabouraud's glucose agar to identify the specific pathogen species, incubated at 28°C for at least 3 weeks 2, 4
Specimen Collection Technique:
- Collect subungual debris from the most proximal part of the infection using a dental scraper, as dermatophyte infections primarily affect the nail bed 2, 4
- Submit as much material as possible due to the relative paucity of fungal elements 2
- For distal lateral subungual onychomycosis, scrape material from beneath the nail plate and from the nail bed itself 2
Advanced Testing When Initial Tests Are Negative:
- Histopathological examination with periodic acid-Schiff (PAS) staining is more sensitive (82-92%) than KOH (48-80%) or culture (53-59%) when clinical suspicion remains high 4
- Real-time PCR assays provide results within 2 days versus 3 weeks for culture, with significantly increased detection rates 3, 4
For Suspected Inflammatory Conditions
When vertical ridges are accompanied by nail thinning, pitting, onycholysis, or subungual hyperkeratosis suggesting psoriasis or lichen planus:
- Inflammatory markers (ESR, CRP) to assess for systemic inflammatory disease 4
- Nail biopsy may be necessary for definitive diagnosis of lichen planus or psoriasis when clinical features are ambiguous 2, 5
For Suspected Bacterial Infection
- Bacterial culture if green discoloration is present to identify Pseudomonas aeruginosa infection 1, 4
For Brittle Nails with Ridging
- Complete blood count to screen for iron deficiency anemia 1
- Thyroid function tests (TSH, free T4) to evaluate for thyroid dysfunction 1
- Consider biotin levels if nutritional deficiency is suspected, though supplementation trials may be more practical 1
Critical Pitfalls to Avoid
- The most common cause of treatment failure is incorrect diagnosis based solely on clinical grounds without laboratory confirmation 2, 4
- False negatives are common in fungal testing—if clinical suspicion remains high despite negative KOH, proceed to calcofluor white staining, PAS histopathology, or repeat testing 4
- Inadequate specimen collection from superficial nail plate rather than proximal subungual debris contributes to false negatives 2, 4
- Never assume fungal infection without laboratory confirmation—the nail surface in non-infectious conditions doesn't become soft and friable as in true fungal infection 1
Clinical Features That Guide Testing Decisions
Test for Fungal Infection When:
- Nail plate thickening with yellow-brown discoloration and friable texture 3
- Onycholysis (nail separation from bed) starting distally 3
- Subungual hyperkeratosis (debris under nail) 2, 3
- Associated tinea pedis 3
Test for Inflammatory Disease When:
- Nail pitting, oil drop sign (psoriasis) 3
- Nail thinning with longitudinal ridging and dorsal pterygium (lichen planus) 1, 3
- Multiple nail involvement with skin lesions 1