Periodic Acid-Schiff (PAS) Nail Stain in Clinical Practice
Direct Answer
PAS staining of nail clippings is the single most sensitive diagnostic method for detecting fungal elements in suspected onychomycosis, with 82-92% sensitivity compared to 48-80% for KOH preparation and 53-59% for fungal culture, and should be used as a second-line test when initial microscopy and culture are negative but clinical suspicion remains high. 1, 2, 3, 4
Primary Diagnostic Role
When to Order PAS Staining:
As a second-line test when direct microscopy (KOH preparation) and fungal culture are negative but clinical findings strongly suggest onychomycosis (nail thickening, discoloration, friable texture) 5, 1, 6
In patients with prior antifungal treatment, where PAS demonstrates 88% sensitivity compared to only 33% for culture and 50% for direct microscopy, as the antifungal agents inhibit fungal growth in culture but do not eliminate fungal elements visible on histopathology 2
When rapid diagnosis is needed, as PAS results are available within days versus the 3-week incubation period required for fungal culture 5, 3
Diagnostic Performance Characteristics
Sensitivity and Specificity:
PAS staining achieves 82-92% sensitivity for detecting fungal elements, significantly higher than KOH preparation (48-80%) or culture (53-59%) 1, 2, 3, 4
The specificity is 72%, comparable to KOH but slightly lower than culture (82%) 4
Negative predictive value of 77% makes PAS superior to other methods when ruling out onychomycosis 4
When used as a second-line test after negative initial workup, PAS identifies fungal elements in 38% of cases, leading to appropriate treatment and complete clinical cure 6
Technical Considerations and Specimen Collection
Proper Sampling Technique:
Collect subungual debris from the most proximal part of the infection using a dental scraper, as dermatophyte onychomycosis primarily affects the nail bed rather than the nail plate 5
For distal lateral subungual onychomycosis (DLSO), obtain material from beneath the nail; if onycholytic, cut back the nail and scrape both the underside and nail bed 5
Submit as much material as possible due to the relative paucity of fungal elements within specimens 5
The same nail clipping specimen can be processed for routine histology and stained with both hematoxylin-eosin and PAS 3, 6
Critical Limitations and Pitfalls
Important Caveats:
PAS cannot distinguish between pathogenic fungi and saprophytes, nor does it provide information about fungal vitality or specific pathogen identification 3, 6
Fungal culture remains necessary for species identification to guide targeted antifungal therapy, particularly when distinguishing dermatophytes from Candida species 5, 3
PAS may detect fungal colonization rather than true infection in some cases, requiring clinical correlation 7, 6
In nail psoriasis, PAS staining reveals fungal elements in 26% of cases, indicating secondary colonization rather than primary pathogenic infection, which can confound diagnosis 7
Algorithmic Approach to Nail Diagnosis
Step 1: Initial Testing
- Perform KOH preparation and fungal culture on Sabouraud's glucose agar for all suspected onychomycosis 5, 1
- Consider calcofluor white staining, which enhances visualization of fungal elements compared to standard KOH 1
Step 2: When Initial Tests Are Negative
- If clinical suspicion remains high (thickening, discoloration, friable texture present), order PAS staining of nail clippings 1, 4, 6
- This is particularly critical in patients with prior antifungal treatment 2
Step 3: Interpretation
- Positive PAS with negative culture: Treat as onychomycosis with empiric antifungal therapy 6
- Positive PAS with positive culture: Use culture results to guide species-specific treatment 5, 3
- Negative PAS after negative initial workup: Consider non-fungal causes (psoriasis, lichen planus, trauma) 1, 7
Additional Diagnostic Utility
Beyond Onychomycosis:
PAS staining can help differentiate nail psoriasis from onychomycosis by revealing histopathological features including hyperkeratosis with parakeratosis (78%), neutrophilic infiltration (63%), and hypergranulosis (58%) 7
The presence of parakeratosis and plasma globules on histological examination is statistically more common in fungal-infected nails, indicating ongoing inflammatory processes 6
For chronic paronychia with secondary nail dystrophy, PAS can identify Candida involvement requiring specific treatment approaches 5, 8
Clinical Impact on Treatment Decisions
Treatment should never be instituted on clinical grounds alone, as 50% of nail dystrophy cases are fungal in origin but cannot be identified accurately by appearance alone 5. Given that toenails require 12 months and fingernails 6 months to grow out completely, confirming diagnosis before initiating long-term systemic antifungal therapy is essential 5. The cost of PAS diagnostic testing is always small relative to inappropriate and unnecessary treatment 5.