What laboratory tests are recommended for evaluating lines in nails?

Laboratory Tests for Evaluating Lines in Nails

For lines in nails, microscopy and culture are the essential laboratory tests to rule out fungal infection, while additional tests should be selected based on the specific appearance and pattern of the nail lines.

Initial Diagnostic Approach

• Direct microscopy with potassium hydroxide (KOH) preparation is the primary test for visualizing fungal elements in nail samples, particularly when fungal infection is suspected 1
• Fungal culture on Sabouraud's glucose agar should be performed to identify the specific pathogen species involved in nail abnormalities 1
• Calcofluor white staining for microscopic examination is recommended as it significantly enhances visualization of fungal elements compared to standard KOH preparation 1

Advanced Diagnostic Tests

• Molecular diagnostic techniques such as real-time PCR should be considered for faster and more sensitive detection of dermatophytes, with results available within 2 days 1
• Histopathological analysis using periodic acid-Schiff (PAS) staining is more sensitive than direct microscopy or culture for detecting fungal elements 1
• Nail biopsy may be necessary when standard tests are inconclusive or when malignancy is suspected, particularly with longitudinal melanonychia (brown-black lines) 2, 3

Specific Tests Based on Nail Line Appearance

For Vertical Lines/Ridges:

• Complete blood count and iron studies if koilonychia (spoon-shaped nails with ridges) is present, as this may indicate iron deficiency anemia 4
• Screening for inflammatory conditions (ESR, CRP) if vertical ridges are accompanied by other nail changes suggestive of lichen planus or psoriasis 5, 3

For Horizontal Lines (Beau's Lines):

• Comprehensive metabolic panel to assess for systemic illness that may have triggered temporary nail matrix arrest 6, 4
• Renal function tests if half-and-half nails (Lindsay's nails) are present, as these can indicate chronic kidney disease 7

For Discolored Lines:

• Bacterial culture if green discoloration is present, particularly to identify Pseudomonas infection 1, 5
• Dermoscopic examination followed by biopsy for longitudinal melanonychia to rule out subungual melanoma, especially in dark-skinned individuals 3, 4

Common Pitfalls and Considerations

• The most common cause of treatment failure for nail abnormalities is incorrect diagnosis based solely on clinical appearance without laboratory confirmation 1
• Proper specimen collection is crucial for accurate diagnosis - samples should be taken from the most proximal part of the infection for dermatophyte infections 1
• False negatives are common in fungal testing; if clinical suspicion remains high despite negative KOH examination, culture or histopathological analysis should be pursued 1
• Nail changes may reflect systemic disease, so laboratory testing should not be limited to local causes when appropriate 3, 4
• For suspected non-infectious causes of nail lines, testing should be directed by the suspected underlying condition (e.g., thyroid function tests, autoimmune markers) 5, 3