What are the possible causes of leukonychia (white lines) on the fingernails and how should it be evaluated?

White Lines in Fingernails (Leukonychia): Causes and Evaluation

White lines or discoloration in the nails (leukonychia) requires systematic evaluation to distinguish between benign nail plate abnormalities and potentially serious systemic disease—the key is determining whether the whiteness originates from the nail plate itself (true leukonychia), the nail bed (apparent leukonychia), or surface deposits (pseudoleukonychia).

Diagnostic Approach: Anatomical Classification First

The most clinically useful approach combines anatomical origin with morphological pattern 1:

Step 1: Perform the Blanching Test

• Press on the nail plate firmly
• If white discoloration disappears with pressure → Apparent leukonychia (nail bed pathology)
• If white discoloration persists with pressure → True leukonychia (nail plate abnormality)
• If white material scrapes off → Pseudoleukonychia (surface deposits)

Step 2: Assess the Distribution Pattern

For True Leukonychia (doesn't blanch):

• Punctate/striated: Usually traumatic injury from manicure or minor nail trauma 2
• Transverse lines (Mees' lines): Consider arsenic poisoning, chemotherapy, systemic illness 3
• Total leukonychia: Hereditary (genetic mutations in PLCδ1 gene) or chronic arsenic exposure 3, 4

For Apparent Leukonychia (blanches with pressure):

• Terry's nails (proximal 80% white, distal pink band): Cirrhosis, heart failure, diabetes
• Muehrcke's lines (paired transverse white bands): Hypoalbuminemia, nephrotic syndrome
• Lindsay's nails (half-and-half): Chronic kidney disease

For Pseudoleukonychia:

• Superficial white onychomycosis (SWO): Fungal infection affecting nail surface 5
• External deposits from nail polish, chemicals

Critical Differential Diagnoses

Infectious Causes to Rule Out

The British Association of Dermatologists guidelines emphasize that fungal infections can present with white discoloration 5:

• Superficial white onychomycosis: Crumbling white lesions on nail surface, most common in children, usually Trichophyton interdigitale 5
• Candida infection: White marks in proximal/lateral portions, associated with paronychia, more common in women with water exposure 5
• Key distinguishing feature: Fungal infections make the nail soft and friable, unlike most non-infectious causes 5

Non-Infectious Mimics

Multiple conditions can produce white nail changes 5:

• Chronic trauma: Repetitive injury causing distal onycholysis
• Psoriasis: Associated with other nail findings (pitting, oil spots)
• Lichen planus: Thinning, subungual hyperkeratosis, pterygium
• Darier disease: Genetic disorder with characteristic nail changes

Essential Workup

When to Investigate Further:

1. If fungal infection suspected (soft, friable nails):

   • Nail clipping for microscopy and culture 5
   • Consider PCR testing for faster diagnosis (2 days vs 2-6 weeks) 5
   • Specimen collection: Cut nail as far back as possible through entire thickness 5

2. If systemic disease suspected (apparent leukonychia):

   • Liver function tests: Albumin, transaminases (Terry's nails)
   • Renal function: Creatinine, BUN, urinalysis (Lindsay's/Muehrcke's)
   • Nutritional assessment: Vitamin B12, zinc levels 6
   • Thyroid function: TSH if other symptoms present 6

3. If toxic exposure suspected (transverse lines):

   • Arsenic levels: Hair, nail, and urine testing 3
   • Heavy metal screen: If occupational/environmental exposure
   • Electrophysiological studies: If neuropathy symptoms present 3

Common Pitfalls to Avoid

• Don't assume all white nails are fungal: Only 15-40% of nail dystrophies are onychomycosis 5
• Don't treat without confirmation: Always obtain mycological confirmation before starting antifungal therapy for suspected fungal infection 5
• Don't miss systemic disease: Apparent leukonychia (blanches with pressure) warrants systemic workup
• Don't overlook arsenic toxicity: Total leukonychia with neuropathy symptoms requires urgent arsenic testing 3

When Benign Observation is Appropriate

Idiopathic true leukonychia (persistent white discoloration, normal systemic workup, negative fungal studies) is often benign and hereditary 6. These cases require:

• Reassurance about benign nature
• Family history documentation
• No treatment necessary unless psychosocial burden significant
• If treatment desired for hereditary cases: Intramatricial triamcinolone has shown success 4

The critical distinction is identifying which patients need aggressive workup for life-threatening conditions (liver failure, kidney disease, arsenic poisoning) versus those with benign nail plate abnormalities or simple fungal infection requiring only topical therapy.