In a 43-year-old woman with normal laboratory studies except a serum iron of 6 µg/dL who presents with darkly discolored toenail plates and longitudinal dark lines, what is the appropriate next step in evaluation and management?

Urgent Dermatology Referral for Nail Biopsy to Rule Out Subungual Melanoma

This patient requires immediate dermatology referral for nail biopsy to exclude subungual malignant melanoma, which is the most critical diagnosis to rule out in a patient presenting with darkly discolored toenails and longitudinal dark lines. The low serum iron is a secondary concern that should be addressed concurrently but does not explain the nail findings.

Critical Diagnostic Reasoning

The presentation of dark discoloration with longitudinal dark lines (longitudinal melanonychia) in an adult raises serious concern for subungual malignant melanoma, which is explicitly listed as a nonfungal cause of nail dystrophy that can mimic infectious processes 1. While the British Association of Dermatologists guidelines discuss various causes of nail discoloration, they specifically emphasize that bacterial infection (particularly Pseudomonas aeruginosa) causes green or black discoloration, but the presence of longitudinal dark lines is more characteristic of melanocytic processes 1.

Key Clinical Features Suggesting Melanoma Risk:

• Longitudinal melanonychia (brown-black streaks) is the presenting sign in the majority of ungual melanomas 2
• Dark discoloration "almost seemed dark" suggests intense pigmentation
• Longitudinal dark lines are specifically associated with melanocytic activity in the nail matrix 3, 4
• Age 43 years places patient in the typical demographic for subungual melanoma (very rare in children) 2

Immediate Next Steps

1. Dermatology Referral Within 2 Weeks

Refer urgently for:

• Onychoscopy (dermoscopy of nail) - provides valuable pre-biopsy information to differentiate melanocytic from non-melanocytic causes 5, 4
• Nail matrix biopsy if concerning features present 6, 2

2. Clinical Red Flags to Document

Before referral, document:

• Width of pigmented band (>5mm suggests malignancy) 2
• Hutchinson's sign (pigment extending onto proximal or lateral nail fold) - strongly suggests melanoma 2
• Nail dystrophy or bleeding mass 2
• Single nail involvement (more concerning than multiple nails) 1
• Irregular borders or color variation within the band 3, 4

Differential Diagnosis Framework

Melanocytic Causes (URGENT):

• Subungual melanoma - most serious, requires immediate exclusion 2
• Benign melanocytic activation (less likely with this presentation)

Non-Melanocytic Causes (Less Likely Given Description):

• Bacterial infection (Pseudomonas) - typically greenish-black, not longitudinal lines 1
• Fungal infection - nails become soft and friable, not typically dark with longitudinal lines 1
• Subungual hematoma - oval shaped, not neat streaks, history of trauma 2

Common Pitfalls to Avoid

1. Do NOT assume this is fungal infection - while onychomycosis is common, the specific description of dark discoloration with longitudinal dark lines does not fit typical fungal patterns, which cause thickening, friability, and discoloration but not longitudinal melanonychia 1

2. Do NOT delay referral for fungal testing - even if fungal studies are ordered, they should not delay dermatology evaluation for possible melanoma 6

3. Do NOT attribute findings to iron deficiency - while the serum iron of 6 µg/dL indicates iron deficiency (well below physiologically-based thresholds of ~25 µg/L for women 7), iron deficiency does not cause dark longitudinal nail lines. Toenail iron levels do not correlate with serum iron biomarkers 8 and iron deficiency typically causes koilonychia (spoon nails), not dark pigmentation 9

Concurrent Management of Iron Deficiency

While awaiting dermatology evaluation:

• Investigate cause of low iron (serum iron 6 µg/dL is severely low)
• Check complete iron panel: ferritin, TIBC, transferrin saturation
• Assess for gastrointestinal blood loss, menorrhagia, dietary insufficiency
• Initiate iron supplementation if indicated by complete workup

Timeline

• Dermatology referral: Within 2 weeks (urgent)
• Do not wait for iron studies or fungal cultures before referring
• If Hutchinson's sign present or band >5mm: same-day/next-day referral

The priority is excluding melanoma, which has significant mortality implications if diagnosis is delayed. Conservative management to preserve the digit is possible when melanoma is caught early 2.