Subungual melanoma is the most important life-threatening nail diagnosis to identify in primary care — it represents approximately 1-3% of all melanomas in Caucasian populations but up to 25% of melanomas in Black and Asian populations (where acral lentiginous melanoma is relatively more common). The median age at diagnosis is approximately 60-65 years. The clinical challenge: benign longitudinal melanonychia (dark streak in the nail) is common in dark-skinned individuals and can be entirely normal — the key discriminating features suggesting malignancy are Hutchinson's sign (pigment extending onto the proximal or lateral nail folds, the cuticle, or the surrounding skin), width above 3 mm, irregular borders, multiple colours within the band, progressive widening or darkening over time, and concurrent nail dystrophy. The ABCDEF rule for subungual melanoma: A (age and affected digit — peak 50-70 years, thumb or great toe), B (band width >3 mm, Blurred borders), C (Colour heterogeneity, Change in size), D (Digit — single), E (Extension to the nail fold — Hutchinson's sign), F (Family or personal history of melanoma).

Beau's lines are one of the most useful retrospective clinical signs in nail examination — they appear as transverse grooves or ridges running across the width of the nail, caused by temporary cessation of nail matrix activity during a significant systemic illness, severe nutritional deficiency, or chemotherapy. Because fingernails grow at approximately 3 mm per month, the distance of the Beau's line from the proximal nail fold can be used to date the precipitating illness — a groove at 6 mm from the base occurred approximately 2 months ago. Multiple parallel Beau's lines indicate multiple separate insults. This retrospective dating can be clinically useful: a patient presenting with fatigue and multiple Beau's lines at 9 mm from the base had a significant physiological stress approximately 3 months ago — this can prompt targeted history-taking. Common causes include: septicaemia, major surgery, severe infection, myocardial infarction, severe nutritional depletion, and chemotherapy cycles.

The nail clipping sample collection technique is critical for accurate mycology results — poor technique is the most common reason for false-negative fungal nail cultures. The correct technique: (1) clean the nail with 70% alcohol and allow to dry (removes contaminant bacteria that can inhibit fungal growth); (2) collect material from the most proximal infected part of the nail (where the fungus is actively growing, not the distal dystrophic free edge which contains mostly dead keratin); (3) include subungual debris with the nail clipping — this material has the highest fungal yield; (4) for white superficial onychomycosis: scrape the white material from the nail surface; (5) place in a dry paper envelope or mycology pot (not swab — swabs dry out and reduce yield); (6) minimum 20-50 mg of material. Clinical Pearl: even with perfect technique, nail mycology culture has approximately 50-70% sensitivity — a negative culture does not exclude fungal nail infection. PCR-based nail mycology testing (where available) has higher sensitivity (approximately 85-90%).

The psoriatic nail oil drop sign is one of the most diagnostically specific physical examination findings in dermatology — it appears as a pink, salmon, or yellowish-brown translucent discolouration in the nail bed, usually in the proximal nail bed, resembling a drop of oil beneath the nail. The pathological correlate is foci of parakeratosis (abnormal keratinisation) in the nail bed epithelium, identical to the psoriatic plaque process occurring in the nail unit. The oil drop sign has a specificity of approximately 90-95% for psoriasis when present — it is far more specific than onycholysis or subungual hyperkeratosis alone. Clinical importance: in a patient with nail dystrophy where the diagnosis is uncertain (fungal vs psoriatic), the presence of oil drop sign should prompt assessment for psoriasis elsewhere (hairline, natal cleft, umbilicus, ears — hidden psoriasis sites) and consideration of psoriatic arthritis (early, when nail changes may precede joint manifestations). Nail psoriasis with onycholysis and pitting should not be treated with systemic antifungals — nail mycology should confirm or exclude concurrent fungal infection before prescribing.

Lichen planus of the nails is a specific condition requiring early dermatology referral to prevent permanent nail destruction — nail LP affects approximately 10% of patients with lichen planus and can occur without skin or mucosal LP. The nail changes include: thinning and brittleness, longitudinal ridging (onychorrhexis), fraying, loss of shine, and crucially, pterygium formation (dorsal pterygium — progressive forward growth of the proximal nail fold over the nail plate, caused by scarring of the nail matrix). Pterygium formation is permanent and irreversible — once the nail matrix is scarred, no further nail can grow in that area. Early treatment with potent topical corticosteroids (triamcinolone 0.1% ointment applied to the proximal nail fold twice daily) or intralesional steroid injections can prevent pterygium formation if initiated before scarring occurs. This is why urgent dermatology referral for nail LP is important — the window of opportunity to prevent permanent damage is typically 1-2 years from symptom onset.

The herpetic whitlow distinction from bacterial paronychia is critical because incising a herpetic whitlow worsens the condition and spreads the virus — herpetic whitlow (HSV-1 or HSV-2 infection of the finger pulp or periungual skin) presents as multiple clusters of vesicles on an erythematous base, with intense burning/throbbing pain that is disproportionate to the appearance, without the central fluctuance of a bacterial abscess. The tell-tale features: the vesicular appearance (multiple small fluid-filled blisters rather than a single pointing abscess), absence of purulent exudate, disproportionate pain, and the characteristic dermatomal-like appearance. Risk groups: dental/medical workers (exposure to oral herpes), patients with genital herpes (autoinoculation). Treatment: aciclovir 200 mg 5 times daily for 5 days (or valaciclovir 1 g BD), analgesia, loose dressing. Do NOT perform incision and drainage — this is the most important error to avoid. If uncertain between bacterial and herpetic: swab for HSV PCR + culture, treat empirically with antiviral while awaiting result.

The biologics for nail psoriasis represent a major advance in treating this difficult-to-treat condition — the NAPSI (Nail Psoriasis Severity Index) score is the validated tool for measuring nail psoriasis severity (0-8 per nail, scoring matrix involvement and nail bed involvement on each nail). Anti-IL-17 agents (secukinumab, ixekizumab) achieve the highest rates of complete nail psoriasis clearance (approximately 50-60% achieve NAPSI = 0 at 26 weeks in phase III trials), compared to anti-TNF agents (adalimumab approximately 40%) and anti-IL-12/23 (ustekinumab approximately 35-40%). The guselkumab (anti-IL-23) PSUMMIT trial showed NAPSI improvement of approximately 60-70% at 24 weeks. Crucially, nail psoriasis responds more slowly to biologics than plaque psoriasis — the first 3-6 months of biologic treatment may show little nail improvement because the nail must physically grow out. Patients starting biologics for psoriasis should be counselled that nail response will lag 3-6 months behind skin response. This expectation-setting prevents premature discontinuation of effective treatment.

The diabetic patient with onychomycosis deserves more proactive treatment than the standard 'wait and see' approach commonly used for nail fungus in non-diabetic patients — the rationale: in diabetic foot syndrome, the normal epidermal barrier in the toe web spaces and periungual skin is already compromised by neuropathy (reduced protective sensation), microangiopathy (impaired wound healing), and xerosis (dry, cracked skin). Onychomycosis adds further disruption by creating a portal of entry for Staphylococcus aureus and gram-negative organisms that can rapidly progress to cellulitis, necrotising fasciitis, osteomyelitis, and ultimately limputations — which diabetes-related foot disease already causes at a rate of approximately 135 amputations per week in England. NICE NG19 (Diabetic Foot) and NICE NG28 (Type 2 Diabetes) both recommend proactive treatment of onychomycosis in diabetic patients. GPs and practice nurses conducting annual diabetic foot checks should actively identify and treat onychomycosis rather than dismissing it as a cosmetic concern.

The documentation of nail examination findings at chronic disease reviews is a simple but important quality and medicolegal standard — GPs who perform annual diabetic foot checks, chronic heart failure reviews, COPD reviews, and IBD reviews are expected to examine relevant physical signs including those visible in the nails. A note stating 'nails: koilonychia noted for first time — ferritin requested' demonstrates appropriate clinical observation. Conversely, a review note that does not mention nail examination when the patient subsequently presents with a missed subungual melanoma (present at the time of the review) creates a defensible medicolegal concern. The nail examination itself takes approximately 30 seconds — inspect both hands and feet systematically at relevant chronic disease reviews, and document findings.