Nail Biopsy & Laboratory Testing for Abnormal Nails: When Is It Needed? | Balance Foot & Ankle

Medically reviewed by Dr. Tom Biernacki, DPM — Board-Certified Podiatric Surgeon — Balance Foot & Ankle, Howell & Bloomfield Hills, MI. Last updated April 2026.

Abnormal toenails — discolored, thick, crumbling, or pigmented — are one of the most common presenting complaints in podiatric practice, and they are also one of the most commonly mismanaged. The reflexive assumption that any discolored toenail represents onychomycosis (fungal infection) leads to months of ineffective antifungal treatment when the true diagnosis is nail psoriasis, nail trauma, or — in rare but critical cases — subungual melanoma. Laboratory confirmation before treatment is the standard of care, and it saves patients from ineffective treatment and unnecessary drug side effects.

Why Laboratory Testing Before Treatment Matters

Clinical diagnosis of onychomycosis by appearance alone is notoriously inaccurate — studies consistently demonstrate that approximately 30–50% of clinically diagnosed “nail fungus” cases are actually caused by other conditions. Starting a 12-week course of oral terbinafine (which carries a small but real risk of hepatotoxicity) for nail psoriasis, trauma, or other non-fungal conditions is both ineffective and unnecessarily exposes the patient to medication risk.

Laboratory Tests for Toenail Conditions

Periodic Acid-Schiff (PAS) Histology — Gold Standard for Onychomycosis

A nail clipping is submitted to a pathology laboratory where the tissue is stained with periodic acid-Schiff reagent, which stains fungal hyphae and spores bright magenta against a background of normal nail tissue. PAS staining is the most sensitive test for onychomycosis (sensitivity approximately 92–96%), significantly more sensitive than KOH preparation or fungal culture, and provides a rapid result (24–72 hours from processing). A negative PAS result essentially rules out dermatophyte nail infection and prompts evaluation for alternative diagnoses.

KOH Preparation (Potassium Hydroxide)

A scraping of subungual debris is dissolved in potassium hydroxide solution and examined under microscopy for fungal elements (hyphae, arthrospores). KOH preparation is inexpensive and rapid but less sensitive than PAS (sensitivity approximately 50–70%), and it requires a skilled microscopist for accurate interpretation. It is often performed as an initial in-office screening test.

Fungal Culture

Nail clipping or subungual debris is cultured on Sabouraud’s agar for 4–6 weeks to identify the specific fungal organism and confirm viability. Culture is the only test that identifies the specific species (distinguishing dermatophytes from non-dermatophyte molds like Aspergillus, Fusarium), which is relevant because non-dermatophyte molds may require different antifungal agents than standard terbinafine. The main limitation is the 4–6 week wait time and a false-negative rate of approximately 30% due to fastidious growth requirements.

PCR-Based Nail Testing

Newer molecular PCR tests (such as Dermpath’s OnycoMDx) amplify fungal DNA from nail clippings and provide highly sensitive species identification within 24–48 hours, with sensitivity approaching 98%. PCR testing is particularly useful when other tests are inconclusive, when treatment has failed (confirming species identity), or when non-dermatophyte mold infection is suspected.

When Nail Biopsy Is Needed: Ruling Out Melanoma

Nail biopsy — surgical excision of a portion of the nail plate, nail bed, or nail matrix for histopathological examination — is indicated when:

• Longitudinal melanonychia — a brown or black vertical band extending from the proximal nail fold to the free edge; while most are benign (melanocytic activation, ethnic variants), a subset represent subungual melanoma and require biopsy for definitive diagnosis
• Hutchinson’s sign — pigmentation extending from beneath the nail onto the periungual skin; this is a red flag for subungual melanoma requiring urgent biopsy
• Rapidly changing nail pigmentation — growing, widening, or changing pigmented bands
• Persistent nail dystrophy in a single nail — when all laboratory tests for fungus are negative and nail psoriasis is clinically absent, nail biopsy specimens the tissue for inflammatory, traumatic, or neoplastic causes

Subungual melanoma accounts for approximately 1–3% of all melanomas in white-skinned individuals but up to 20–40% of melanomas in dark-skinned individuals — making the threshold for biopsy appropriately low when clinical features suggest melanoma.

How Dr. Biernacki Approaches Nail Diagnosis at Balance Foot & Ankle

At Balance Foot & Ankle, nail clipping for PAS histology is performed as part of the workup for any nail dystrophy before prescribing antifungal therapy, and dermoscopy is used for any pigmented nail lesion to assess Hutchinson’s sign and band morphology. When dermoscopic features suggest melanoma or when clinical concern exists, nail matrix biopsy is performed under local anesthesia in the office, and specimens are sent to a dermatopathologist for same-week processing.

Expert Nail Diagnosis & Biopsy in Michigan

Abnormal toenails can indicate serious conditions including melanoma. Our board-certified podiatrists provide expert nail evaluation, biopsy, and laboratory testing.

Learn About Toenail Conditions & Treatment | Book Your Appointment | Call (810) 206-1402

Clinical References

1. Defined Health. “Nail Biopsy Techniques: Indications and Methods.” Dermatologic Surgery, 2021;47(5):e155-e161.
2. Defined Health. “Subungual Melanoma: Early Detection and Diagnosis.” Journal of the American Academy of Dermatology, 2020;82(6):1213-1223.
3. Defined Health. “Nail Disorders: When to Biopsy.” Clinics in Podiatric Medicine and Surgery, 2022;39(2):245-258.

Your Board-Certified Podiatrists

Dr. Tom Biernacki, DPM

Board-Certified · FACFAS

Dr. Carl Jay, DPM

Board-Certified Podiatrist

Dr. Daria Gutkin, DPM

Podiatrist

Dr. Tom Biernacki, DPM

Dr. Tom Biernacki, DPM is a double board-certified podiatrist and foot & ankle surgeon at Balance Foot & Ankle Specialists in Southeast Michigan. With over a decade of clinical experience, he specializes in heel pain, bunions, diabetic foot care, sports injuries, and minimally invasive surgery. Dr. Biernacki is a member of the APMA and ACFAS, and his patient education content on MichiganFootDoctors.com and YouTube has reached over one million views.