Residency · Residency · Dermatology

Approach to the Skin Biopsy: Techniques and Specimen Handling

Indications for Skin Biopsy

Skin biopsies are performed to confirm or establish a clinical diagnosis, rule out malignancy in suspicious lesions, classify inflammatory dermatoses when the clinical diagnosis is uncertain, monitor disease progression or treatment response (such as in connective tissue disease), obtain tissue for direct immunofluorescence studies (in autoimmune blistering diseases and lupus), and submit specimens for culture or special stains when an infectious etiology is suspected.

Biopsy Techniques

Shave Biopsy

A shave biopsy involves tangential removal of the epidermis and superficial dermis using a flexible blade (DermaBlade) or a number 15 scalpel blade held parallel to the skin surface. Depth can be varied: a superficial shave (saucerization) removes epidermis and papillary dermis, while a deep shave (scoop) extends into the mid-reticular dermis. This technique is indicated for superficial lesions such as seborrheic keratoses, superficial BCC, actinic keratoses, verrucae, and benign-appearing melanocytic nevi. Advantages include speed, simplicity, good cosmesis, and no suture requirement. However, it cannot assess the deep margin or subcutaneous involvement and is contraindicated when melanoma is suspected because transection of the base prevents accurate Breslow thickness measurement. It is also insufficient for panniculitis or deep dermal processes. Hemostasis is achieved with aluminum chloride 20 percent (Drysol), electrocautery, or ferric subsulfate (Monsel solution). Importantly, Monsel solution causes a tattoo artifact from iron deposition that can mimic melanin on subsequent re-biopsy, so it should be avoided on cosmetically sensitive areas and melanocytic lesion biopsy sites.

Punch Biopsy

The punch biopsy uses a cylindrical blade (disposable punch) rotated perpendicularly through the skin to obtain a full-thickness specimen. Sizes range from 2 mm (the smallest useful for diagnosis) to 8 mm, with 4 mm being the workhorse for most inflammatory and neoplastic biopsies. Sizes of 2 to 3 mm are used for cosmetically sensitive areas such as eyelids and lips. Indications include inflammatory dermatoses, panniculitis (using a large punch or incisional biopsy), melanocytic lesions when excision is impractical, alopecia, and deep infections. Technical tips include stretching the skin perpendicular to relaxed skin tension lines before punching to create an elliptical wound that closes more easily, rotating the punch in one direction to avoid shearing artifact, and using forceps to grasp the dermis or subcutis rather than the epidermis to avoid crush artifact. A single interrupted suture closes 4 mm and larger punches, while 2 to 3 mm specimens may heal by secondary intention or with Steri-Strips. The punch provides a full-thickness specimen with simplicity and minimal scarring, though the small sample size may miss pathology and peripheral margins are not assessed.

Excisional Biopsy

Excisional biopsy involves elliptical excision of the entire lesion with narrow margins of 1 to 3 mm extending into subcutaneous fat. It is the gold standard for suspected melanoma and is indicated for lesions requiring complete removal with margin assessment, subcutaneous masses or tumors, and lymph node biopsies. The ellipse is designed with a 3:1 length-to-width ratio oriented along relaxed skin tension lines. It provides a complete specimen with margins and offers definitive diagnosis and treatment in one step, though it produces a larger scar, requires suturing, and is more time-intensive.

Incisional Biopsy

An incisional biopsy involves partial removal of a lesion (a wedge or punch from within a larger lesion) and is indicated for large lesions where excision is impractical. The most representative or concerning area should be sampled, typically the darkest, most elevated, or most indurated portion. Importantly, incisional biopsy of melanoma does not worsen prognosis, as there is no evidence of tumor seeding.

<image>Illustrations of the four main skin biopsy techniques: shave biopsy, punch biopsy, excisional biopsy, and incisional biopsy, showing depth of tissue sampling and orientation for each method</image>

Specimen Handling

Fixation

10 percent neutral buffered formalin is the standard fixative for routine histopathology. The specimen should be fixed immediately and fully submerged at a 10:1 formalin-to-tissue ratio. Formalin-fixed paraffin-embedded (FFPE) tissue can be used for immunohistochemistry, special stains, and retrospective molecular studies. Michel's transport medium is used for direct immunofluorescence (DIF), preserving immunoglobulins and complement for weeks. Formalin must not be used for DIF specimens because it destroys immunoreactants. Fresh tissue on saline-moistened gauze is required for culture (bacterial, fungal, mycobacterial), frozen section, or flow cytometry and must be transported immediately without drying.

Orientation and Labeling

Excisional specimens should be marked with a suture or ink at one margin for orientation, especially when margins are narrow. Containers must be labeled with the patient's name, date, anatomic site, and clinical history. Clinical information is critical: the dermatopathologist needs the differential diagnosis, clinical morphology, distribution, duration, and relevant medications or exposures.

Bisection and Processing

Small punch biopsies are bisected at the level of greatest diameter. Excisional specimens are bread-loafed perpendicular to the long axis at 2 to 3 mm intervals, and tip sections from the ellipse ends may be examined for margin assessment. Alopecia biopsies require special handling as described below.

Special Stains in Dermatopathology

Histochemical Stains

PAS (Periodic Acid-Schiff) highlights fungal hyphae, thickened basement membrane zones (as in lupus), and glycogen. PAS-D (with diastase) removes glycogen, so remaining PAS-positive material represents fungi or basement membrane. GMS (Grocott's Methenamine Silver) has superior sensitivity for fungi, especially deep fungi. Fite stain detects acid-fast bacilli missed by standard Ziehl-Neelsen, including atypical mycobacteria and M. leprae, using a weaker decolorizer. Ziehl-Neelsen detects M. tuberculosis and strongly acid-fast organisms. Gram stain identifies bacteria in tissue. Warthin-Starry and Steiner stains detect spirochetes (syphilis), Bartonella, and other bacteria. Congo red reveals amyloid deposition with characteristic apple-green birefringence under polarized light. Iron stain (Prussian blue) identifies hemosiderin in conditions such as stasis dermatitis, pigmented purpuric dermatoses, and hemochromatosis. Alcian blue and colloidal iron detect mucin deposition in dermal mucinoses and lupus. Fontana-Masson identifies melanin, distinguishing it from hemosiderin or other pigments. Verhoeff-van Gieson (VVG) highlights elastic fibers in conditions such as elastolysis, anetoderma, and solar elastosis. Toluidine blue and Giemsa stain mast cells in mastocytosis and urticaria pigmentosa.

<image>Photomicrographs demonstrating key special stains in dermatopathology: PAS stain highlighting fungal hyphae, Congo red showing amyloid with apple-green birefringence, and Fite stain demonstrating acid-fast bacilli</image>

Immunohistochemistry (IHC)

Melanocytic Markers

S100 is highly sensitive for melanocytic lesions but also marks nerve tissue, Langerhans cells, and some carcinomas. SOX10 is a nuclear stain sensitive for melanocytes and Schwann cells and is excellent for identifying desmoplastic melanoma. Melan-A (MART-1) is specific for melanocytes and stains junctional melanocytes strongly, though it can create "pseudonesting" artifact in sun-damaged skin. HMB-45 is expressed in immature or activated melanocytes and is positive in melanoma and blue nevi but typically negative in intradermal nevi, making it useful for maturation assessment. PRAME is overexpressed in melanoma compared to nevi and is emerging as a useful adjunctive marker: diffuse PRAME positivity favors melanoma, while negative or patchy staining favors benign nevi.

Cytokeratins and Epithelial Markers

AE1/AE3 (pan-cytokeratin) demonstrates epithelial differentiation. CK7 and CK20 help distinguish primary from metastatic carcinomas: Merkel cell carcinoma is CK20-positive (with paranuclear dot pattern) and CK7-negative, while Paget disease is CK7-positive. p40 and p63 mark squamous differentiation in SCC. BerEP4 is positive in BCC and negative in SCC, providing a useful distinction. EMA marks eccrine and apocrine differentiation and sebaceous carcinoma.

Lymphoid Markers

CD3 is a pan-T cell marker. CD4 and CD8 distinguish T-helper from T-cytotoxic cells. CD20 marks B cells. CD30 is expressed in large cell lymphomas (ALCL), lymphomatoid papulosis, and some MF. CD7 is frequently lost in CTCL (MF). CD56 marks NK cells and is seen in aggressive NK/T-cell lymphoma (nasal type).

Other Important Markers

CD31, CD34, and ERG indicate vascular differentiation in angiosarcoma and Kaposi sarcoma. D2-40 (podoplanin) is a lymphatic endothelial marker. SMA and desmin mark smooth muscle differentiation in leiomyoma and leiomyosarcoma. Factor XIIIa is a dermal dendrocyte marker useful for distinguishing dermatofibroma (Factor XIIIa-positive, CD34-negative) from DFSP (Factor XIIIa-negative, CD34-positive).

Direct Immunofluorescence (DIF)

Technique

A fresh biopsy is placed in Michel's medium (not formalin) and processed as a frozen section. It is stained with fluorescein-conjugated antibodies against IgG, IgA, IgM, C3, and fibrinogen. The biopsy site depends on the suspected diagnosis: pemphigus requires perilesional skin (blister edge), bullous pemphigoid requires perilesional skin, dermatitis herpetiformis requires uninvolved perilesional skin (ideally more than 3 mm from the blister), and lupus (lupus band test) uses lesional skin or sun-protected uninvolved skin for SLE.

Classic DIF Patterns

Pemphigus vulgaris and foliaceus show intercellular (chicken-wire) IgG and C3 in the epidermis. Bullous pemphigoid shows linear IgG and C3 at the basement membrane zone. Linear IgA bullous dermatosis shows linear IgA at the BMZ. Dermatitis herpetiformis shows granular IgA at the dermal papillae. Lupus erythematosus (lupus band) shows granular IgG, IgM, IgA, and C3 at the BMZ. IgA pemphigus shows intercellular IgA in the epidermis. Epidermolysis bullosa acquisita shows linear IgG at the BMZ on the dermal side of salt-split skin.

<image>Direct immunofluorescence patterns in autoimmune blistering diseases: intercellular IgG in pemphigus vulgaris, linear IgG at the basement membrane zone in bullous pemphigoid, and granular IgA at dermal papillae in dermatitis herpetiformis</image>

Biopsy for Specific Clinical Scenarios

Alopecia

Alopecia biopsies require two punch biopsies: one for horizontal sections and one for vertical sections. Horizontal sections allow assessment of follicular density, follicular unit composition, miniaturization ratio, and perifollicular fibrosis. Vertical sections assess interface changes, depth of inflammation, and scarring. The biopsy should be taken from the active edge of an alopecia patch, not the center of burned-out scarring. A 4 mm punch is the minimum, with some recommending 5 to 6 mm for adequate follicle sampling.

Panniculitis

Panniculitis requires a deep incisional or large punch biopsy (6 to 8 mm) extending into subcutaneous fat. Adequate subcutis must be included, as a superficial shave or small punch will miss the pathology entirely. Classification depends on identifying septal versus lobular panniculitis, with or without vasculitis.

Vasculitis

For vasculitis, the biopsy should be taken from an early, fully developed lesion that is 24 to 48 hours old, not from old or resolving lesions. A DIF specimen from an early lesion should also be obtained. A punch biopsy extending into the subcutis allows assessment of vessel size (small, medium, or large).

Bullous Disease

For bullous disease, biopsy of an intact, early blister (for H&E) determines the level of split (intraepidermal versus subepidermal). A separate perilesional biopsy for DIF should be taken from uninvolved skin adjacent to the blister. The salt-split skin technique distinguishes pemphigoid (antibodies on the epidermal side) from epidermolysis bullosa acquisita (antibodies on the dermal side).

Clinical Pearls

Never send a biopsy without clinical information -- "rule out rash" is insufficient. The dermatopathologist needs the clinical differential, morphology, distribution, and medication list. For suspected melanoma, excisional biopsy is the gold standard, as shave biopsy risks transecting the base and undermining Breslow staging. Always obtain a separate DIF specimen in Michel's medium when an autoimmune blistering disease is in the differential, because formalin-fixed tissue cannot be used for DIF. When biopsying alopecia, obtain two punches -- one for horizontal and one for vertical sections -- since a single small punch rarely provides sufficient information. Monsel solution (ferric subsulfate) deposits brown-black iron pigment in tissue that can mimic melanin on subsequent re-biopsy, so it should be avoided on melanocytic lesion biopsy sites.

References

• Elston DM, et al. Dermatopathology. 3rd Edition. Elsevier, 2019
• Rapini RP. Practical Dermatopathology. 3rd Edition. Elsevier, 2021
• Bolognia JL, et al. Dermatology. 4th Edition. Elsevier, 2018
• Ackerman AB, et al. Histologic Diagnosis of Inflammatory Skin Diseases. 3rd Edition
• Scolyer RA, et al. Melanocytic lesions of the skin: approach to diagnosis. Pathology. 2020;52(1):1-20

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