Consider the working diagnosis and issues related to the individual case.
Record adequate data on the pathology request form (duration of the disease, site of the lesions, biopsy site).
Take a biopsy from the correct place using the correct technique.
Indications for skin biopsy
Tumours
All tumours that are removed should be sent for a pathological examination. Seborrhoeic keratosis (senile warts) and typical epidermoid cysts (containing light-coloured keratin mass) can be removed without a histopathological investigation if they can be identified with certainty (videos Excision of an Epidermal CystExcision of an Epidermal Cyst from the Earlobe). If the benign nature of a skin lesion is not obvious, a biopsy is preferable to watchful waiting.
Malignant skin tumours are not common in children. A child with an unidentified skin tumour must be referred directly to a specialist.
Psoriasis is usually diagnosed clinically, a biopsy is rarely needed
Skin infiltrates that resemble tumours (a biopsy is usually needed to confirm diagnosis)
It may be necessary to confirm with a biopsy specimen that a chronic ulcer is benign.
In chronic blistering skin disease (e.g. pemphigoid and dermatitis herpetiformis Chronic Bullous Diseases (Dermatitis Herpetiformis, Pemphigoid)), an immunofluorescence test (IF) is also usually necessary. The specimen should contain an entire blister fixed in formalin and a small fresh sample of perilesional skin for IF testing. If transport in liquid nitrogen is not available, the fresh tissue sample can be sent in transport medium (Michelin's fixative), which will preserve it for 5 days.
A biopsy may be indicated even from a harmless looking skin lesion if the lesion fails to react to appropriate treatment (e.g. eczematous chronic rash may prove to be cutaneous lymphoma or carcinoma in situ of the skin).
Skin biopsy technique
An ordinary skin biopsy is taken with a scalpel (no. 15); the cut is made through the entire skin down to the subcutaneous fat layer. The biopsy should have a fusiform shape (elliptical with sharp ends) with a flat bottom. The site is chosen so that no underlying structures limit the depth of the biopsy. The incision should be in the direction of skin folds in order to minimise scar formation . The standard size for a biopsy sample is 0.5 × 1.5 cm with the thickness dependent on the site.
A punch biopsy (usually 3-6 mm) is most suited for isolated tumours. The diagnosis will be confirmed and any necessary further action can be planned. For rashes and larger tumours, punch biopsies from several sites may be needed to improve diagnostic accuracy. If the skin lesion is very small, the biopsy specimen should also be small to aid the localisation of the lesion.
It is also possible to take a punch biopsy from skin tumours that are suspected to be malignant. Furthermore, a punch biopsy may be taken when melanoma is suspected, but the degree of atypia may vary significantly within the pigmentary change, and therefore the punch biopsy is associated with uncertainty. In other skin tumours, a punch biopsy specimen is usually very reliable.
The punch biopsy specimen should include the subcutaneous fat layer. Then it is a little raised from the skin and easy to remove. Use small forceps or a needle and scissors to handle the biopsy specimen. The specimen must not be squeezed or torn. A successful punch biopsy specimen is a cylindrical and unbroken piece where skin layers are discernible and there is also a little fat.
The biopsy site usually needs no sutures. A bleeding wound can be dressed with a suitable small dressing. A round cut shrinks noticeably with healing. Size 4-0 or 5-0 absorbable suture may be used to close a cut in more visible areas. The suture will fall out by itself within 1-2 weeks. Skin tape is used for protection during the first week.
Skin biopsy site
The biopsy is taken from a fully developed lesion, but areas with broken skin due to scratching should be avoided. In blistering skin diseases and suspected cases of vasculitis, the best specimen, however, is obtained from a lesion that is as newly developed as possible.
A biopsy is always taken from the centre of a lesion. A rash with a well-defined border is the only exception, and the biopsy should be taken so that the border is at the centre of the specimen. If the rash leaves a scar, a biopsy of the scar tissue is also beneficial.
Small blisters should be collected whole (punch biopsy is not a suitable method as the roof of the blister may become detached rendering diagnosis impossible!).
Small tumours may be removed in their entirety if easily achieved. A margin of 1-2 mm is sufficient. A larger margin, 3-5 mm, may be indicated if a malignant tumour is suspected (e.g. basal cell carcinoma) and the site is easy to take a biopsy from. A punch biopsy is a good alternative if further surgery is anticipated in any case. A small biopsy cut will heal easily, and the actual procedure may be as extensive as determined by the diagnosis.
Suspicious pigment changes are preferably removed in their entirety with a margin of 1-2 mm. An excision at the diagnostic phase that is too wide may hinder subsequent sentinel lymph node mapping.
If there is a discrepancy between the histopathological diagnosis and clinical presentation, a new biopsy is warranted or the patient should be referred to a specialist. The histopathological diagnosis does not always tell the whole story, and the ultimate responsibility for diagnosis and treatment rests with the treating doctor.