Biopsy of skin lesions and rashes
What is a skin biopsy?
A skin biopsy is the removal and histopathological examination of a sample of skin to identify the presence, cause, or extent of a disease or condition.
Why have a skin biopsy?
One or more skin biopsies are performed to make or confirm a diagnosis, which often helps determine the correct treatment.
- There may be several distinct skin conditions with similar clinical appearance but different time course and management
- Complex treatment may be under consideration
- It may be unclear if a skin lesion is benign, and can be left alone, or if it is skin cancer, when it should be completely removed
- The clinician may be attempting to find the edge of a tumour
Choosing the site for a biopsy
It is crucial that the site of a biopsy is chosen carefully, or the pathological diagnosis could be incorrect or misleading.
- Scratched lesions will show nonspecific inflammation and wound healing
- Ulcerated areas are often very inflamed and secondarily infected, whatever the original cause of the ulcer
- A fresh lesion is often the most suitable for evaluation of an inflammatory skin condition, particularly if vasculitis is a consideration
- The edge of an enlarging lesion is often the best site to detect basal cell or squamous cell skin cancer
- A small intact blister may show more useful information than the corner of a large one
- Diagnostic clues are found in the skin adjacent to a scarred or ulcerated area
Completing the request form
The clinician should ensure the pathology request form includes patient information (including age and identification details), the site and type of biopsy, time and date, clinical information and a range of possible diagnoses.
The sample pot should be labelled with patient identification details, the body site of the biopsy, time and date.
What happens to the biopsy sample?
Most skin biopsies are placed in formalin in a small pot and are sent to the lab for paraffin fixation, processing and histopathology.
- If considering deep fungal infection or mycobacteria, the sample may be divided so that one part of the sample is sent in formalin for histopathology and the other is placed on a saline-soaked gauze swab for microbiology.
- Samples for direct immune fluorescence are placed in transport media, snap frozen in liquid nitrogen, or sent “fresh” (eg placed on a moistened gauze swab).
Types of skin biopsy
Skin biopsy is usually undertaken using a local anaesthetic injection into the surrounding skin to numb the area. The injection stings transiently. After the procedure, a dressing will usually be applied to the site of the biopsy. This should be left in place for the first 24 hours and replaced if necessary.
A punch biopsy is quick to perform, convenient, and only produces a small wound. The pathologist can evaluate the full thickness of skin.
The disposable skin biopsy punch has a round stainless steel blade ranging from 2–6 mm in diameter; 3 and 4 mm are the most common sizes used for inflammatory skin disease. The clinician holds the instrument perpendicular to the skin and rotates it to pierce the skin and removes a cylindrical core of epidermis, dermis and sometimes, subcutaneous tissue.
A suture is often used to close the wound, or, if the wound is small, it may heal adequately without it.
A tangential shave of skin is taken using a scalpel, special shave-biopsy instrument or razor blade. No stitches are required. The wound forms a scab that should heal in 1–3 weeks.
A scoop biopsy is a deep form of shave biopsy, used to remove a skin lesion such as a benign mole by "scooping" it out. It is also called “saucerisation” or “tangiential excision.”
A skin curette may be used to scrape off a superficial skin lesion, such as basal cell carcinoma or seborrhoeic keratosis. Some of the curettings are sent for histopathology. These samples are not suitable for determining if a lesion has been completely removed.
Incisional biopsies refer to removal of a larger ellipse of skin for diagnosis, using a scalpel blade. Stitches are usually required after an incisional biopsy.
Excision biopsy refers to complete removal of a skin lesion, such as a skin cancer. A margin of surrounding skin is taken, as a precaution. Smaller lesions are most often removed using a scalpel blade as an ellipse, with primary closure using sutures. Larger excisions may be repaired using a skin flap (moving adjacent skin to cover the wound) or graft (skin taken from another site to patch the wound).
Complications of skin biopsy
Skin biopsy is usually straightforward and complications are uncommon.
Intraoperative or postoperative bleeding can occur in anyone, but can be particularly troublesome in those with a bleeding tendency, or taking blood-thinning medications such as warfarin or aspirin.
Bacterial wound infection affects about 1–5% of surgeries. It is more likely in ulcerated or crusted skin lesions. The risk of infection is greater than usual in diabetics, elderly patients, and in people taking immunosuppressive medicines.
Delayed healing is most likely in biopsies taken from the lower legs, especially if the leg is swollen, the arterial or venous circulation is impaired, or there is a skin condition that predisposes to ulceration after skin injury (eg pyoderma gangrenosum).
The blade may cut a superficial nerve causing pain or numbness. This is most likely to occur where the skin is thin, for example on the face or back of hand.
It is usual for a biopsy site to form a permanent scar. Some people form excessive or hypertrophic scars, particularly in certain body sites such as the centre of the chest.
Persistence or recurrence of the skin lesion
Many biopsies are deliberately partial, so that the underlying skin condition remains. In other cases, complete removal is intended but not achieved; in time, the lesion may recur at the same site.
Obtaining the results of the biopsy
It usually takes about one week to obtain the result from the pathology laboratory, but can sometimes take longer if special stains are required. The pathologist describes what is observed under light microscopy in several sections of the biopsy sample, and provides the likely diagnosis. Sometimes it is not possible to make an exact diagnosis on the biopsy sample provided.
Skin diseases and conditions can at times be very difficult to diagnose accurately. The various specialists involved may need to consult with each other. This is called clinicopathological correlation (CPC). Larger organisations hold regular multidisciplinary meetings (MDMs) at which clinical information, clinical photographs, and pathology slides are reviewed by a team of experts to determine the best diagnosis and treatment for the patient.