Do You Need a Skin Biopsy to Diagnose Psoriasis?

Your doctor can usually tell if you have psoriasis without a biopsy -- a procedure that collects skin cells and checks them under a microscope. But there are a few rare cases where you might need to get one.

Psoriasis causes your body to make new skin cells too quickly. These cells tend to pile up and form spots, bumps, and thick scaly patches called plaques. Your doctor generally makes a diagnosis by examining your skin.

A biopsy does not typically show any helpful new information about psoriasis. But when the spots -- "papules" of psoriasis -- have not yet joined together to form a plaque, it may be hard for your doctor to tell whether you have psoriasis or another skin condition.

There are a number of skin conditions, like eczema, lichen planus, and cutaneous lupus, that cause lesions that sometimes look like psoriasis. By checking skin cells under a microscope, your doctor might be able rule out some of these other conditions.

What to Expect Before a Biopsy

A biopsy is a relatively minor procedure and shouldn't take more than 15 minutes, including any preparation. Your doctor, usually a skin specialist known as a dermatologist, will remove some of your skin cells.

The procedure can cause some minor bleeding, so it's a good idea to tell your doctor if you:

• Have problems with extra bleeding
• Take blood-thinning medication like warfarin or heparin
• Have lots of skin infections
• Use medicine that curbs your immune system -- your body's defense against germs

If the skin that your doctor wants to check is in a hard-to-reach area, you may have to undress and change into a hospital gown. Your doctor will clean the area and may mark the biopsy area with a special pen.

Then, your doctor will give you a shot of medicine that numbs the area so that you don't feel anything when they remove a sample of skin. The area may burn for a few seconds before it gets numb.

What to Expect During a Biopsy

For a "punch biopsy," the most common type when checking for psoriasis, the doctor will take an instrument that is about the size and shape of a pencil. Once you are numbed up, the doctor applies the instrument to your skin, and as it turns, it removes a small section down through deeper layers, including the epidermis, dermis, and superficial fat.

In some cases, instead of a punch biopsy, your doctor may shave just the top layer of skin (shave biopsy) or cut a section of skin away with a knife (excisional biopsy).