Diagnosis of Psoriasis

Doctors usually diagnose psoriasis after a careful exam of the skin. However, diagnosis may be difficult because psoriasis often looks like other skin diseases. A pathologist may assist with diagnosis by examining a small skin sample (biopsy) under a microscope.

There are several forms of psoriasis.

• Plaque psoriasis (psoriasis vulgaris), the most common form. In plaque psoriasis, lesions have a reddened base covered by silvery scales.

• Guttate psoriasis. Small, drop-like lesions appear on the trunk, limbs, and scalp. Guttate psoriasis is most often triggered by bacterial infections.

• Pustular psoriasis. Blisters of noninfectious pus appear on the skin. Medicines, infections, emotional stress, or exposure to certain chemicals can bring on attacks of this form of psoriasis. It may affect either small or large areas of the body.

• Inverse psoriasis. Large, dry, smooth, vividly red plaques occur in the folds of skin near the genitals, under the breasts, or in the armpits. Inverse psoriasis is related to increased sensitivity to friction and sweating and may be painful or itchy.

• Erythrodermic psoriasis. Itching or pain occurs along with widespread reddening and scaling of the skin. A severe sunburn, use of oral steroids (such as cortisone), or a drug-related rash can trigger this type of psoriasis.