Your doctor can often recognize by the appearance and location of the patches on your skin, scalp, or nails. Psoriasis usually involves bright red areas of raised patches that are often covered with loose, silvery, scaling skin and are commonly located on the knees, elbows, scalp, hands, feet, or lower back.
Special tests are usually not needed. If it is hard to diagnose the condition by looking at your skin, your doctor may remove a small skin sample (biopsy) and send it to a lab for analysis. If you have joint pain, X-rays may be taken to diagnose psoriatic arthritis. Blood work may help rule out other forms of arthritis.
Guttate psoriasis is a less common type of psoriasis. The small [less than 0.4 in. (1 cm) in diameter], scaly, circular elevations (papules) that occur with guttate psoriasis appear more on the trunk of the body than on the arms or legs. If you have these papules, your doctor may perform a throat culture to check for strep throat.
Sometimes a skin is done to rule out a fungal infection.