Psoriasis - Diagnosis

Diagnosis

A microscopic examination of tissue taken from the affected skin patch is required to make a definitive diagnosis of psoriasis and to distinguish it from other skin disorders. Usually in psoriasis, the examination will show a large number of dry skin cells, but without many signs of inflammation or infection. Changes in the nails typical of psoriasis are often strong indicators of psoriasis.

Ruling Out Other Conditions

Distinguishing Psoriasis Patches from Other Disorders. A number of conditions produce symptoms that resemble those of psoriasis. Examples include:

Seborrheic psoriasis is hard to distinguish from seborrheic dermatitis (dandruff is one form of this condition). Seborrheic dermatitis patches are usually greasy, yellowish, and crusty. Nail involvement may also help differentiate psoriasis.
Generalized erythrodermic psoriasis may be confused with drug allergic reactions, atopic eczema, and symptoms of lymphomas.
Fungal infections, other skin conditions, or circulation problems may also cause nail changes typical of psoriasis.

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Distinguishing Psoriatic Arthritis from Other Conditions. Psoriatic arthritis may resemble the following:

Rheumatoid arthritis (RA). As in rheumatoid arthritis, psoriatic arthritis can cause pain or tenderness in one or more joints and morning stiffness is common. People with psoriatic arthritis, however, lack a particular antibody, called rheumatoid factor, which is found in the blood of many people with rheumatoid arthritis.
Systemic lupus erythematosus (SLE). Symptoms of SLE may include both a psoriasis-like rash and arthritis, which could make the diagnosis difficult.
Reiter's disease. Reiter's disease is a syndrome that includes arthritis and inflammation in the eyes and urinary tract. It also causes skin lesions that are very similar to psoriasis, which are usually raised patches on the lips, penis, palms, and soles.
Gout. Gout causes pain, often in the fingers and toes.

Some evidence now indicates that inflammation in psoriatic arthritis may be distinguished from other arthritic conditions by its occurrence in sites where muscle tissue inserts into the bone (called enthesitis) rather than in the joint, which is a common site in other inflammatory arthritic conditions.