Diagnosis

A microscopic examination of tissue taken from the affected skin patch is needed 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. Specific changes in the nails are often strong signs of psoriasis.

Ruling Out Other Conditions

Several conditions produce symptoms that resemble those of psoriasis. For example:

• 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.

Symptoms of psoriatic arthritis may also 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.

Disease Severity

Severity of psoriasis itself ranges from one or two flaky inflamed patches to widespread pustular psoriasis that, in rare cases, can be life threatening. To help determine the best treatment for a patient, doctors usually classify the disease as mild to severe. The classification depends on how much of the skin is affected:

• Mild psoriasis affects less than 3% of the body surface. Most cases of psoriasis are limited to less than 2% of the skin.
• Moderate psoriasis covers 3 - 10% of the skin.
• If more than 10% of the body is affected, the disease is considered severe.

The palm of the hand equals 1% of the body. The severity of the disease is also measured by its effect on a person’s quality of life. However, the National Psoriasis Foundation has proposed a new classification method. The group suggests a new two-tiered system that classifies patients as needing either local or body-wide (systemic) treatment.

While disease severity impacts treatment success, some forms of psoriasis can be very resistant to treatment even though they are not categorized as severe. They include:

• Any psoriasis on the palms and soles (hand and foot psoriasis)
• Inverse psoriasis (which occurs in the folds of the skin)
• Scalp psoriasis
• Psoriatic arthritis