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.
The severity of psoriasis 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:
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.
The National Psoriasis Foundation has proposed a new classification method. The group suggests a two-tiered system that classifies patients as needing either local or body-wide (systemic) treatment.
Although 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:
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