Lichen planus usually appears slightly more purple than psoriasis and typically involves the mouth, flexural surfaces of the wrists, genitals, and ankles.
Other conditions in the differential include pityriasis lichenoides chronica, which may be identified on skin biopsy. Inverse psoriasis can be difficult to differentiate from candida intertrigo, erythrasma, or tinea cruris.
A potassium hydroxide (KOH) preparation can help differentiate psoriasis from candida or tinea. In psoriasis, a KOH test will be negative for fungal elements. Mycology culture on skin scrapings may be performed to rule out fungal infection. Erythrasma may exhibit a coral red appearance under Wood lamp examination.
If a lesion fails to respond to appropriate treatment, a careful drug history and biopsy can help clarify the diagnosis.
It’s important to thoroughly document the extent and severity of the psoriasis and to monitor the impact of treatment. The Psoriasis Area and Severity Index is a commonly used method that calculates a score based on the area (extent) of involvement surrounding 4 major anatomical regions (head, upper extremities, trunk, and lower extremities), as well as the degree of erythema, induration, and scaling of lesions. The average redness, thickness, and scaling are graded on a scale of 0 to 4 and the extent of involvement is calculated to form a total numerical score ranging from 0 (no disease) to 72 (maximal disease).
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