Differentiation of psoriatic arthritis from rheumatoid arthritis and gout can be facilitated by the absence of the typical laboratory findings of those conditions. Overlap with other arthritic syndromes is possible, however.
The diagnosis of psoriasis is clinical, and histopathologic examination is rarely necessary. However, it may be useful in certain cases in differentiating the condition from other dermatoses. Histopathologic findings include the following:
Regular acanthosis of the epidermis
Kogoj spongiotic pustules
Laboratory studies that should be done and findings in patients with psoriasis may include the following:
The erythrocyte sedimentation rate is usually normal (except in pustular and erythrodermic psoriasis, in which it may be elevated).
Test result for rheumatoid factor is negative.
The uric acid level may be elevated in psoriasis (especially in pustular psoriasis), causing confusion with gout in psoriatic arthritis.
Fluid from pustules is sterile, with neutrophilic infiltrate.
Perform fungal studies. This is especially important in cases of hand and foot psoriasis that seem to be worsening with the use of topical steroids.
If starting systemic therapies such as immunologic inhibitors, obtain baseline laboratory studies.
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Cite this: William James. Fast Five Quiz: Psoriasis - Medscape - Nov 08, 2021.