The diagnosis of psoriasis is based on a physical examination of the skin, scalp, and nails. Skin biopsy and blood testing are rarely necessary (UMMC 2014a; Usatine 2013).
Other skin diseases with a similar appearance are ruled out by differential diagnosis. These include eczema, dermatitis (contact, atopic, seborrheic, nummular), tinea, lichen planus, lichen simplex, pityriasis rosea, cutaneous lupus erythematosus, syphilis, drug reactions, squamous cell carcinoma in situ, and mycosis fungoides (cutaneous T-cell lymphoma) (Armstrong 2014; Usatine 2013; Schalock 2014). If psoriatic arthritis is suspected, rheumatoid arthritis and gout are screened for using laboratory tests and x-rays (Bosmansky 1983; Sankowski 2013; Busse 2010).