Psoriasis is diagnosed through observation of the lesions during a physical examination. There are no associated blood tests, although x-rays (images of a body part on film paper or fluorescent screens) may be used to look for early signs of arthritis (joint inflammation). If a diagnosis is not definite, it can be confirmed with a skin biopsy.

Psoriasis can be hard to diagnose because many other conditions may be confused for psoriasis. These may include:

• Seborrheic dermatitis. A scaly form of dermatitis. This is the most common condition confused with psoriasis.
  
  
• Eczema. Skin condition with itchy and inflamed lesions, including allergic contact dermatitis and atopic dermatitis.

  

• Sézary syndrome. An itchy, shedding dermatitis.
  
  
• Pityriasis rubra pilaris. A scaly eruption of the hair follicles.
  
  
• Pityriasis rosea. Dry, scaly eruptions that disappear spontaneously and typically do not recur.
  
  
• Parapsoriasis. A generally painless, scaly dermatitis.
  
  
• Secondary syphilis. Syphilis marked by skin rash.
  
  
• Impetigo. Bacterial skin infection marked by crusty lesions.
  
  
• Superficial candidiasis. Yeast infection of the skin.