Testing and Diagnosis of Psoriatic Arthritis

Distinguishing psoriatic arthritis from other types of arthritis

Doctor reviewing x-ray with patient
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Psoriatic arthritis is classified as an inflammatory type of arthritis. It belongs to a group of conditions, known as spondyloarthropathies. Accurately diagnosing the disease is important so that appropriate treatment can be initiated.

Distinguishing Psoriatic Arthritis From Other Types

Psoriatic arthritis can mimic other types of arthritis, so distinguishing it from other types is the focus of diagnosis. It is sometimes confused with and misdiagnosed as gout, rheumatoid arthritis, or osteoarthritis

There is no single test which identifies psoriatic arthritis. Diagnosis is primarily based on your medical history, physical examination, and x-rays or other imaging studies which look for evidence of joint damage caused by inflammation. Blood tests and synovial fluid analysis may be used to rule out other types of arthritis.  

Basically, patients with psoriatic arthritis have musculoskeletal aspects of the disease and skin manifestations in the form of psoriasis. Typically, psoriatic arthritis patients are negative for rheumatoid factor. The rheumatoid factor test is used to distinguish psoriatic arthritis from rheumatoid arthritis, although 20 percent of patients with rheumatoid arthritis are also seronegative (negative for rheumatoid factor). It is considered one piece of evidence, but more must be considered.

Diagnostic Criteria

Criteria which serves as evidence of psoriatic arthritis include:

  • Inflammatory arthritis with joint, spine, or entheseal involvement.
  • Evidence of psoriasis (current, as part of patient's medical history, or as part of their family history).
  • Psoriatic nail dystrophy (onycholysis [separation of the nail from the nail bed], pitting, or hyperkeratosis [thickening] observable during physical examination).
  • Dactylitis (current swelling of a whole digit, or history of dactylitis).
  • X-ray evidence of new bone formation near or around joints of the hand or foot (i.e., the joint margins) but excluding osteophytes. (X-ray is the preferred mode of imaging used to detect changes associated with psoriatic arthritis. MRI and ultrasound can be used if more detail is needed.)
  • Negative or low-level rheumatoid factor. (While a negative rheumatoid factor is considered important evidence in distinguishing psoriatic arthritis from rheumatoid arthritis, a low level of rheumatoid factor may be found in some patients with typical features of psoriatic arthritis. Between 5 to 16 percent of psoriatic arthritis patients have a low-level rheumatoid factor.)
  • Cyclic citrullinated peptide antibodies (anti-CCP), once thought to be specific to rheumatoid arthritis, are present in about 5 percent of psoriatic arthritis patients.
  • Acute phase reactants (sedimentation rate, C-reactive protein [CRP], and serum amyloid A) may be elevated in patients with psoriatic arthritis, but to a lesser degree than occurs with rheumatoid arthritis. Usually, these markers are elevated with polyarticular disease and are indicative of a poor prognosis.
  • Hyperuricemia can occur with psoriatic arthritis in conjunction with metabolic abnormalities

According to Kelley's Textbook of Rheumatology, approximately 77 percent of patients with established psoriatic arthritis have abnormalities on x-ray. About 47 percent of patients with new or recent onset psoriatic arthritis develop erosions within two years. Distinctive features observed on x-ray are what define the type of psoriatic arthritis:

  • Asymmetric joint involvement
  • Involvement of interphalangeal joints of the fingers or toes
  • Involvement of entheseal sites
  • Spinal involvement (less severe and asymmetric compared to ankylosing spondylitis)   

Generally, the radiographic (x-ray) progression of psoriatic arthritis in its early stages occurs slowly. Aside from typical involvement of the spine and interphalangeal joints of the fingers and toes, psoriatic arthritis typically affects the large joints of the body.

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Article Sources
  • Kelley's Textbook of Rheumatology. Ninth edition. Elsevier. Chapter 77 Psoriatic Arthritis. Oliver Fitzgerald. Accessed 03/08/2016.
  • Patient information: Psoriatic arthritis (Beyond the Basics). UpToDate. Gladman and Ritchlin. Updated 04/09/2015.
  • Psoriatic Arthritis. American College of Rheumatology. Emery and Ash. Updated September 2013.