Psoriatic Arthritis Workup

Updated: Oct 25, 2019
  • Author: Anwar Al Hammadi, MD, FRCPC; Chief Editor: Herbert S Diamond, MD  more...
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Workup

Approach Considerations

No specific diagnostic tests are available for psoriatic arthritis. [4] Diagnosis of the disease is instead based on clinical and radiologic criteria in a patient with psoriasis. Radiologic features can, for example, help to distinguish psoriatic arthritis from other causes of polyarthritis, such as RA.

The most characteristic laboratory abnormalities in patients with psoriatic arthritis are elevations of the erythrocyte sedimentation rate (ESR) and C-reactive protein level. The results from these laboratory tests help to track the activity of the disease by measuring inflammation.

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Laboratory Studies

Erythrocyte sedimentation rate

An elevated ESR is found in approximately 40% of patients with psoriatic arthritis. An ESR of greater than 15 mm/h, along with medication use before the first clinical visit, evidence of radiologic damage, and absence of nail lesions, has been associated with increased mortality in patients with psoriatic arthritis.

Rheumatoid factor

Patients with psoriatic arthritis are typically seronegative for RF, although RF is detected in 5-9% of patients. RF testing is usually associated with a high false-positive rate; thus, RF-positive and RF-negative patients should undergo the same treatment.

Antinuclear antibodies

Antinuclear antibody titers in persons with psoriatic arthritis do not differ from those of age- and sex-matched controls. In 10-20% of patients with generalized skin disease, the serum uric acid concentration may be increased and, on occasion, may predispose patients to acute gouty arthritis. Low levels of circulating immune complexes have been detected in 56% of patients with psoriatic arthritis but do not appear to parallel disease activity.

Immunoglobulin

Serum IgA levels are increased in two thirds of patients with psoriatic arthritis and in one third of patients with psoriasis.

Synovial fluid

Synovial fluid is inflammatory in psoriatic arthritis, with white blood cell (WBC) counts ranging from 5000-15,000/µL and with more than 50% of cells being polymorphonuclear leukocytes. Within the synovium, the infiltrate consists predominantly of T lymphocytes. Glucose levels are within reference ranges, and synovial fluid complement levels are either within reference ranges or increased.

Psoriatic versus rheumatoid arthritis

The table below compares laboratory values in psoriatic arthritis with those in RA.

Table. Comparison of Expected Laboratory Values in Psoriatic Arthritis and Rheumatoid Arthritis (Open Table in a new window)

Laboratory Studies

Psoriatic Arthritis

Rheumatoid Arthritis

Erythrocyte sedimentation rate

Elevated (< 100)

Elevated (< 100)

Rheumatoid factor

Negative

Positive (85% of patients)

Antinuclear antibody

Negative

Positive (30% of patients)

C-reactive protein

Elevated

Elevated

Synovium

WBC count 5000-15,000/µL, >50% polymorphonuclear leukocytes

WBC count 2000/µL

Histologic findings

The histopathology of psoriatic synovitis is similar to that observed in other inflammatory arthritides, with a notable lack of intrasynovial immunoglobulin and RF production and a greater propensity for fibrous ankylosis, osseous resorption, and heterotopic bone formation.

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Imaging Studies

Radiologic features have helped to distinguish psoriatic arthritis from other causes of polyarthritis. For full discussion, see Psoriatic Arthritis Imaging. In general, the common subtypes of psoriatic arthritis, such as asymmetrical oligoarthritis and symmetrical polyarthritis, tend to result in only mild erosive disease. Early bony erosions occur at the cartilaginous edge, and cartilage initially is preserved, with maintenance of a normal joint space.

Juxta-articular osteopenia, which is a hallmark of RA, is minimal in persons with psoriatic arthritis. Asymmetrical erosive changes in the small joints of the hands and feet are typical of psoriatic arthritis and have a predilection (in decreasing order) for the DIP, proximal interphalangeal, metatarsophalangeal, and metacarpophalangeal joints. (See the images below.)

Swelling and deformity of the metacarpophalangeal Swelling and deformity of the metacarpophalangeal and distal interphalangeal joints in a patient with psoriatic arthritis.
Psoriatic arthritis involving the distal phalangea Psoriatic arthritis involving the distal phalangeal joint.
Psoriatic arthritis involving the distal phalangea Psoriatic arthritis involving the distal phalangeal joint.
Psoriatic arthritis involving the distal phalangea Psoriatic arthritis involving the distal phalangeal joint.

Erosive disease frequently occurs in patients with either DIP involvement or progressive deforming arthritis and may lead to subluxation and, less commonly, to bony ankylosis of the joint.

Erosion of the tuft of the distal phalanx, and even of the metacarpals or metatarsals, can progress to complete dissolution of the bone. Although this form of acro-osteolysis is not diagnostic, it is highly suggestive of psoriatic arthritis.

The pencil-in-cup deformity observed in the hands and feet of patients with severe joint disease usually affects the DIP joints but also may involve the proximal interphalangeal joints.

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Radiography

Traditional methods of monitoring patients with rheumatic conditions include, along with clinical assessment for joint inflammation or damage, radiographic evaluations.

Radiography shows a combination of erosion (unlike in ankylosing spondylosis) and bone growth (unlike in RA) in affected joints. [54] The following radiographic abnormalities are suggestive of psoriatic arthritis:

  • Pencil-in-cup deformity (seen in the image below)

    Arthritis mutilans (ie, "pencil-in-cup" deformitie Arthritis mutilans (ie, "pencil-in-cup" deformities).
  • Joint-space narrowing in the interphalangeal joints, possibly with ankylosis

  • Increased joint space in the interphalangeal joints as a result of destruction

  • Fluffy periostitis

  • Bilateral, asymmetrical, fusiform soft-tissue swelling

  • Unilateral or symmetrical sacroiliitis

  • Large, nonmarginal, unilateral, asymmetrical syndesmophytes (intervertebral bony bridges, seen in the image below) in the cervical, thoracic, and lumbar spine, often sparing some of the segments

    Lateral radiograph of the cervical spine shows syn Lateral radiograph of the cervical spine shows syndesmophytes at the C2-3 and C6-7 levels, with zygapophyseal joint fusion. Courtesy of Bruce M. Rothschild, MD.

A study validated the use of radiologic scoring methods—ie, the original Steinbrocker method, a modified Steinbrocker method, and the Larsen method—for the assessment of peripheral joint radiographs in patients with psoriatic arthritis. The latter 2 methods can now be used to assess the disease's progression.

CT scanning and MRI

Computed tomography (CT) scanning and MRI may be useful for detecting early signs of joint synovitis.

MRI is particularly sensitive for detecting sacroiliitic synovitis, enthesitis, and erosions; it can also be used with gadolinium to increase sensitivity. MRI may show inflammation in the small joints of the hands, involving the collateral ligaments and soft tissues around the joint capsule, a finding not found in persons with RA.

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Ultrasonography

Ultrasonography has an emerging role in the diagnosis and management of psoriatic arthritis. Its uses include predicting progression to psoriatic arthritis in patients with psoriasis by detecting subclinical signs of synovitis and enthesitis, diagnosing psoriatic arthritis, providing accurate and objective monitoring of disease activity, and predicting clinical and structural outcome. [55]

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