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      Drugs & Diseases > Rheumatology

      Psoriatic Arthritis Workup

      Updated: Jan 24, 2022
      • 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 rheumatoid arthritis (RA).

      The most characteristic laboratory abnormalities in patients with psoriatic arthritis are elevations of the erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) 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 rheumatoid factor (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 receive 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 polymorphonuclear leukocytes comprising more than 50% of cells. 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 may help 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 distal interphalangeal (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.
      View Media Gallery
      Psoriatic arthritis involving the distal phalangea Psoriatic arthritis involving the distal phalangeal joint.
      View Media Gallery
      Psoriatic arthritis involving the distal phalangea Psoriatic arthritis involving the distal phalangeal joint.
      View Media Gallery
      Psoriatic arthritis involving the distal phalangea Psoriatic arthritis involving the distal phalangeal joint.
      View Media Gallery

      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

      Radiographic evaluations, along with clinical assessment for joint inflammation or damage, are a traditional method for monitoring patients with rheumatic conditions. Radiography shows a combination of erosion (unlike in ankylosing spondylosis) and bone growth (unlike in RA) in affected joints. [63] 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).
        View Media Gallery
      • 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.
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      A study by Tillett et al of four radiographic scoring methods for psoriatic arthritis reached the following conclusions [64] :

      • Modified Sharp score (MSS) and modified Sharp/van der Heijde score (SHS) - The most reliable and sensitive to change, but take longer to perform
      • Modified Steinbrocker score - The most feasible but lacks the sensitivity of the SHS
      • Ratingen score - Smallest detectable change (SDC) close to that of the SHS and MSS, but this score is quicker to perform

      Salaffi et al have reported preliminary validation of a novel radiographic scoring system for psoriatic arthritis, the Simplified Psoriatic Arthritis Radiographic Score (SPARS). SPARS correlated strongly with the SHS and Ratingen scores, and proved quicker to calculate (4.5 min, versus 14.4 min for SHS and 10.1 min for Ratingen). [65]  

      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, which is not seen in RA.

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      Ultrasonography

      Ultrasonography has an emerging role in the diagnosis and management of psoriatic arthritis. Its uses include the following [66] :

      • Predicting progression to psoriatic arthritis in patients with psoriasis by detecting subclinical signs of synovitis and enthesitis [67]
      • Diagnosing psoriatic arthritis
      • Providing accurate and objective monitoring of disease activity
      • Predicting clinical and structural outcome
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