Laboratory Tests and Ancillaries
Serology
No specific serologic tests are done to confirm the diagnosis of psoriatic arthritis. The absence of rheumatoid factor aids in the diagnosis, but some patients with psoriatic arthritis can still have a positive rheumatoid factor.
Imaging
Plain Radiography
Psoriatic Arthritis_Diagnostics 1
Plain radiography helps in the diagnosis of psoriatic arthritis. This demonstrates the extent and location of joint damage and distinguishes psoriatic arthritis from other types of arthritis. The X-ray features of psoriatic arthritis are as follows: Joint erosions that usually start at the margins of the joints and then progress toward the center; joint space narrowing; bony proliferation, including periarticular and shaft periostitis; osteolysis, which includes “pencil-in-cup” deformity, acro-osteolysis, ankylosis, spur formation, and spondylitis; distal interphalangeal erosive changes; and dactylitis affecting the feet in an asymmetric distribution. Changes occur early in the course of psoriatic arthritis. The spectrum of joint inflammation is great, ranging from axial to peripheral disease, synovial and adjacent soft tissue inflammation, enthesitis, osteitis, new bone formation, and severe osteolysis, along with overlapping findings. This can be used to monitor the efficacy of treatment.
Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) Imaging
Magnetic resonance imaging and computed tomography imaging may be done for detecting patients with asymptomatic disease. These help in the diagnosis of early sacroiliitis with no radiographic changes in plain X-ray. These can detect erosions earlier in the course of disease and are more sensitive than plain radiography in detecting articular, periarticular, and soft-tissue inflammation.
Ultrasonography
Psoriatic Arthritis_Diagnostics 2
Ultrasonography is an alternative method to estimate the degree of joint inflammation and volume of inflamed tissue. This is more sensitive in detecting knee synovitis and lower extremity enthesopathy. This may also be used during ultrasound-guided aspiration or injection.