Can laboratory tests help in diagnosing Psoriatic arthritis?
PsA is classified as a “seronegative” arthritis, meaning that the rheumatoid factor is typically negative. However, low-titer rheumatoid factor can be detected in 2% to 10% and anticitrullinated peptide antibodies in 8% to 16% of PsA patients. This can make it difficult to separate from coexistent rheumatoid arthritis. However, the presence of DIP involvement, enthesitis, and dactylitis supports a diagnosis of PsA regardless of serologies. Antinuclear antibodies are reported in 10% to 15%. As in other inflammatory diseases, erythrocyte sedimentation rates (ESR), C-reactive protein (CRP), and anemia may vary with disease activity. Patients with an elevated ESR and CRP are more likely to have polyarticular disease and a worse prognosis. It is important to note that only 40% of patients with PsA have elevated ESR and/or CRP. Hyperuricemia is seen in 20% and is not due to the extent of skin involvement, but related to the increased incidence of the metabolic syndrome seen in patients with psoriatic disease (this is important to keep in mind when distinguishing between PsA and gout; hence, synovial fluid analysis is paramount). Analysis of synovial fluid typically reveals inflammatory fluid with a neutrophilic predominance. HLA-B27 is positive in 25%.