Sensitivity and specificity of the classification of psoriatic arthritis criteria in early psoriatic arthritis

Objective: To assess the sensitivity and specificity of the Classification of Psoriatic Arthritis (CASPAR) Study Group criteria in early psoriatic arthritis (PsA) and to compare them with the sensitivity and specificity of the Moll and Wright criteria.

Methods: The CASPAR Study Group criteria were applied to patients with early PsA (<24 months symptom duration) and to control patients with other new-onset inflammatory arthritides. Both groups were naive to all disease-modifying antirheumatic drugs. The gold standard diagnosis was confirmed by the consulting rheumatologist using radiography and magnetic resonance imaging where required. Proportions of patients and control patients meeting the criteria were compared using McNemar's tests.

Results: We recruited a total of 111 patients with early PsA and 111 control patients with other forms of inflammatory arthritis (82 with rheumatoid arthritis, 13 with undifferentiated arthritis, 9 with spondylarthritis, 4 with inflammatory osteoarthritis, and 3 with crystal arthritis) to the study. The sensitivity of the CASPAR Study Group criteria in classifying early PsA was 87.4% compared to 80.2% for the Moll and Wright criteria. The specificity for both criteria was 99.1%. When considering different cut points for the CASPAR Study Group criteria, the best cut point for classification remained a score of ≥ 3 as in the original CASPAR Study Group analysis. Considering a score of ≥ 2 gave a higher sensitivity of 99.1% but resulted in a drop in specificity to 94.6%. Regression analysis determined that psoriasis and rheumatoid factor negativity were the most important features that differentiated PsA, followed by nail psoriasis and current or previous dactylitis.

Conclusion: The CASPAR Study Group criteria are more sensitive than the Moll and Wright criteria in classifying early PsA. Although their sensitivity for early PsA is lower than that for established disease, the CASPAR Study Group criteria are valid for use as inclusion criteria for trials in early PsA.