Objective: To compare the reliability of 3 different simplified joint counts with the gold standard 66 swollen/68 tender joint count (JC66/68) for assessing clinical response in patients with polyarticular psoriatic arthritis (PsA).
Methods: The 28-joint count (JC28), in the same way that it is used in rheumatoid arthritis, and 2 measures including distal interphalangeal (DIP) joints (the 32-joint count [JC32], including all finger joints as well as wrists and knees, and 36-joint count [JC36], which additionally included elbows and ankles), were compared with the JC66/68 in 182 patients using data from the Infliximab Multinational Psoriatic Arthritis Controlled Trial 2 trial database. Pearson's correlation coefficients were calculated to compare the swollen and tender JC28, JC32, and JC36 with the corresponding results of the total JC66/68. American College of Rheumatology (ACR) responses based on the individual measures were compared, and their ability in predicting a clinical response of ACR 20% improvement (ACR20) based on the JC66/68 was assessed by calculating the area under the receiver operating characteristic curve via logistic regression and the maximum Youden indices at weeks 14 and 24.
Results: All simplified joint counts were highly correlated to the standard JC66/68 both for tenderness and swelling at each individual visit (Pearson's correlation coefficients consistently >0.8, n = 182-200; P < 0.0001). Logistic regression for ACR20 response showed that area under the curve was constantly >0.91, with comparable results for Youden indices of the simplified joint counts.
Conclusion: All simplified joint counts considered seemed sufficiently sensitive and specific to measure clinical response in trial patients with polyarticular PsA when compared with the JC66/68, no matter whether DIP joints were included (the JC36 and JC32) or excluded (the JC28). Further research will be needed to clarify this issue.
Trial registration: ClinicalTrials.gov NCT00051623.