Measuring Joint Involvement in Polyarticular Psoriatic Arthritis: An Introduction of Alternatives

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.