Measuring dactylitis in clinical trials: which is the best instrument to use?

Until recently there were no validated tools to assess and measure dactylitis, but a quasi-objective measure of dactylitis (the Leeds Dactylitis Index, LDI, and a simplified version, the LDI basic) has now been developed. We undertook an open-label observational trial to test the responsiveness of the LDI and other measures previously used in clinical trials.

Twenty-eight patients with a diagnosis of psoriatic arthritis (as defined by the new ClASsification criteria for Psoriatic ARthritis, CASPAR) and active disease including new-onset dactylitis were enrolled. The patients underwent clinical assessment at baseline, 2 weeks, and 1, 3 and 6 months after change of disease modifying therapy, usually to methotrexate. Comparator dactylitis tools were taken from the literature and denoted IMPACT1 (Infliximab Multinational Psoriatic Arthritis Controlled Trial), Clegg, and Salvarani.

All 5 measures of dactylitis showed significant change from baseline and a large effect size (effect sizes first to last clinic visit: LDI 0.99, LDI basic 0.9, IMPACT1 1.63, Clegg 0.77, Salvarani 1.27). The correlation with clinical measures was strongest for the IMPACT1 score, but all the indices except Clegg had a significant positive relationship with tender joint counts, swollen joint counts, Disease Activity Score 28, and patient and physician global measures. When considering the 5 measures of dactylitis within the Outcome Measures in Rheumatology [Clinical Trials] filter, the LDI and the LDI basic showed the best overall fit for the domains of truth, discrimination, and feasibility.

With the important points in its development examined, the LDI is now ready to be used in larger randomized controlled trials both as an outcome measure and to allow further assessment of its utility.