Comparing the Patient-Reported Physical Function Outcome Measures in a Real-Life International Cohort of Patients With Psoriatic Arthritis

Objective: We evaluated the psychometric properties of 3 patient-reported outcome measures to assess the physical function in psoriatic arthritis (PsA). Methods: Data were available for the Health Assessment Questionnaire disability index (HAQ DI), the 12-item Short Form instrument physical component summary (SF-12 PCS), and the Psoriatic Arthritis Impact of Disease instrument functional capacity score (PsAID-FC). Data came from a longitudinal study in 14 countries of consecutive adults with definite PsA with ≥2 years of duration. The score distribution, construct validity, responsiveness, and thresholds of meaning of the patient-reported outcome measures were evaluated. Results: At baseline, 414 subjects (52% male) were analyzed. The mean ± SD age was 52.4 ± 12.5 years and duration of illness was 10.9 ± 8.1 years. Ceiling effects were noted in 31% and 21% of patients for HAQ DI and PsAID-FC, respectively; floor effects were minimal. All 3 patient-reported outcome measures met a priori hypotheses for construct validity. After a median follow-up of 4.1 (interquartile range 2.7) months in 350 patients, 27%, 54%, and 18% of patients reported themselves improved, not changed, and worsened, respectively. Change scores were statistically different for groups for worsening versus no-change for all patient-reported outcome measures. PsAID-FC was more sensitive to change than the other 2 patient-reported outcome measures. Comparing groups with worsening condition to no-change, the standardized response mean square ratios were HAQ DI 29.9, SF-12 PCS 16.7, and PsAID-FC 40.1. Conclusion: HAQ DI, SF-12 PCS, and PsAID-FC are valid measures of physical function for PsA. PsAID-FC, a single question, performed similarly to the other patient-reported outcome measures and may be an additional option to measure PsA-specific physical function.