Measurement of global functional performance in patients with rheumatoid arthritis using rheumatology function tests

Outcome assessment in patients with rheumatoid arthritis (RA) includes measurement of physical function. We derived a scale to quantify global physical function in RA, using three performance-based rheumatology function tests (RFTs). We measured grip strength, walking velocity, and shirt button speed in consecutive RA patients attending scheduled appointments at six rheumatology clinics, repeating these measurements after a median interval of 1 year. We extracted the underlying latent variable using principal component factor analysis. We used the Bayesian information criterion to assess the global physical function scale's cross-sectional fit to criterion standards. The criteria were joint tenderness, swelling, and deformity, pain, physical disability, current work status, and vital status at 6 years after study enrolment. We computed Guyatt's responsiveness statistic for improvement according to the American College of Rheumatology (ACR) definition. Baseline functional performance data were available for 777 patients, and follow-up data were available for 681. Mean +/- standard deviation for each RFT at baseline were: grip strength, 14 +/- 10 kg; walking velocity, 194 +/- 82 ft/min; and shirt button speed, 7.1 +/- 3.8 buttons/min. Grip strength and walking velocity departed significantly from normality. The three RFTs loaded strongly on a single factor that explained >or=70% of their combined variance. We rescaled the factor to vary from 0 to 100. Its mean +/- standard deviation was 41 +/- 20, with a normal distribution. The new global scale had a stronger fit than the primary RFT to most of the criterion standards. It correlated more strongly with physical disability at follow-up and was more responsive to improvement defined according to the ACR20 and ACR50 definitions. We conclude that a performance-based physical function scale extracted from three RFTs has acceptable distributional and measurement properties and is responsive to clinically meaningful change. It provides a parsimonious scale to measure global physical function in RA.