Measurement of Global Functional Performance in Patients with
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 ≥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.

Measurement of physical functional limitations in patients with rheumatoid arthritis (RA) is a time-honored strategy to assess the disease's outcome. Performance-based tests of physical function such as grip strength and walking velocity were included in some of the earliest trials of antirheumatic therapy. These tests provide reproducible, quantitative information about a patient's current status and about the prognosis. In a paper describing the behavior of functional tests over time in RA, Pincus and Callahan made the analogy between them and commonly used laboratory tests of other organs, referring to performance- and questionnaire-based measures as 'rheumatology function tests' (RFTs).

It is useful to consider RFTs within an overarching conceptual framework of the disease's outcome. We have proposed a disablement framework for studying the development of disability, and possibly other outcomes, in RA. The framework consists of a main disease–disability pathway, which describes the sequential development of pathology, impairment, functional limitation, and, finally, disability. Within this framework, performance-based functional tests are well suited to quantify functional limitations, because they entail measurement of physical actions performed by the intact person. A number of different tests are available, and researchers often include more than one in studies. However, the clinical literature is sparse in guiding how to analyze or report research findings when multiple tests are used. The need for data parsimony may sway investigators to report findings on less than the full set of tests available. We are concerned that if researchers choose this route, important information may be lost.

In an earlier analysis, we used principal component factor analysis to extract the underlying latent variable from three primary disability scales. The distributional and measurement characteristics of the latent disability scale were better than those of the primary scales. In the present analysis, we used a similar approach to extract a global physical performance scale from three primary performance-based RFTs: grip strength; walking velocity over 50 feet; and the timed shirt button test. The resulting latent functional performance scale reflects overall physical function in RA. This data reduction approach may assist investigators who wish to quantify functional limitations in RA.