(Wolfe, Arthritis Rheum 43:2751-2761) A fundamental paradigm of RA is that disease activity acting over time produces functional disability. Patient self-report assessment (rather than physician assessment) of disability is the standard in RA and is routinely used in clinical trials. The Health Assessment Questionnaire (HAQ) is the most common assessment tool for functional disability, and is a 20-item questionnaire in which scores range from 0 to 3 and increase in steps of 0.125. Wolfe reexamined the hypothesis that HAQ disability is a good model of disability, using data from a large, prospective, long-term study of disability in RA.
The course of HAQ disability was assessed in 32,525 observations (1,843 patients) in which the HAQ was administered. In addition, the course of HAQ disability in individual patients was studied in a subset of 2,189 visits from 50 patients followed for an average of 17 years, in order to model the effect of disease duration on the course of HAQ disability.
Group linear and nonlinear models of the effect of disease duration on HAQ disability shared 3 characteristics: 1) HAQ disability scores are high at disease onset rather than gradually increasing; 2) HAQ disability increases very slowly over time (0.03 units per year); and 3) all such models fit very poorly, explaining only 5% of the variance in HAQ disability scores. In contrast, application of nonlinear models to individual patient courses (as opposed to groups of patients) explains 37% of the HAQ disability score variation. In some patients, the course of HAQ disability was either: 1) chaotic (scores change without any pattern); 2) determinable, but unrelated to time. When covariates were added to the group model, 51% of the variance in the disability score could be explained statistically by pain, depression, erythrocyte sedimentation rate, and disease duration.
Individual RA patients have differing, characterizable courses: 1) nonlinear, 2) chaotic, or 3) non-time determined. The model that self-reported physical disability occurs as a function of disease acting over time did not fit the data well and was an inadequate model. Individual patient models may be a more effective way to examine the course of RA than are conventional group-based models.
This is an important study that calls into serious question the long-held notion that duration of disease is linearly related to disability. As the author points out, current disease activity, pain and psychological factors probably play a much more important role in determining self-reported disability than years of disease or structural damage to the joints. This information should be considered in the design and analysis of data from clinical trials which assume a linear increase in disability over time in RA.