Disability in patients with early inflammatory polyarthritis cannot be 'tracked' from year to year: An examination of the hypothesis underlying percentile reference charts

Objective. To determine whether disability in patients with early inflammatory polyarthritis (IP) can be charted or 'tracked' over time. The disability score was also adjusted in an attempt to exclude the influence of current disease activity, with the aim of ascertaining that component of disability relating to other factors such as psychosocial factors and joint damage. Methods. Four hundred thirty-three patients with early IP referred to the Norfolk Arthritis Register (NOAR) were followed annually using the Health Assessment Questionnaire (HAQ) for 5 years. HAQ scores at each year were divided into quartiles. The number of patients remaining in the same quartile from year to year was examined. The relationship between disease activity, assessed by swollen and tender joint counts, and HAQ was modelled, with the residual HAQ attributed to psychosocial factors and joint damage. Results. From year to year, there was considerable within-individual variation in quartile, with only 48-65% of patients remaining in the same quartile. With increasing time, a greater proportion of patients remained in the same quartile. A statistically valid activity-adjusted HAQ score could not be computed. Conclusion. Disability in patients with early IP cannot be easily tracked over time. It is therefore not appropriate to construct longitudinal reference charts for disability in the early years, although it may be feasible for more established disease.