Background Disease activity influences functional disability [1,2]. This influence constitutes a direct effect (through signs and symptoms) and an indirect effect through the effect on joint damage over time. It has been found that the direct relation between disease activity and functional disability weakens with longer disease duration , but it has been questioned whether this still is the case with current intensive treatment strategies. The indirect effect has not been studied.
Objectives To explore the relationship between disease activity and functional disability over time considering indirect and direct associations and the influence of treatment strategy.
Methods Data from 3 consecutive randomized clinical trials studying increasing intensive (tight control) treatment strategies were used. Functional capacity (HAQ) and disease activity (DAS28) were measured every 12 months. Linear mixed modeling was used to study the relation of DAS28 (current and lagged DAS28), other variables on HAQ. Interaction terms were introduced to study the influence of lagged DAS28 on HAQ was constant over time and differed with treatment strategy.
Results Current DAS28 was positively associated with HAQ over time. Also, independently lagged DAS28 was positively associated with HAQ. When specifically looking at the longitudinal relation, the association of a change in DAS28 with a change in HAQ decreased over disease duration, whereas the influence of a change in the lagged DAS28 with a change in HAQ increased with duration of the disease. HAQ progression was less in the more intensively treated patients, but did not modify the relation between (lagged) DAS28 and HAQ.
Conclusions Although the influence of current DAS28 on HAQ decreases with disease duration, the influence of preceding disease activity on progression of functional disability actually increases. This further underpins the importance of disease activity over time as an important treatment target to prevent long term functional decline in RA patients over the complete course of the disease. These relationships should also be taken into account in health economic modeling studies simulating the progression of RA.
van der Kooi E, et.al. Ann Rheum Dis 2010;70:168-71.
Welsing PM, et al. Arthritis Rheum 2001;44:2009-17.
Disclosure of Interest None Declared
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