Abstract
For rheumatic diseases, Minimal Disease Activity (MDA) is usually defined as a composite outcome which is a function of several individual outcomes describing symptoms or quality of life. There is ever increasing interest in MDA but relatively little has been done to characterise the pattern of MDA over time. Motivated by the aim of improving the modelling of MDA in psoriatic arthritis, the use of a two-state model to estimate characteristics of the MDA process is illustrated when there is particular interest in prolonged periods of MDA. Because not all outcomes necessary to define MDA are measured at all clinic visits, a partially hidden multi-state model with latent states is used. The defining outcomes are modelled as conditionally independent given these latent states, enabling information from all visits, even those with missing data on some variables, to be used. Data from the Toronto Psoriatic Arthritis Clinic are analysed to demonstrate improvements in accuracy and precision from the inclusion of data from visits with incomplete information on MDA. An additional benefit of this model is that it can be extended to incorporate explanatory variables, which allows process characteristics to be compared between groups. In the example, the effect of explanatory variables, modelled through the use of relative risks, is also summarised in a potentially more clinically meaningful manner by comparing times in states, and probabilities of visiting states, between patient groups.
Highlights
For studies in rheumatic diseases, and in other medical contexts, the outcome variable of interest is often composite
For psoriatic arthritis (PsA), an inflammatory arthritis associated with the skin disease psoriasis, this has been operationally defined in terms of 7 criteria related to physician, patient and laboratory measures of disease activity, that is, disease symptoms that are potentially reversible through treatment or other factors
Short term minimal disease activity (MDA) is of little clinical interest as it is MDA of extended duration, typically one year or more, that has been linked with reducing the risk of permanent joint damage, a major aspect of disease progression in PsA (Coates et al 2010a)
Summary
For studies in rheumatic diseases, and in other medical contexts, the outcome variable of interest is often composite. The concept of minimal disease activity (MDA) in rheumatic disease was conceptually defined as “that state of disease activity deemed a useful target of treatment by both the patient and physician, given current treatment possibilities and limitations” by The Outcome Measures in Rheumatology Clinical Trials 6 Conference (Wells et al 2005). Coates et al (2010a) divided a longitudinal cohort of patients into two groups, those who achieved the criteria for MDA at consecutive visits for a minimum of 12 months and those who did not over their periods of followup These two groups were compared in various ways in terms of explanatory variables. Age and disease duration prior to clinic entry were included as continuous variables
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