Abstract
In psoriatic arthritis, many patients do not develop permanent joint damage even after a prolonged follow‐up. This has led several authors to consider the possibility of a subpopulation of stayers (those who do not have the propensity to experience the event of interest), as opposed to assuming the entire population consist of movers (those who have the propensity to experience the event of interest). In addition, it is recognised that the damaged joints process may act very differently across different joint areas, particularly the hands, feet and large joints. From a clinical perspective, interest lies in identifying possible relationships between the damaged joints processes in these joint areas for the movers and estimating the proportion of stayers in these joint areas, if they exist. For this purpose, this paper proposes a novel trivariate mover‐stayer model consisting of mover‐stayer truncated negative binomial margins, and patient‐level dynamic covariates and random effects in the models for the movers and stayers, respectively. The model is then extended to have a two‐level mover‐stayer structure for its margins so that the nature of the stayer property can be investigated. A particularly attractive feature of the proposed models is that only an optimisation routine is required in their model fitting procedures. © 2016 The Authors. Statistics in Medicine Published by John Wiley & Sons Ltd.
Highlights
Psoriatic arthritis (PsA) is an inflammatory arthritis associated with the skin condition psoriasis
Motivated by the aforementioned clinical considerations, we develop a novel trivariate model with mover-stayer truncated negative binomial margins, as a flexible alternative to zero-inflated Poisson (ZIP) models, and incorporate patient-level dynamic covariates and random effects in the models for the movers and stayers, respectively
This research was clinically motivated by the need to understand the relationship between damage progression in the hands, feet and large joints, under the assumption that the stayer property is inherent, and relaxing this assumption by allowing for the possibility of a clinic-induced stayer population
Summary
Psoriatic arthritis (PsA) is an inflammatory arthritis associated with the skin condition psoriasis. It would be of interest to investigate the nature of the stayer property, to distinguish between patients who are inherent stayers (true stayers) or attain the stayer property through management/treatment strategies employed by the clinic (clinic-induced stayers) These clinical considerations motivate the need for the development of new methodology that allows the relationship between three damaged joints counting processes to be investigated whilst simultaneously allowing for the possibility that there could be two stayer populations associated with each process. The dynamic covariates allow asymmetric relationships to be identified, whilst the random effects provide information across processes to estimate the stayer proportions We extend this model to have a two-level mover-stayer structure for its margins so that inherent and clinic-induced stayers can be investigated for each marginal process. The section introduces the PsA data on which this analysis is undertaken
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