Use of multidimensional composite scores in rheumatology: parsimony versus subtlety.

Rheumatic and musculoskeletal diseases (RMDs) form a diverse group of diseases. Proper disease assessment is pivotal, for instance to make treatment choices and for optimising outcome in general. RMDs are multidimensional diseases, entrenching many, sometimes very different aspects. Composite outcome measures ('composites') have become very popular to assess RMDs, because of their claim to catch all relevant dimensions of the disease into one convenient measure.In this article we discuss dimensionality of RMDs in the context of the most popular conceptual framework of RMDs, being an inflammatory process leading to some sort of damage over time. We will argue that multidimensionality not only refers to heterogeneity in disease manifestations, but also to heterogeneity in possible outcomes. Unlike most unidimensional measures, multidimensional composites may include several disease manifestations as well as several outcome dimensions into one index. We will discuss fundamental problems of multidimensional composites in light of modern strategies such as treat-to-target and personalised medicine.Finally, we will disentangle the use of multidimensional composites in clinical trials versus their use in clinical practice, and propose simple solutions in order to overcome problems of multidimensionality and to avoid harm to our patients due to overtreatment.