Composite outcomes at OMERACT: Multi-outcome domains and composite outcome domains.

The OMERACT Technical Advisory Group recognises that working groups during the process of creating a core outcome set may identify an outcome domain that would be best represented as a composite that encapsulates these component outcome domains by bringing them together into a single outcome. A multi-outcome domain (MOD) is a within-patient combination of component outcomes, and an individual patient's evaluation depends on the observation of all of the components in that patient with a single overall rating determined according to a specified rule; which is often applicable when we consider a disease activity score. A composite outcome domain (COD) consists of a number of component outcomes and is defined as the occurrence in a patient of one, some or all of these specified components; which is often applicable when we consider the risk of adverse events or remission criteria. We review the general benefits, challenges, reporting and interpretation of using MODs and CODs. The development of the MOD or COD instrument for an OMERACT core outcome measurement set is considered through four distinct steps: choosing relevant outcome domains; finding high quality instruments for each of these outcome domains; weighting the outcome domain instruments in the MOD/COD instrument; and putting MOD/COD instrument through the OMERACT Filter. Guidance and training are in preparation for working groups who will be completing the OMERACT Instrument Selection Algorithm (OFISA). As for other initiatives in OMERACT, we will seek feedback from OMERACT working groups who complete the development of their MOD/COD, which will then be incorporated into the refinement of the guidance and training.