Core Outcome Set (COS)

A “Core Outcome Set” is an agreed minimum set of outcomes or outcome measures. It is a recommendation of ‘what’ should be measured and reported in all trials in a specific area. Researchers also need to consider ‘how’ these outcomes should be measured, and work is ongoing to develop “Core Outcome Measurement Instrument Sets”, which will include details on the instruments or tools to use to measure the outcomes in a Core Outcome Set. The COMET team welcomes comments on these definitions, and ideas on how to describe outcomes, outcome measures and outcome measurement instruments.

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(i) For example, the OMERACT RA core outcome set consists of tender joints, swollen joints, pain, physician global assessment, patient global assessment, acute phase reactants (APR), functional status.

(ii) Some groups prefer the term ‘outcome domain’ to describe the ‘what’.

(iii) The term ‘outcome measure’ has been used differently by various groups. Sometimes what is defined as an outcome measure is often (loosely) called an outcome. Here, we use the term outcome measure to mean ’what’ is being measured. However, it should be noted that some groups use outcome measure to mean an outcome measurement instrument i.e. ’how’ an outcome is being measured. An outcome measurement instrument is a tool to measure an outcome, where the tool can be a single question, a questionnaire, a score obtained through physical examination, a laboratory measurement, a score obtained through observation of an image, etc. Sometimes, most obviously for death, the outcome and the outcome measure are synonymous. But for very familiar outcomes such as blood pressure, it is blood pressure that is the core item in a core outcome set (i.e. the ‘what’) rather than the specific machine chosen to measure it (i.e. the ‘how’).

(iv) Once agreement is reached about the Core Outcome Set, the next step is to achieve agreement on ’how’ each outcome should be measured.