a supplemental measure, instrument, or set of indicators that can be attached to an existing Core Outcome Set (COS) to make it more comprehensive, context-specific, or applicable to a particular population. It extends the standard set of outcomes with additional content without altering the core itself.
Synonyms: Add-on, Supplement, Extension, Module, Optional Component
Example
A Core Outcome Set for osteoarthritis includes pain, physical function, and quality of life. A bolt-on on “sleep disturbance” could be added for studies involving patients with comorbid insomnia to capture this important outcome without changing the original COS.