Cognitive testing of the Children's Palliative Outcome Scale (C-POS) with children, young people and their parents/carers.

Background: The Children's Palliative Outcome Scale (C-POS) is being developed using best methodological guidance on outcome measure development, This recommends cognitive testing, an established method of item improvement, prior to psychometric testing.

Aim: To cognitively test C-POS within the target population to establish comprehensibility, comprehensiveness, relevance and acceptability.

Design: Cross-sectional cognitive interview study following COnsensus-based Standards for the selection of health Measurement INstruments (COSMIN) methodology and Rothrock guidance on outcome measure development. Cognitive interviews were conducted using 'think aloud' and verbal probing techniques.

Setting/participants: Children 5-⩽17 years old with life-limiting conditions and parents/carers of children with life-limiting conditions were recruited from 14 UK sites.

Results: Forty-eight individuals participated (36 parents; 12 children) in cognitively testing the five versions of C-POS over two to seven rounds. Content and length were acceptable, and all questions were considered important. Refinements were made to parent/carer versions to be inclusive of non-verbal children such as changing 'share' to 'express' feelings; and 'being able to ask questions' to 'having the appropriate information'. Changes to improve comprehensibility of items such as 'living life to the fullest' were also made. Parents reported that completing an outcome measure can be distressing but this is anticipated and that being asked is important.

Conclusion: Cognitive interviewing has facilitated refinement of the C-POS, especially for non-verbal children who represent a large proportion of those with a life-limiting condition. This study has enhanced the face and content validity of the measure and provided preliminary evidence for acceptability for use in routine practice.