Background: The comprehensive ICF Core Set for rheumatoid arthritis (RA) is a selection of 96 categories from the International Classification of Functioning, Disability and Health (ICF), representing relevant aspects in the functioning of RA patients.

Objectives: To study the reliability of the ICF Core Set for RA in rheumatological practice, and to explore the metric of the qualifiers’ scale.

Methods: 25 RA patients from an outpatient department of rheumatology were interviewed using the ICF Core Set for RA (76% females, mean (SD) age 57.5 (12.5) years, disease duration 15.9 (14.6) years). Interviews were performed independently by both a physiotherapist and an occupational therapist on the same day and again after one week by one of them. The severity of the patients’ problems was quantified on a qualifier scale ranging from 0 (no problem) to 4 (complete problem). Analyses of intra-rater and inter-rater agreement, kappa statistics, and Rasch analyses were applied.

Results: Mean intra-rater (inter-rater) complete agreement for all categories was seen in 59% (47%) of observations, ranging from 29% (0%) to 96% (80%) for individual categories. Weighted kappa statistics with

value >0.4 showed reliability in 86% of categories within raters, and in 43% of categories between raters. Improved inter-rater and intra-rater reliability was observed with a reduced number of qualifiers for the categories.

Conclusions: Inter-rater and intra-rater reliability of the ICF Core Set of RA was low to moderate. The metric of the qualifiers’ scale may be improved by reducing the number of qualifiers to three for all components.