Aim is to develop a common reference metric of functioning incorporating generic and health condition-specific disability instruments and to test whether this common metric is invariant across two health conditions belonging to a musculoskeletal (rheumatoid arthritis = RA) and a neurological (stroke) disorder.

A psychometric study using secondary data analysis of 487 people; 61.4% with RA and 38.6% with stroke was performed. In RA sample 25.4% were male, and in stroke sample 53.7%. Generic instrument, included in the analysis was World Health Organization Disability Assessment Schedule (WHODAS 2.0) whereas condition-specific instruments were Health Assessment Questionnaire (HAQ) in RA, and Functional Independence Measure (FIM™) motor scale in stroke. The International Classification of Functioning, Disability and Health (ICF) Linking Rules were used to examine the concept equivalence between the included instruments, and a scale-banking approach based on the Rasch measurement model was applied to examine score equivalence between these instruments. Only scales (or domains) that resulted in content and score equivalence were included in the reference metric.

Three sub-domains of WHODAS 2.0 and all items of HAQ and FIM™ motor scale were linked to the ICF chapters d4-Mobility, d5-Self-care and d6-Domestic life. The scale banking of these items resulted in good model fit ( χ ² _(df=21) = 22.755; P = 0.357) with good reliability (PSI = 0.891), indicating that a reference metric across these three instruments was available. Operational range of each scale in reference to the common metric is shown in Fig. 1 which supports that co-calibration of 3 scales resulted in an improved coverage of the measurement continuum.

This study provides evidence for, and the transformation table to enable direct comparisons among instruments measuring physical functioning commonly used in RA (HAQ) and stroke (FIM™ motor), as well as in people with disability in general (WHODAS 2.0).