Measurement of disability in rheumatoid arthritis is often used to support treatment decisions and outcome assessments, but is used without reference to the impact of disability on individual patients.
To develop and validate a scale to measure individual values for functions, which is used to weight the level of an individual patient's functional loss and thus calculate the personal impact of disability.
In four linked studies, first the phraseology for values was explored to develop a stem question for the value scale couched in terms patients understand (face validity). Then short and long versions of the value scale were compared (content validity) and tests of internal consistency and short term reliability undertaken (criterion validity). Finally, the value scale was examined for long term reliability and agreement with expected variables (criterion and construct validity), after which personal impact scores were calculated and their construct validity examined.
Patients understand the concept of values, and a positively phrased stem question was developed for the value scale, for which a short version was reasonably equivalent to a long version. The value scale was reliable over one week (96% changed by <1 point) with positive interitem correlation. Reasonable six and 12 month reliability was shown (52% changed by <0.5 points), and the value scale was independent of disability and clinical, psychological, personality, and social support variables. Personal impact scores were then calculated by using the value scores to weight disability scores. Impact scores varied widely between patients of similar disability. Personal impact for disability showed convergent validity with dissatisfaction with disability, perceived increase in disability, increased disease activity, worse psychological status, low social support, and time trade off for disability. It discriminated between patients with low and high dissatisfaction with disability, life satisfaction, depression, pain, and helplessness.
This individualised personal impact scale should lend meaning to disability scores, improving the interpretation of clinical and research data.