Patient reported outcomes (PROs) have become standard study endpoints. However, little attention has been given to using item improvement to advance PRO performance which could improve precision, clarity, patient relevance, and information content of "physical function/disability" items and thus the performance of resulting instruments.
The present study included 1860 physical function/disability items from 165 instruments. Item formulations were assessed by frequency of use, modified Delphi consensus, respondent judgement of clarity and importance, and item response theory (IRT). Data from 1100 rheumatoid arthritis, osteoarthritis, and normal ageing subjects, using qualitative item review, focus groups, cognitive interviews, and patient survey were used to achieve a unique item pool that was clear, reliable, sensitive to change, readily translatable, devoid of floor and ceiling limitations, contained unidimensional subdomains, and had maximal information content.
A "present tense" time frame was used most frequently, better understood, more readily translated, and more directly estimated the latent trait of disability. Items in the "past tense" had 80-90% false negatives (p<0.001). The best items were brief, clear, and contained a single construct. Responses with four to five options were preferred by both experts and respondents. The term physical function may be preferable to the term disability because of fewer floor effects. IRT analyses of "disability" suggest four independent subdomains (mobility, dexterity, axial, and compound) with factor loadings of 0.81-0.99.
Major improvement in performance of items and instruments is possible, and may have the effect of substantially reducing sample size requirements for clinical trials.