Functional Status Index: reliability of a chronic disease evaluation instrument.

The reliability of 3 methods of assessing respondents' perceptions of their functional ability was tested. The Functional Status Index defines function as including 3 distinct but related dimensions: the degree of dependence, the degree of difficulty and the amount of pain experienced in performing specific activities of daily living. A total of 149 adults with rheumatoid arthritis were studied. Exploratory factor analyses of scores on 18 specific daily activities across the 3 hypothesized dimensions yielded the following functional categories: gross mobility, hand activities, personal care, home chores and interpersonal activities. The resultant indexes achieve internal consistency reliability levels ranging from 0.66 to 0.91 across all but 1 functional category. Average test-retest and interobserver reliability values range from 0.65 to 0.81. Levels of interobserver reliability generally equal or surpass levels of test-retest reliability. The findings suggest that it is feasible to quantify level of function using self-report methods. The Functional Status Index is recommended for use in investigations where changes in functional ability are of interest.