Functional index-2:: Validity and reliability of a disease-specific measure of impairment in patients with polymyositis and dermatomyositis

Objective. To revise the content of the Functional Index in myositis (FI) and to evaluate measurement properties of a revised FI. Methods. Previously performed FI (n = 287) were analyzed for internal redundancy and consistency, and ceiling and floor effects. Content was evaluated and a preliminary revised FI was developed. To evaluate the construct validity of the preliminary revised FI, it was compared with isokinetic measurements of muscular strength and endurance, the Myositis Activities Profile, disease impact on general wellbeing, and creatine phosphokinase levels. Minor adjustments were made and the revised FI was investigated for interrater reliability and intrarater reliability over a 1-week period. After this, some minor, additional adjustments were made leading to the final version, FI-2. Results. Five tasks were removed from the original FI due to ceiling effects. Performance pace and number of repetitions were modified for the remaining tasks. A moderate correlation (r(s) = 0.58) was found between the shoulder flexion task of the preliminary revised FI and isokinetic measurements of shoulder flexion endurance. Intraclass correlation coefficient (ICC) for interrater reliability of the revised FI varied from 0.86-0.99 with no systematic differences. ICC for intrarater reliability varied from 0.56-0.99 with systematic differences (P < 0.05) between test and retest in 3 of the tasks. The sit-up task was excluded due to low intrarater reliability resulting in the final 7-item FI-2. There was a good correlation between tasks on the right and left side suggesting that the FI-2 could be performed unilaterally. Conclusion. The FI-2 is a valid and reliable outcome measure of impairment for patients with polymyositis or dermatomyositis. It is well tolerated and the unilateral FI-2 requires a maximum of 20 minutes to perform. Further evaluation of sensitivity to change and, testing in healthy individuals needs to be conducted.