Measuring fatigue in persons with multiple sclerosis: Creating a crosswalk between the modified fatigue impact scale and the PROMIS fatigue short form

Purpose To create cross-walk tables to associate scores for the Modified Fatigue Impact Scale (MFIS) with scores for the Patient Reported Outcome Measurement Information System (PROMIS) Fatigue Short Form (SF) in persons with Multiple Sclerosis (MS). Methods Cross-walk tables were created using equipercentile linking and based on data collected at one time point in a longitudinal study of persons with MS (N = 458). Validation of the tables was conducted using data collected at a subsequent time point (N = 444). Deviations between estimates and actual scores were compared across levels of fatigue. The impact of sample size on the precision of sample mean estimates was evaluated using bootstrapping. Results Correlations between deviations and fatigue level for the PROMIS Fatigue SF and MFIS were (-0.31) and (-0.30), respectively, indicating moderately greater deviations with lower fatigue scores. Estimated sample means were impacted by sample size. Conclusions Cross-walk tables allow data from studies using different measures of fatigue to be combined to achieve larger sample sizes and to compare results. These tables are valid for group-level analyses with sample sizes of 150 or greater.