Objective: To evaluate the interrater reliability and the construct validity of a specially constructed functional index and to assess functional status in 23 patients with myositis; and to describe the occurrence, intensity, and distribution of pain in these patients.
Methods: The functional test scored muscle endurance in extremities, neck, and trunk, grip strength, peak expiratory flow, turning in bed, and transferring from lying in bed to sitting. The scores of the different functions were added to create a functional index. The patients' subjective gradings of disability and pain intensity were indicated on a category scale. Blood samples for analyses of muscle enzyme levels were obtained in connection with the functional tests.
Results: The interrater reliability of the functional index was high. Grip strength and muscle endurance in shoulder abduction and flexion, hip flexion, and a step test and heel and toe lifting were significantly reduced compared to controls. Peak expiratory flow was lowered in 4 patients. The functional index did not correlate significantly with the laboratory tests, but there was significant correlation between the weakness of the lower extremities and the subjective grading of disability. All patients but one had experienced severe pain during the course of the disease.
Conclusion: The described functional index has high interrater reliability, but its construct validity and sensitivity as a monitor of response to medication and training need further confirmation.