The General Function Score: a useful tool for measurement of physical disability. Validity and reliability.

The General Function Score (GFS) is a disease-specific instrument consisting of nine items focusing on strict physical activities of daily living. It is intended as an alternative to the more complex scores of disability (such as the Oswestry Disability Index), serving as a complement to the quality of life instruments in the study of low back pain (LBP). It was developed from an original 17-item questionnaire, of which 11 of the items were tested for criterion validity in an observer-supervised performance test. Two items were excluded from further analysis because of too low a validity. The remaining nine-item GFS was tested for construct validity, reliability, feasibility and responsiveness in six different cohorts: 297 patients with chronic low back pain (cohort 1), an age- and sex-matched control group of 287 randomly allocated Swedish citizens (cohort 2), three separate groups of patients admitted for surgery due to low back disorders (cohorts 3-5) and outpatients with spinal disorders (cohort 6). Correlations were tested with the Spearman Rank correlation coefficient, differences between groups with the Mann-Whitney test and the internal consistency with the Cronbach's coefficient alpha. The GFS total scores showed correlations of 0.78, 0.81 and 0.88 in the three aspects of the performance test. The response rate was 98.3-100%. The mean time to complete the questionnaire was 1.2 min. The internal consistency was 0.69 and 0.86 in cohorts 1 and 2 respectively. The test-retest correlation was 0.88. The GFS showed a high responsiveness to difference and change. The effect size was 0.82-0.96 in surgically treated disc herniation and 0.55-0.85 in spondylolisthesis. The GFS is a highly valid and reliable instrument with good responsiveness and feasibility, useful for evaluation of physical disability.