Validation for the reduced Western Ontario and McMaster Universities Osteoarthritis Index function scale.

PURPOSE To assess the criterion validity, repeatability, and the missing value protocol of the reduced Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) function scale ('reduced scale'). METHODS Three separate studies were performed: a cross-over study to compare the full- and reduced-scale scores in 66 UK patients for validity, a test-retest study for repeatability in 103 Australian patients, and a study for the missing value protocol of the reduced-scale questionnaire. RESULTS There was no significant difference between scores for the full- and reduced-scale questionnaires in both cross-over and test-retest studies. For the missing value protocol of the reduced-scale questionnaire, when 3 or more of the 7 items were missing, the patient's response was regarded as invalid and the sub-scale score was eliminated from further analysis; when 2 or fewer items were missing, the mean value of the sub-scale was substituted for the missing values. CONCLUSIONS The reduced-scale questionnaire retains excellent validity and repeatability. Its use is recommended along with the original pain dimension in studies of total joint replacement.