[Validation of a short form of the function dimension of the WOMAC questionnaire]. 
The aim of this prospective study was to evaluate the validity, reliability and responsiveness of the Spanish version of the 7-item short-form of the function dimension of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC).  We conducted a prospective 1-year cohort study that included 459 patients on the waiting list for total knee or hip replacement.  The WOMAC and EQ-5D questionnaires were sent at four time points: baseline and at 3, 6 and 12 months post-surgery.  Based on a previously shortened scale, we performed a validation process with Rasch analysis and agreement measured by Bland-Altman plots.  The values of Spearman's rho and Lin concordance coefficients between the full and short-form ranged from 0.96 to 0.98.  Differences were statistically significant (p < 0.001) among the three groups of severity measured by a item in the EQ-5D. The two Bland-Altman plots showed no systematic differences and agreement was not dependent on the severity score.  The short form fitted the Rasch model.  Cronbach's α coefficients were higher than 0.85.  Responsiveness was higher than 1.27 and 1.72 in knee and hip replacement, respectively.  The Spanish 7-item short form of the WOMAC function dimension is valid, reliable and responsive for patients with total joint replacement.  The use of this version will reduce the burden on patients in both clinical studies and clinical practice.