Objective: To achieve a reliable tool for measuring health related quality of life among Iranians with knee osteoarthritis, by translating and culturally adapting the Knee injury and Osteoarthritis Outcome Score(KOOS) to Persian and testing the reliability and internal consistency of the Iranian version.

Materials & Methods: It was a non experimental methodology study. KOOS was translated and adapted culturally to Persian language and culture in three phases with respect to IQOLA project. For examining test-retest reliability Iranians version of KOOS was corresponded twice with in at least two days or at most one week interval, by 30 Iranian people with knee OA whom were referred to Municipality and 110 physiotherapy clinics of Tehran with PT order by physicians. It was a non experimental methodological research and we used sample of convenience and non probability design for sampling. Psychometric evaluation: the collected data from the questionnaires was rated and analyzed with SPSS software from the aspects of test-retest reliability, absolute reliability, subscale and item internal consistency.

Results: Internal consistency which was calculated by Cronbach '&alpha was high for all the subscales (at least 0.76), except for "symptom" subscale which was moderate, and showed that items of each subscale measured the same construct. Item internal consistency after correction for overlap, was higher than optimal value (0.4), except for the items of" symptom" subscale , which demonstrated good item internal consistency. SEM and ICC which were used for evaluating the absolute and test-retest reliability in respect showed that all the subscales had good test-retest reliability (0.7) and the absolute reliability was also very good in such away that the highest calculated SEM for Persian version was 7.44 which was less than Minimal Perceptible Clinical Improvement (MPCI) that is estimated 8 to 10 for the KOOS questionnaire.

Conclusion: With the Persian version of the Knee Injury and osteoarthritis outcome score (KOOS) a multidimensional instrument is available now to measure health status as well as therapeutic effects in patients with knee OA. Modifying the subscale “symptom” could further optimize this questionnaire in patients with advanced knee OA.