Objective: To conduct the cross-cultural adaptation of the Functional Rating Index (FRI) and to test the reliability and validity of the Thai version of FRI (Thai FRI).
Material and Method: The cross-cultural adaptation process was used to develop the Thai FRI. The two groups of patients
comprised low back pain (LBP) and neck pain (NP). Each patient was asked to complete the questionnaires twice: at the first and second visits. The patients with LBP completed the Thai FRI, Roland-Morris Disability, modified Oswestry Low Back
Pain Disability and multi-level Roland-Morris Disability, while the patients with NP completed the Thai FRI and Thai Neck
Disability Index. Each patient was also asked to rate a Global Perceived Effect Scale at the second visit. Reliability and crosssectional construct validity of the Thai FRI were evaluated. Minimal detectable change (MDC95%) was calculated.
Results: The FRI was cross-culturally adapted to Thai and the adapted version was validated. In total, 161 patients with LBP
and 84 patients with NP completed the questionnaires. Cronbach’s alpha for the Thai FRI equaled 0.86 for LBP and 0.83 for NP, ICC2,1 equaled 0.82 for LBP and 0.89 for NP, correlations between the Thai FRI and other questionnaires ranged from 0.68 to 0.78 for both groups. The MDC95% equaled 2.5 for LBP and 2.3 for NP.
Conclusion: The Thai FRI was developed and validated. Its measurement properties demonstrated acceptable internal
consistency, good test-retest reliability and moderate to high cross-sectional construct validity.

Keywords: Functional rating index, Low back pain, Neck pain, Reliability and validity, MDC