The study aimed to translate the foot function index (FFI) questionnaire to Thai and to determine psychometric properties of the questionnaire among individuals with plantar foot complaints. The Thai version of the FFI (FFI-Th) was adapted according to a forward and backward translation protocol by two independent translators and analyzed by a linguist and a committee. The FFI-Th was administered among 49 individuals with plantar foot complaints to determine internal consistency, reliability, and validity. Cronbach’s alpha and the Intraclass Correlation Coefficient (ICC3,1) were used to test the internal consistency and test-retest reliability. The Principal Component Analysis with varimax rotation method was used to test the factor structure and construct validity. Furthermore, the criterion validity was tested using Pearson’s correlation coefficient (rp) between the FFI-Th and the visual analogue pain scale (pain-VAS) as well as the EuroQol five-dimensional questionnaire (EQ-5D-5L). The FFI-Th showed good to excellent internal consistency and test-retest reliability in the total score, pain, disability, and activity limitation subscales. The Principal Component Analysis produced 4 principal factors from the FFI-Th items. Criterion validity of the FFI-Th total score showed moderate to strong correlations with pain-VAS and EQ-5D-5L, and EQ-VAS scores. The FFI-Th was a reliable and valid questionnaire to assess the foot function in a Thai population. NCT03161314 (08/05/2017).
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