Background: This study aimed to examine the psychometric performance of the Foot and Ankle Outcome Score (FAOS) used in Indonesian patients with chronic lateral ankle instability (CLAI). Methods: The FAOS was translated into Indonesian through standardized procedures. Among 224 patients with unilateral CLAI recruited from 14 physical therapy clinics during a 1-year period, reliabilities, construct validities, and responsiveness levels of the FAOS were examined. Active and passive range of motion of ankle dorsiflexion or plantiflexion, figure-of-eight, numeric pain rating scale (NPRS), and Short Form (SF)-36 were used to test the construct validities. Results: The five subscales indicated adequate internal consistency (Cronbach’s alpha, 0.74 ~ 0.96) and interrater test-retest reliabilities (interclass correlation coefficients, 0.80 ~ 0.94). Subscales of the FAOS moderately converged with those selected measures with similar constructs (r values, 0.32 ~ 0.53), with the exception of the correlation of pain with the NPRS (r, -0.06). Results of the principal component analysis showed that the five-factor structure of the FAOS was appropriate for the Indonesian data, although six items (four in the pain and two in the other symptoms (OSs) subscales) did not perfectly fit their original subscales. Guyatt’s responsiveness index for the FAOS’s subscales changed in the SF-36’s physical function over a 1-month period and ranged 0.37 to 1.27. Conclusions: The Indonesian version of the FAOS demonstrated acceptable reliabilities and responsiveness, and fair construct validities among CLAI patients, although certain items in the pain and OSs subscales may need to be further explored and improved.
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