Although Foot and Ankle Outcome Score (FAOS) is a widely used patient-reported outcome measure (PROM) for foot and ankle conditions, research on its longitudinal validity is still needed. Minimal important change (MIC) values for the FAOS were determined using predictive modeling.

Overall, 134 patients that underwent operative treatment for foot and ankle conditions were included. An anchor based predictive logistic modeling method was used for estimating the MIC values for the FAOS subscales after surgery.

Mean score changes in the improved and the unimproved groups were 17.7 and 0.43 points for Pain, 3.9 and −3.3 points for Symptoms, 21.3 and 1.8 points for Activities and daily living (ADL), 8.7 and −2.8 points for Sport, and 12.5 and −3.3 points for quality of life subscale, respectively. MIC was successfully determined to four out of five subgroups as follows: Pain 9.5 (94% CI −6.4 to 24.6); ADL 11.7 (95% CI −19.6 to 46.6); Sport (95% CI −10.4 to 15.4); QoL 5.0 (95% CI −2.6 to 12.9). The Symptoms subgroup presented with low MIC of 0.3 (95% CI −11.7 to 13.4) fitting to the measurement error.

ADL, Sports, Pain, and QoL subscales of the FAOS presented logical MIC values. The MIC can be further evaluated for specific conditions.