Polish Cross-cultural Adaptation of the Lower Limb Functional Index (LLFI) demonstrates a Valid Outcome Measure for the Lower Limb Region and Joints

Abstract

Background: Patient reported outcome measures (PROMs) are recommended to enable the standardization of collected data and provide accurate representation of the patients’ subjective opinions of their functional capabilities. The purpose of this study was to perform linguistic and cross-cultural adaptation to establish a Polish version of the Lower Limb Functional Index (LLFI), and to evaluate the psychometric properties of internal consistency, reliability, error score, validity, and factor structure with standardized criteria PROMs in a population with lower limb problems.Methods: Linguistic and cultural adaptation complied with the International Society for Pharmacoeconomics and Outcomes Research (ISPOR) guidelines to produce the Lower Limb Functional Index - Polish version (LLFI-PL). This was a two-stage, cross-sectional study with repeated measures on two variables during retest examination. The study recruited n=125 subjects (age =52.86±19.53 years, range 20-87, 56% female, injury duration =17.69±18.39 weeks, range 5-71). Baseline reliability and construct validity included the LLFI-PL, Western Ontario and McMaster University Osteoarthritis Index (WOMAC), Euroqol Health Questionnaire 5-Dimensions (EQ-5D-5L), and an 11-point pain Numerical Rating Scale (NRS), with retest at 3-7 days. Practicality for readability was considered within the face and content validity. Completion and scoring time were also calculated. Results: Statistical analysis showed high internal consistency (α=0.94) that is below the 0.95 threshold limit, and excellent test-retest reliability (ICC2.1=0.96). The error score found the SEM=4.85% with MDC90=11.3%. Validity analysis showed strong correlations between the LLFI-PL with the WOMAC (r=-0.81) and moderate with the EQ-5D-5L (r=-0.63). Exploratory factor analysis confirmed a single-factor structure. Times for completion (172±33 seconds) and scoring (20±9 seconds) were determined.Conclusions: The LLFI-PL is a psychometrically sound questionnaire for Polish-speaking patients with lower limb musculoskeletal conditions. The results support the findings of previous original-English, Spanish, and Turkish versions for internal consistency, validity, reliability, error score, and factor structure.