Evaluation of the Structural Validity, Internal Consistency, and Measurement Invariance of the TENDINopathy Severity Assessment – Achilles (TENDINS-A): A Rasch Measurement Theory Analysis

OBJECTIVE: To evaluate the structural validity, internal consistency, and measurement invariance of the TENDINopathy Severity assessment – Achilles (TENDINS-A) using Rasch measurement theory. DESIGN: Cross-sectional evaluation of the measurement properties of the TENDINS-A. METHODS: Participants with Achilles tendinopathy completed the TENDINS-A. Unidimensionality (achieved when the eigenvalue is <2), observed average category measure, threshold order, and item fit (achieved when 0.5 ≤ Infit/Outfit ≤ 1.5) assessed structural validity for each TENDINS-A subscale (Pain, Symptoms, and Function). Standardized 50-item reliability assessed internal consistency (achieved when standardized reliability is >0.7). Differential item functioning assessed measurement invariance (achieved when differential item functioning contrast is <0.5 and P>.01). RESULTS: One-hundred and forty-seven participants (52% female) were included. Each TENDINS-A subscale (Pain, Symptoms, and Function) had sufficient structural validity. A single total score of all TENDINS-A subscales is multidimensional and scoring should not combine subscales. The TENDINS-A displayed sufficient internal consistency for all subscales. The TENDINS-A displayed no measurement bias for age, sex, body mass index, education level, income level, or physical activity level. CONCLUSION: Our Rasch analysis of the 3 subscales of the TENDINS-A has shown that they have adequate structural validity, internal consistency, and measurement invariance. The TENDINS-A can be considered a valid tool to assess the pain, symptoms, and function in Achilles tendinopathy. JOSPT Methods 2025;1(1):30-39. Epub 4 December 2024. doi:10.2519/josptmethods.2024.0114