Objective: To further simplify the Simple Erosion Narrowing Score (SENS) by removing scored areas that contribute least to its measurement precision according to Item Response Theory (IRT) based analysis and to compare the measurement performance of the simplified version to the original. Methods: Baseline and 18 months data of the ‘Combinatietherapie bij reumatoide artritis’ (COBRA) trial were modelled using longitudinal IRT methodology. Measurement precision was evaluated across different levels of structural damage. SENS was further simplified by omitting the least reliably scored areas. Discriminant validity of SENS and its simplification was studied by comparing their ability to differentiate between the COBRA and SSZ arm. Responsiveness was studied by comparing standardized change scores between versions. Results: SENS data showed good fit to the IRT model. Carpal and feet joints contributed least statistical information to both erosion and joint space narrowing scores. Omitting the joints of the foot reduced measurement precision for the erosion score in cases with below average levels of structural damage (relative efficiency compared with the original version ranged 35%-59%). Omitting the carpal joints had minimal effect on precision (relative efficiency range 77%-88%). Responsiveness of a simplified SENS without carpal joints closely approximated the original version (i.e., all ∆ standardized change scores ≤0.06). Discriminant validity was also similar between versions for both the erosion score (relative efficiency = 97%) and the SENS total score (relative efficiency = 84%). Conclusion: Our results show that the carpal joints may be omitted from the SENS without notable repercussion for its measurement performance.