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Objectives: Assessing external validity and clinical relevance of modified radiographic union score (mRUS) to predict delayed union in closed humeral shaft fractures initially treated with conservative management., Design: Retrospective cohort., Setting: Single urban academic level 1 trauma center., Patients: Patients undergoing initial nonoperative treatment of a humeral shaft fracture with a minimum of 3 months follow-up and at least one set of follow-up orthogonal x-rays within 12-weeks of injury., Main Outcome Measurements: Interobserver and intraobserver reliability of the (mRUS) system for humeral shaft fractures, and establishing an mRUS threshold at 6 and 12 weeks postinjury to predict surgery for delayed union., Results: mRUS demonstrated substantial interobserver agreement on all assessments. Intraobserver agreement was nearly perfect for all reviewers on repeat assessment. mRUS of ≤7 at 6 ± 1 weeks follow-up was associated with surgery for delayed union with an odds ratio of 4.88 (95% CI, 2.52-9.44, P < 0.01), sensitivity of 0.286, and specificity of 0.924. At 12 ± 1 weeks follow-up, the same threshold demonstrated a stronger association with an odds ratio of 14.7 (95% CI, 4.9-44.1, P < 0.01), sensitivity of 0.225, and specificity of 0.981., Conclusions: The mRUS for humeral shaft fractures is reliable and reproducible providing an objective way to track subtle changes in radiographs over time. An mRUS of ≤7 at 6 or 12 weeks postinjury is highly specific for delayed union. This can be helpful when counseling patients about the risk of nonunion and potential early surgical intervention., Level of Evidence: Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence., Competing Interests: The authors report no conflict of interest., (Copyright © 2022 Wolters Kluwer Health, Inc. All rights reserved.)