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Background: The Knee injury and Osteoarthritis Outcome Score (KOOS) has demonstrated inferior psychometric properties when compared with the International Knee Documentation Committee (IKDC) subjective knee form when assessing outcomes after anterior cruciate ligament (ACL) reconstruction. The KOOS, Joint Replacement (KOOS, JR) is a validated short-form instrument to assess patient-reported outcomes (PROs) after knee arthroplasty, and the purpose of this study was to determine if augmenting the KOOS, JR with additional KOOS items would allow for the creation of a short-form KOOS-based global knee score for patients undergoing ACL reconstruction, with psychometric properties similar to those of the IKDC. Hypothesis: An augmented version of the KOOS, JR could be created that would demonstrate convergent validity with the IKDC but avoid the ceiling effects and limitations previously noted with several of the KOOS subscales. Study Design: Cohort study (diagnosis); Level of evidence, 2. Methods: Based on preoperative and 2-year postoperative responses to the KOOS questionnaires from a sample of 1904 patients undergoing ACL reconstruction, an aggregate score combining the KOOS, JR and the 4 KOOS Quality of Life subscale questions, termed the KOOSglobal, was developed. Psychometric properties of the KOOSglobal were then compared with those of the IKDC subjective score. Convergent validity between the KOOSglobal and IKDC was assessed with a Spearman correlation (ρ). Responsiveness of the 2 instruments was assessed by calculating the pre- to postoperative effect size and relative efficiency. Finally, the presence of a preoperative floor or postoperative ceiling effect was defined with the threshold of 15% of patients reporting either the worst possible (0 for KOOSglobal and IKDC) or the best possible (100 for KOOSglobal and IKDC) scores, respectively. Results: The newly developed KOOSglobal was responsive after ACL reconstruction and demonstrated convergent validity with the IKDC. The KOOSglobal significantly correlated with the IKDC scores (ρ = 0.91, P < .001), explained 83% of the variability in IKDC scores, and was similarly responsive (relative efficiency = 0.63). While there was a higher rate of perfect postoperative scores with the KOOSglobal (213 of 1904, 11%) than with the IKDC (6%), the KOOSglobal was still below the 15% ceiling effect threshold. Conclusion: The large ceiling effects limit the ability to use several of the KOOS subscales with the younger, more active ACL population. However, by creating an aggregate score from the KOOS, JR and 4 KOOS Quality of Life subscale questions, the 11-item KOOSglobal offers a responsive PRO tool after ACL reconstruction that converges with the information captured with the IKDC. Also, by offering the ability to calculate multiple scores from a single questionnaire, the KOOSglobal may provide the orthopaedic community a single PRO platform to be used across knee-related subspecialties. Registration: NCT00478894 (ClinicalTrials.gov identifier). [ABSTRACT FROM AUTHOR]