OBJECTIVES: Over 30 instruments are in current, active use for delirium identification. In a recent systematic review, we recommended 4 commonly used and well-validated instruments for clinical and research use. The goal of this study is to harmonize the four instruments on the same metric using modern methods in psychometrics. DESIGN: Secondary data analysis from 3 studies, and a simulation study based on the observed data. SETTING: Hospitalized (non-ICU) adults over 65 years old in the United States, Ireland, and Belgium. PARTICIPANTS: The total sample comprised 600 participants, contributing 1,623 assessments. MEASUREMENTS: Confusion Assessment Method (long-form and short-form), Delirium Observation Screening Scale, Delirium Rating Scale-Revised-98 (DRS-R-98) (total and severity scores), and Memorial Delirium Assessment Scale. RESULTS: Using item response theory, we linked scores across instruments, placing all four instruments and their separate scorings on the same metric (the propensity to delirium). Kappa statistics comparing agreement in delirium identification among the instruments ranged from 0.37 to 0.75, with the highest agreement between the DRS-R-98 total score and MDAS. After linking scores, we created a harmonized item bank, called the Delirium Item Bank (DEL-IB), consisting of 50 items. The DEL-IB allowed us to create six crosswalks, to allow scores to be translated across instruments. CONCLUSIONS: With our results, individual instrument scores can be directly compared to aid in clinical decision-making, and quantitatively combined in meta-analyses.