Interruptions contribute to catastrophic errors in health care. Interruptions are breaks in the performance of a human activity initiated by a source internal or external to the recipient. Errors during the initial triage assessment can lead to errors in estimating the acuity of a patient and resources required for appropriate care. To advance the science, a valid and reliable instrument to measure this phenomenon is required. The purpose of this mixed-methods/exploratory sequential study was to develop an instrument sensitive to the uniqueness of the triage assessment that would categorize and measure the number and causes of triage interruptions. The study included 3 phases. Phase 1: Qualitative focus groups were used to qualitatively explore interruptions in triage. In Phase 2: Content Validity Assessment, an online survey was used and a content validity index was calculated for each item to determine which items should be modified or removed. A descriptive correlational design was used to assess interrater reliability in Phase 3. Many of the items identified during the focus group sessions were already on the study instrument; some new items were added. Content validity for the entire instrument was 0.82 and increased to 0.91 once irrelevant items were removed. Interrater reliability for the entire instrument demonstrated substantial agreement at 0.773. The study instrument was shown to have strong psychometrics and can be used in practice to better understand what interruptions are occurring in triage and how they affect the triage process.