BackgroundPortable breath ketone sensors may help people with Type 1 Diabetes Mellitus (T1DM) avoid episodes of diabetic ketoacidosis; however the design features preferred by users have not been studied. We aimed to elucidate design preferences associated with breath ketone sensors among young people with T1DM aged 12-16 years, their parents, and diabetes educators.Research Designs and MethodsTwo commercially available breath ketone sensors, designed for ketogenic diet monitoring, were assessed over one week by ten young people with T1DM following a usability evaluation approach. Participants interacted with the devices at least twice daily for five days, taking breath ketone, blood ketone and blood glucose measurements. Semi-structured interviews were conducted post-testing with participants, parents and two diabetes educators to elicit design preferences to inform the co-design of diabetes breath ketone sensor prototypes.ResultsParticipants acknowledged the non-invasiveness of breath sensors as compared to blood testing. Affordability, reliability and accuracy were identified as prerequisites for breath ketone sensors used for diabetes management. Design features valued by young people included portability, ease of use, sustainability, readability and suitability for use in public. The time required to use breath sensors was similar to that for blood testing. The requirement to maintain a 10-second breath exhalation posed a challenge for users.ConclusionsBreath ketone sensors for diabetes management bear potential to facilitate ketone testing in young people. Our study affirms features for young people that drive usability of breath sensors among this population, and provides a model of usability assessment of medical devices among young users.Highlights/new findingsPortable breath ketone sensors potentially offer a non-invasive alternative to blood ketone testing in diabetes management, especially for young people with diabetes.A usability assessment of portable non-medical breath ketone devices among young people with Type 1 Diabetes Mellitus demonstrates the key values associated with usability.The time to test for breath ketones was similar to that of blood ketone testing.Young people can and should be engaged in co-creation of medical devices that they will use.