a | Defining disease requires a composite understanding of both the signs (observable to the patient and others) and symptoms (reported by the patient). While these quantifiable events are not identical, they are physiologically linked (for example, increased joint pain results in a reduction in overall activity and mobility and diminished sleep quality). So, understanding how different functional domains are altered by disease and treatment is crucial in defining how an individual feels, functions and survives. b | Biometric monitoring devices (BMDs) can link quantifiable data collected from biosensors that measure a biological response using wearables, smart phones, clothing, implants ingestible sensors or remote biosensors (see 'Future market trends for wearables' in Further information). Improving our understanding of real-time changes in function in health and disease needs to be a focus (see 'Maximizing the potential of real world evidence to support health care innovation' in Further information). Although not yet formally validated as a clinical outcome assessment (COA), medication adherence has been shown to be decreased by as much as 40% in those with mild cognitive impairment. c | For many chronic diseases, the clinical instruments used to quantify disease are validated only during manifest disease, and there is a strong need to begin treatment earlier in the disease course, as indicated in the figure, which could be enabled by BMDs . Claims regarding the benefits of BMDs should include the analytical and clinimetric validity of the devices and the clinical utility of the measured end points (see 'What the Fitbit lawsuit means for clinical researchers' in Further information). With the increased emphasis on prevention therapies, the need to identify sensitive outcome assessments is growing. Otherwise, long (5+ year) trials will be necessary before the effects of early interventions can be determined. Both the health care community and regulators are embracing this need, and analysis of BMD data is anticipated to provide patient-centred, real-world evidence to support regulatory and clinical decision-making (see 'Use of real-world evidence to support regulatory decision-making for medical devices: draft guidance for industry and Food and Drug Administration staff' in Further information). PRO, patient-reported outcome.