The rating of patients’ impairments, for example in the arm and hand after a neurological injury, through experienced clinical personnel is a fundamental pillar of the global healthcare system. Such information is essential to guide the definition of a therapy plan and evaluate its efficacy. However, the established rating process leaves room for improvement, as its subjectivity implies that different assessors can reach varying rating results regarding the type and severity of the impairments. In addition, the employed rating scales are often coarse, provide only few rating options with a lower and upper limit (e.g., severe, moderate, mild impairment), thereby challenging the detection of subtle changes.
Digital health metrics promise to address these limitations by providing an objective and sensitive characterization of impairments as complementary information for clinical personnel to support evidence-based decision making. Such metrics offer high resolution, enabling the evaluation of subtle changes in impairment severity. They can be extracted by applying multiple mathematical processing steps to health-related sensor data, for example to describe hand movement and grip force control.
Despite the great potential of digital health metrics, their clinical integration is still scarce as a result of two key factors: first, researchers who intend to develop digital health metrics are confronted with the demanding task of selecting the metrics to extract from their health-related sensor data. This is challenging, as a heterogeneous landscape of metrics with varying physiological interpretation and statistical properties (e.g., reliability) exists. In the example of analyzing movement data in neurological injuries, over 150 metrics have been proposed . Second, clinical personnel expect that the rating scales that guide their clinical decisions have been thoroughly validated from a statistical point of view. This is often not the case for digital health metrics, as typically no accurate ground truth about the targeted impairment exists, which challenges the methodological implementation of the validation process.
In our work presented in npj Digital Medicine , we address these two challenges by proposing a data-driven framework that automatizes the selection and validation of digital health metrics based on clinically-accepted statistical properties. The approach starts with clear physiological hypotheses motivating an initial set of digital health metrics. Through multiple validation steps, metrics that do not fulfill all relevant statistical properties according to clinical standards are discarded, yielding a reduced set of validated application-specific metrics (Figure 1).
As an exemplary use-case, this framework was applied to 77 digital health metrics gathered in 120 neurologically intact and 89 affected individuals with a technology-aided assessment of upper limb sensorimotor impairments (Figure 2). This allowed establishing a set of 10 validated digital health metrics sensitively characterizing sensorimotor impairments in arm and hand (Figure 3). These metrics provided additional clinical value by detecting impairments in neurological subjects that did not show any deficits according to conventional scales.
Given that the proposed framework (source code) is based on a data-driven, application-dependent evaluation of clinically-accepted statistical criteria, it overcomes the caveats of expert groups providing recommendations about the choice of metrics, as these typically cannot account for dependency of the metrics’ statistical properties to each application. Moreover, the transparent and clinically-motivated framework complements existing data-driven machine learning algorithms, which are often black-box approaches that do not consider all clinically-accepted statistical criteria.
Hence, this work makes an important contribution to the implementation of digital health metrics as complementary endpoints for clinical trials and routine. We urge researchers and clinicians to capitalize on the promising properties of digital health metrics and contribute to their validation and clinical integration, which in the long-term will lead to a more thorough understanding of disease mechanisms and enable novel therapeutic approaches with the potential to improve healthcare quality.
 Schwarz, A., Kanzler, C.M., Lambercy, O., Luft, A.R., & Veerbeek, J. M. (2019). Systematic Review on Kinematic Assessments of Upper Limb Movements After Stroke. Stroke, 50(3), 718–727. https://doi.org/10.1161/STROKEAHA.118.023531
 Kanzler, C.M., Rinderknecht, M.D., Schwarz, A., Lamers, I., Cynthia, G., Held, J.P.O., Feys, P., Luft, A.R., Gassert, R., & Lambercy, O (2020). A data-driven framework for selecting and validating digital health metrics: use-case in neurological sensorimotor impairments. npj Digital Medicine, 3, 80. https://doi.org/10.1038/s41746-020-0286-7
This project received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 688857 (SoftPro). The authors would like to thank Stefan Schneller for support in the graphics design.