One implementation described herein includes a system for providing at least an estimate of substance concentrations in a bodily fluid. The system comprises a first emitter for generating signals using a first sensing modality, a first detector configured to detect signals from said first emitter that have travelled through the bodily fluid and output a corresponding first detector signal； a second emitter for generating signals using a second sensing modality different to said first sensing modality, a second detector configured to detect signals from said second emitter that have travelled through the bodily fluid and output a corresponding second detector signal； and a processor configured to receive the detector signals from said first and second detectors, to process said signals and to output an indication of at least an estimate of the concentration of said substance in said bodily fluid.
