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Continuous monitoring of specific metabolite and drug levels within a patient's blood can contribute to shorter hospital stays and more successful treatment of both chronic and acute diseases. Intravenous microdialysis is an attractive technology for continuous venous blood sampling that can be used to manage tight glucose control and to sample a large variety of molecules from human blood. In combination with lab-on-a-chip architectures, microdialysis could provide continuous monitoring of important diagnostic and therapeutic substances. Unfortunately, microdialysis is inherently variable and non-transparent, i.e., errors in sampling cannot be detected and corrected in real-time. A portable microdialysis system is presented that gauges membrane diffusive capacity by using a fluorescent tracer, providing a method to track the intrinsic variability. An embedded systems controller and CMOS image sensor is used to measure and wirelessly communicate fluorescent tracer levels. The controller has the capability to generate alarms when probe performance deteriorates, making microdialysis both more accurate and robust for clinical use. The potential to integrate a microparticle-based, turbidimetric vancomycin immunoassay with microdialysis is also demonstrated by using a CMOS image sensor to detect changes in turbidity.