In today's hospital setting, especially in intensive care units (ICUs), it is routine to access a patient's circulatory system, both to retrieve blood samples for analysis and for infusion of a variety of fluid formulations, such as but not limited to hydration fluids, medicaments and nutrients. For example, patients having heart, vascular, neurologic and orthopedic surgeries may have 2, 3 or more catheters in place, for fluid infusion and sample withdrawal. In some circumstances, a hospital patient is continuously monitored for changes in some blood analyte levels, such as for diagnosing, monitoring and/or prognosticating a patient's medical status. In some circumstances, blood samples are collected at regular intervals, and sent to a laboratory analysis. In other circumstances, a bedside analyte monitor is used to monitor the levels of the analyte.