Methods and apparatus are provided for determining the concentration of an analyte in a sample, such as an analyte in a sample of bodily fluid. In some embodiments, a method for maintaining clear passageways in an extracorporeal blood flow system includes intermittently providing one or more anti-clotting agents to a passageway of the extracorporeal blood flow system. In some embodiments, an extracorporeal blood flow system includes a passageway and a device operatively connected to provide one or more anti-clotting agents to a least a portion of said passageway.
