An apparatus for automatically and periodically measuring a patient's blood glucose level wherein the patient has a catheter in a blood vessel and is receiving infusion fluid through the catheter. A testing unit is provided which has a main infusion channel and, in a preferred embodiment, the testing unit includes a side channel. The side channel has a restrictive cross-sectional area compared with the main infusion channel. A glucose test chamber is formed in the side channel. First and second valves are placed adjacent the test chamber in the side channel to isolate a blood sample for testing. The isolation of the sample provides more accurate blood glucose measurements than those provided by systems of the prior art. A second embodiment of the invention has an isolatable test chamber placed in the main infusion line and no side channel is utilized.