A method and apparatus for periodically and automatically testing and monitoring a patient's blood glucose level. A disposable testing unit is carried by the patient's body and has a testing chamber in fluid communication with infusion lines and a catheter connected to a patient blood vessel. A reversible peristaltic pump pumps the infusion fluid forwardly into the patient blood vessel and reverses its direction to pump blood into the testing chamber to perform a glucose level test. The presence of blood in the testing chamber is sensed by a LED/photodetector pair or pairs and when the appropriate blood sample is present in the test chamber, a glucose oxidase electrode is energized to obtain the blood glucose level. The resulting glucose level is displayed on a monitor. The unit is typically used for a patient in an intensive care unit.