From the Maine Medical Center, Portland, ME (Dr. Kramer, Ms. Palmeri, Dr. Robich, Mr. Groom, Dr. Hayes, Ms. Janoushek, Dr. Rappold, Dr. Swarz, and Dr. Quinn), and the Center for Outcomes Research and Evaluation, Maine Medical Center Research Institute, Portland, ME (Dr. Lucas).
Abstract
- Objective. To determine the accuracy of the glucometer currently used for point-of-care testing (POCT) of blood glucose in our cardiothoracic surgery intensive care unit (CTICU).
- Design. Prospective cohort study.
- Setting. Tertiary care community hospital affiliated with a school of medicine.
- Participants. Coronary artery bypass graft (CABG) surgery patients.
- Measurements. Blood glucose levels obtained via POCT with a glucometer using fingerstick and radial artery blood samples were compared with values obtained via central laboratory testing of radial artery blood samples (gold standard) in 106 CABG patients on continuous insulin infusions (CII) upon arrival to the CTICU from the operating room and 102 CABG patients on CII in the CTICU 6 hours later.
- Results. Fingerstick POCT and central lab blood glucose values correlated well (r = 0.83 for admission and 0.86 for 6-hour values), but the mean values were significantly different as determined by paired t-tests. Upon arrival, the fingerstick POCT mean value was 120.9 mg/dL, while the central laboratory value was 127.9 mg/dL (P value = 0.03). At the 6-hour time point, the mean value for fingerstick POCT was 129.7 mg/dL compared to a central laboratory value of 137.3 (P value = 0.02).
- Conclusion. The blood glucose POCT values correlated well with central laboratory values, but the values were statistically significantly different. Nevertheless, accurate clinical decisions were made despite the inaccuracies of POCT glucose testing, as experienced bedside nurses were able to use the glucometer successfully and safely. The device’s results informed them when the blood glucose was out of a prescibed range and the direction of the change, and they were able to adjust the CII accordingly.
Keywords: quality improvement; glucose management; point-of-care testing; critical care.
Achieving glycemic control in patients with and without diabetes during coronary artery bypass graft (CABG) surgery is associated with reduced perioperative morbidity and mortality and improved long-term survival.1 Hyperglycemia has detrimental effects on the cardiovascular system and insulin has beneficial effects on the ischemic myocardium.2 The current recommendations of the Society of Thoracic Surgery regarding blood glucose management include the use of continuous insulin infusions (CII) during and after surgery in the critical care unit,3 keeping blood glucose in a moderate range. Glucometers are commonly used in the critical care perioperative setting for point-of-care testing (POCT) for timely determinations of blood glucose levels for patients on CII.
POCT for glucose monitoring is a valuable tool for managing patients with diabetes in the outpatient setting. Evolving from urinary test strips that depended on a colorimetric model, glucometers now incoroporate digital technology that allows patients to determine their blood glucose using a drop of blood from a fingerstick. The US Food and Drug Administration’s approval for most glucose POCT technology includes home use by diabetic patients and use in the hospital setting, with the exception of critically ill patients, who may be affected by hypoxemia, poor capillary perfusion, tissue edema, severe anemia4 or other pathophysiologic states that could impact the accuracy of the devices. For example, poor peripheral perfusion related to shock or vasoconstrictors and interstitial edema are variables that could contribute to an erroneous reading. Therefore, many glucometers used in the critical care setting are being used off-label. Because much of the current POCT technology for glucose monitoring may provide erroneous results in certain ranges and in some clinical settings, the safety of most glucometers has been called into question.5,6
Given the concern regarding the potential inaccuracies of commonly used glucometers in the critical care setting, we undertook a quality improvement project to analyze the clinical performance of the glucometer currently used in our critically ill postoperative cardiac surgery population. The cardiac surgery division policy at our institution is to place all patients, both diabetic and nondiabetic, on a CII intraoperatively and to continue the infusion for at least 24 to 48 hours postoperatively. The CII start rate is determined utilizing the division’s Insulin Start Chart, and then the CII is adjusted according to the nomogram through the postoperative course. Both the Insulin Start Chart and nomogram have been previously described by Kramer et al.7
Currently, POCT of glucose in all post cardiac surgery patients is done hourly or more frequently in the first 24 to 48 hours after surgery in order to adjust the CII. In patients undergoing the stress of cardiac surgery, the action of insulin is counter-regulated by glucagon, epinephrine, norepinephrine, cortisol, and growth hormone. The resulting varying degrees of insulin resistance in this population of patients requires close monitoring of blood glucose, keeping it in a prescribed range, which in our center is 110 to 150 mg/dL, both in diabetic and nondiabetic patients. Frequent laboratory and POCT determinations of glucose are made. Providers and bedside nurses adjust the CII according to central laboratory values, POCT values, and trends, as previously described.7