(POC) glucose monitors. A few centers are using POC blood-gas analyzers to measure glucose in critical care units, and a small but slowly growing number of centers are using continuous glucose monitoring systems (CGMS) in specific settings.23–25 At present, most inpatient centers rely heavily on POC glucose monitors for the vast majority of their glucose measurements, in large part because of the ease of use and timeliness. It should be remembered, however, that the central hospital laboratory method is highly accurate and relatively free of interfering substances, and should always be used when a POC glucose level needs confirmation or when the accuracy of the POC meter is in question.
From a systems perspective (see Figure 4.1), several key aspects of POC glucose monitoring should be evaluated constantly:
1. The choice of a POC glucose monitoring system;
2. The frequency of validation of the glucose strips and the monitors;
3. The robustness of the staff education programs and proficiency testing; and
4. The degree to which glucometrics are used to improve in-hospital glycemic control and to reduce errors in insulin therapy.
Figure 4.1—Systems perspective for medical errors.
These systemic issues are key because a significantly inaccurate glucose level may lead to relatively large insulin dosing errors. Additionally, the vulnerability of the particular POC glucose meter may make it unsafe in selected circumstances, and no POC glucose meter should be used without a clear understanding of its limitations and when it is unsafe to use that meter to monitor glucose levels.
Glucose Monitoring Errors
When insulin is to be administered, the glucose level at that time point is important data needed to select the correct dose and the optimal route of administration, and an important safety check to avoid iatrogenic hypoglycemia. Other factors that should be considered to avoid hypoglycemia include the clinical context of the patient and the factors that can be expected to influence insulin resistance of the patient and the expected effect of the dose of insulin. The patient’s vulnerability to hypoglycemia also needs to be considered. For example, an 80-year-old patient with chronic renal failure and hypoglycemic unawareness will be extremely vulnerable to fasting hypoglycemia.26 This patient requires careful and conservative insulin dosing and frequent glucose monitoring. To do otherwise would be an error. Likewise, if a patient receives intravenous (IV) insulin, but the frequency of glucose monitoring is every 6 h, the infrequent monitoring is an error, because the change in glucose levels after IV insulin can be rapid. In this case, the probability of harm increases greatly when the glucose measurements are too far apart.
Often the health-care team uncritically assumes that the monitoring method is accurate. In 2009, the U.S. Food and Drug Administration (FDA) reported the death of 13 patients with diabetes on peritoneal dialysis who were being monitored with a widely used glucose meter that followed a glucose dehydrogenase method, using a pyrrolo-quinoline quinone (PQQ) coenzyme to measure glucose concentrations. This method is less sensitive to ambient oxygen levels and some other interfering substances and will falsely measure maltose as glucose. Because patients who receive icodextrin in their peritoneal infusate will slowly metabolize the icodextrin to maltose, this process can lead to high maltose levels in the patient’s blood up to 2 weeks after the last peritoneal dialysate. In each of the cases reported by the FDA, the health-care providers used a POC glucose meter of the PQQ type, resulting in an erroneously measured and markedly elevated POC glucose, which was used to order inappropriately large doses of insulin and resulted in severe hypoglycemia, coma, seizures, and ultimately death. Despite the 2009 FDA report, however, a recently published report indicates patients in some hospitals and outpatient centers are still using meters of this type with patients who either are or recently have been on peritoneal dialysis with similar lethal outcomes.
The FDA, in response to strong concerns presented by multiple organizations and individuals, most recently tightened the standards for accuracy of POC glucose meters being used in the hospital setting. The new standards recommended by the FDA are expected to be implemented by the end of 2015, and state that 99% of the glucose values must be within ±10% of the glucose levels ≥70 mg/L and within ±7% of values <70 mg/dL. Also, 100% of the values must be within ±20% of the glucose levels ≥70mg/dL or within ±15% of those values <70mg/L. The latter standard is crucial, as other authors have shown the potential for severe errors in clinical decision making when the result obtained by the POC glucose meter is so far from the true value of glucose so as to be misleading.27
Recently Scott et al.28 pointed out the weaknesses of the POC meters in use, especially in the operating room29 and in intensive care settings. Variations in hematocrit, blood pH, oxygen saturation, fever, dehydration, or ketosis all had varying and sometimes significant effects on the results obtained with a POC glucose meter,30 and in critical care, most POC glucose meters were clearly inferior in accuracy to the results obtained by intra-arterial samples tested for glucose levels on POC blood-gas analyzers. Arterial samples are used in preference to fingerstick glucose samples in patients who have circulatory compromise, and the capillary values of glucose may diverge significantly from the arterial values when acidosis or circulatory collapse alters the capillary blood flow.
Errors in monitoring may be due to sampling errors when the site from which the blood sample is obtained does not accurately represent the true blood glucose value. In the operating room, severe hypoglycemia has resulted when the anesthesiologist did not realize the arterial line that they were sampling from was being flushed with 5% glucose solution.31 As a result, insulin was given in error to a hypoglycemic patient. Similar errors can occur when the line from which the specimen is being drawn has “dead space” and that line cannot be flushed adequately, diluting the sample and giving an erroneous result.31
Each hospital must be aware of the strengths and the weaknesses of the POC meters used in their institution. POC meters that use a glucose-oxidase method will tend to give falsely elevated glucose values when the patient is hypoxic, and lower glucose levels when high levels of acetaminophen are present. Glucose-dehydrogenase meters will give more accurate levels in the presence of hypoxia, but falsely lower results in the presence of dopamine infusions and falsely higher glucose levels when higher levels of acetaminophen are present.
In addition, the storing and the care of the glucose monitoring strips are crucial. If the bottle containing the strips is left open, the strips may deteriorate rapidly. If the strips are stored at a temperature ≥39.2°C, the strips may give falsely low blood glucose values, and when the strips are stored <20°C, the strips may give falsely high values. Strips also may be destroyed by extremes in temperature or humidity.
Some of the strips hospitals now use do not have good corrections for hematocrit levels. This is particularly crucial in intensive care and operating room suites, where wide variations of hematocrit are common. Falsely elevated glucose levels in POC meters when patients are anemic occur and most meters do not have a specific method to deal with hematocrit variation. Cembrowski, a NICE-SUGAR investigator,32 found that some of the batches of his POC glucose meter strips did not correct for hematocrit variations as claimed by the manufacturer. He concluded that the error due to the faulty strips may have misled his team and caused the diagnosis of hypoglycemia to be either delayed or missed entirely.32
In addition, proper training of staff on the proper technique of performing POC glucose monitoring is crucial. Too often, POC glucose monitoring is done in hospitals by the most poorly trained and poorly supervised staff. The resultant errors can be quite large. For example, if a person fails to clean the finger properly before obtaining a glucose sample, the result may be as much as 35% or more above the true glucose level, while failure to adequately dry the finger can dilute the sample and give a falsely low value that may be as much as 25% below the true glucose level. Such large deviations from the true blood glucose may cause an outlier that can mislead the prescribing health-care provider.33
Glucometrics
Evidence-based metrics can be used to evaluate not only the