Alternate site (i.e., nonfinger) testing is now widely available. Low-volume sampling with a variety of improvements in collection technique (e.g., capillary action wicking and ability to add additional sample over time) and decrease in specimen size requirement has enabled this. This greatly increases the surface area of the body available to patients who test frequently. For those, such as musicians, who use their fingers at work, this is a significant advantage. However, since the measurements are made in a compartment more distal to blood (i.e., interstitial fluid), there is an inherent delay. Figure 2 shows an example of this
This delay is of particular importance when glucose levels are rapidly decreasing (which could lead to a false overestimation) or increasing (which could lead to a false underestimation of actual glucose levels). Patients are instructed to perform standard capillary testing when this is suspected. A situation similar to this occurs with continuous glucose monitoring (CGM).
Since the publication of the DCCT (3) in 1993, HbAlc test has been elevated from a laboratory test to an actual patient outcome. This is justified by the strong association of HbAlc elevation with microvascular complications. Diabetes complications occur too slowly in most patients with diabetes to be studied as an endpoint in clinical trials of new therapies. HbAlc, on the other hand, reflects ambient glucose levels in the past 1 to
3 months. It is currently the most acceptable surrogate for predicting whether a therapy being tested is likely to result in a reduction of the chronic complication of diabetes.
Following the publication of the DCCT, there was a movement to standardize the various glycosylated hemoglobin tests that were being performed. The majority of testing is now traceable to DCCT, thanks to the National Glycohemoglobin Standarization Program (NGSP). However, the International Federation of Clinical Chemistry has long been working on an improved assay which provides values that are up to 2% lower than the NGSP values clinicians are accustomed to working with (16).
The results of changing HbAlc reporting methodology on HbAlc outcomes were found in a study in Sweden. Because of a change in testing methodology patients' glycemic control appeared to improve after conversion to a test that reported higher values. Following a subsequent change that reported lower values, patients' glucose regulation deteriorated. This happened as a result of the psychological effects of apparent worsening and improvement that was due solely to the reporting methodology (17). There is concern that any change in reference range to values that are lower than those in current use will lead similarly to worsening glycemic control.
Other proposed changes are to report HbAlc in terms of predicted mean glucose levels or as DCCT risk equivalents. The optimal method to report HbAlc should allow health-care providers to have a common goal level to work toward that is supported by the large epidemiologic studies of diabetes complications.
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