Blood glucose measurements in the critically ill: more than just a blood draw

Abstract

A crucial determinant for the success of intensive insulin therapy in critically ill patients is the frequent and accurate measurement of blood glucose values with immediate feedback of results. In general, therefore, this is achieved by point-of-care testing, raising the question of the best way of monitoring blood glucose. Corstjens and coworkers, in the previous issue of Critical Care, demonstrate that, in spite of good correlation to "conventional" laboratory glucose assessment, absolute glucose levels may differ systematically. This commentary reviews the problems of glucose measurements arising from matrix effects, interferences and the use of different assays.

Figure 1

Glucose concentration: water content expressed as percent volume. With a hematocrit of 0.4 and a water content for the fraction 'Cells' of approximately 70%, the total water content will be 28% of total volume (whole blood). From the 60% plasma volume, 90% will be water, thus giving a water content of 54% for the plasma portion of whole blood. The total water content of whole blood then is 82% (54% + 28%). For a hematocrit of 0.4 the plasma/whole blood ratio of water content is 0.9/0.82 = 1.10, which reflects the 10% higher glucose values in plasma compared to whole blood.