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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Diabetes In Pregnancy Series
Sponsored by The Indian Health Service Clinical Support Center
PART 1: Screening and Diagnosis
9. Venous versus capillary and plasma versus whole blood samples
The following section is only meant for those of you
who used to go for 'extra credit' in high school. Remember that?
Have you ever been confused about the differences in glucose results between
venous, whole blood, plasma, and serum or between venous and capillary (fingerstick)
samples?
Whole blood versus plasma or serum
The glucose concentration measure in plasma or serum of a given specimen is approximately 14-15% higher than the concentration in the same sample before separation of the cells (whole blood).
This may be more than you want to know, but.......the reason for this discrepancy is that 35-45% of the volume of whole blood (depending on the hematocrit) is composed of red blood cells. Approximately 35% of the red blood cell is hemoglobin, and glucose is not distributed in this part of the cell. Therefore, for example, glucose is excluded from about 15% of a blood sample with a hematocrit of 43% (0.35 x 43 = approximately 15%). In other words, only 85% of the volume of used for the assay contains glucose, but the results are expressed for the entire volume. Thus, values obtained with whole blood are approximately 15% lower than with plasma or serum, in which glucose is distributed throughout the entire volume.
One further wrinkle is that the original GDM studies by Dr. O'Sullivan utilized Somogyi-Nelson whole blood technique. An enzymatic process analyzes most glucose samples now. These enzymatically derived values have been shown to be about 5mg/dL lower than the Somogyi-Nelson because the enzymatically derived values exclude reducing sugars.
Sample site (fingerstick or venous)
Arterial glucose levels are higher than venous concentrations because the peripheral tissues have not yet had the opportunity to extract the glucose. Capillary blood contains a mixture of arterial and venous blood and is often obtained for sampling from the pads of the fingers. If these areas are warmed or stimulated in order to increase blood flow, the arterial contribution to the sample is enhanced. (Davidson)
In the fasting state, peripheral tissues (with the exception of red blood cells in the brain) do not use glucose to a large extent; hence there is little difference between capillary and venous samples.
After a glucose challenge, depending on the amount of glucose administered and the timing of the sample, there are appreciable differences between capillary and venous samples. As would be expected, the more glucose given and the earlier the interval at which the sample is obtained, the greater the difference. For example, 1 hour after the ingestion of 100g of glucose, capillary values are 30-40mg/100ml higher than venous levels.
On average, arterial glucose is 7% greater than venous glucose.
Reflectance photometers
As technology has advanced the manufacturers have taken all the above into consideration. For example, Accu-Chek(tm) is calibrated to deliver plasma like results.
Murphy et al 1994 found a reflectance photometer to be both accurate and precise in a single operator situation. On the other hand, one most consider both the accuracy and the precision of reflectance photometers among individuals with a wide-variety of skill levels.
At this time reflectance photometers are helpful in monitoring the clinical care of a GDM patient, but they not felt to display enough accuracy and precision in a wide variety of settings to be recommended for diagnosis or screening for GDM.
Bottomline
If you are using a reflectance photometer with fingerstick samples to monitor your GDM patient, then check with the manufacturer's specifications to see to what kind of sample the device is calibrated, e.g., Accu-Chek is calibrated to deliver plasma like results.
