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November 23, 2001

Blood Tests and Insulin Injections

Question from the United Kingdom:

I have just changed from a meter which measures whole blood to one which measures plasma, and I can only find charts for plasma-to-whole-blood conversion which use mg/dl units, and I use mmol/L. Could you point me at a plasma-to-whole-blood conversion chart for mmol/L? On whole blood, my targets are 5-8 mg/dl [90-144 mmol/L] before meals, so I'm guessing that will be 5.6-9 mmol/L [101-162 mg/dl] and 5-10 mmol/L [90-180 mg/dl] two hours after meals (now 5.6 - 11.2 mmol/L [101-202 mg/dl] ).


There are several good reasons for expressing glucose levels in terms of plasma rather than the slightly lower whole blood values. The meter actually measures whole blood of course; but makes an electronic adjustment to the read out and the package insert with your new meter should give the conversion figure which varies between 10-15%. I imagine though that your need to convert mmol/l in plasma to the old whole blood values will be temporary until you get used to the new system. For this reason I would depend on something very simple either using a hand calculator to multiply the plasma reading by the actual conversion factor or much more simply and accurate enough for the purpose just convert to whole blood in your head by reducing the plasma level by 10% — e.g., a reading of 5.0 mmol/L [90 mg/dl] would be approximately equivalent to 4.5 mg/dl [81 mmol/L] in whole blood.

Additional comments from Dr. Tessa Lebinger:

I don’t know of any mmol conversion chart to point you to, but if you use the mg conversion chart you can still calculate whole blood in mmol. The ratio of plasma to whole blood should be the same whether you use mmol or mg. Most of the time, it isn’t really necessary to convert, but if you really want to, the following equation should work. Whole blood (mg)/ plasma(mg) = whole blood (mmol)/ plasma (mmol) or whole blood (mmol) = whole blood (mg) x plasma (mmol) divided by plasma (mg)