How do you convert between blood sugar units?
· 2 min
A blood sugar number with no unit is incomplete information, because the two scales differ many times over. Blood sugar’s two writings convert into each other by a fixed factor. HbA1c’s two scales convert not by a factor but by a published relationship. Which gets printed is decided at the country level.
A blood sugar device and a lab’s method both measure one thing: the blood’s glucose concentration. That concentration is written on paper two ways. One divides mass by volume, giving the result in milligrams per deciliter. The other counts the amount of substance, writing it in millimoles per liter. Both ways describe the same concentration.
The split is historical. Some countries kept the mass-based writing; the rest, moving to the international unit system, began printing results as an amount of substance. The step between them is fixed, because a glucose molecule weighs the same whichever country measures it. Dividing the mass-based number by 18 gives the amount-of-substance scale; multiplying by 18 brings it back. The conversion does not shift by country, is not adjusted per device, not updated over the years.
On paper, the divergence is sharper in the line labeled HbA1c, A1c, or glycated hemoglobin. One scale reports a percentage, tied to the older writing the large clinical trials used. The other counts directly how much of the hemoglobin carries sugar, writing it in millimoles per mole. So the number’s order of magnitude shifts completely. The conversion here is not one multiplication but a relationship with slope and starting point, published by international agreement and checked regularly. The lab printing the result and the health care team can both state the same result on the other scale; no information escapes the reader.
| Measure | One scale | Other scale | Relationship |
|---|---|---|---|
| Blood sugar | Milligrams per deciliter (mg/dL) | Millimoles per liter (mmol/L) | Converted by a fixed factor |
| HbA1c | Percent (NGSP/DCCT scale) | Millimoles per mole (IFCC scale) | Converted by a published relationship |
Which scale gets printed is decided at the country level. In some places both writings go on the paper together, elsewhere only one. Seeing one scale on a foreign report is therefore ordinary, and that one scale may not be the reader’s usual one.
Most often, the paper itself says which scale it uses. The unit sits right beside the result or in the column heading; that small print is among the page’s most decisive details. A second number may appear in parentheses. It is either the same result on the other scale, or the lab’s reference range, or a note about the method. The unit printed beside the parentheses tells you which.
One more number carries the lab result into the language of everyday measurements: estimated average glucose. Its own article stands outside this page.
On a test done abroad, the small print right beside the result carries one of four abbreviations: mg/dL, mmol/L, the percent sign, or mmol/mol. Every value on the page is read on that abbreviation’s scale.
Sources
- D’Orazio P, Burnett RW, Fogh-Andersen N, et al. Approved IFCC recommendation on reporting results for blood glucose (abbreviated). Clin Chem. 2005;51(9):1573-1576.
- Hanas R, John G; International HbA1c Consensus Committee. 2010 consensus statement on the worldwide standardization of the hemoglobin A1c measurement. Clin Chem. 2010;56(8):1362-1364.
- Weykamp C. HbA1c: a review of analytical and clinical aspects. Ann Lab Med. 2013;33(6):393-400.
- Sacks DB, Arnold M, Bakris GL, et al. Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus. Clin Chem. 2011;57(6):e1-e47.