Prefixes are written in lower case — m, d — while the litre takes a capital L so that it is not mistaken for the digit one; mmol/L and mg/dL read the same in every language. The notation mg% is an old name for the same milligram per decilitre, from “per cent” in the sense of per hundred millilitres, and new lab forms no longer use it.
A number without a unit is dangerous: 5.5 is a normal fasting glucose in mmol/L, but the most severe hypoglycaemia if it were mg/dL. Do not confuse it with HbA1c in per cent or with glucose in urine, where the thresholds are different; and plasma and whole-blood results differ by about a tenth.
A glucose meter lies on the table: a drop of blood at the edge of the strip trembles and is drawn into the capillary, while the screen alternately shows the same test in two notations — crimson 5.5 mmol/L and blue 99 mg/dL. Above the meter a ring-shaped glucose molecule slowly rotates with particles floating around it, and on the left a marker climbs the scale to the 7.0 threshold, below which the fasting normal range is marked in pink.
Quantity data sheet
concentration · blood test sheet 1/1 · rev. 2026-09Blood glucose mmol/L · mg/dL
Submultiple of mol/L (the litre is accepted for use with the SI) · quantity: amount-of-substance concentration; mass notation — 1 mmol/L = 18.016 mg/dL
One test, two numbers and a border in the middle of the Atlantic
A person with diabetes who has flown from Toronto to Chicago takes out their meter and sees 6 on the screen, while their American doctor calls “about a hundred” normal — and both are talking about the same blood. Canada, the United Kingdom, Australia, China and Scandinavia report glucose in millimoles per litre, whereas the USA, Japan, India and most of Latin America use milligrams per decilitre, and in France lab reports often give grams per litre — so the same test goes by three names around the world.
The difference lies not in accuracy or in the instruments but in what is counted: the number of glucose molecules in a litre, or their mass in a tenth of a litre. Hence the factor 18 — the molar mass of glucose, 180 grams per mole, divided by the ten decilitres in a litre — which is why the factor for cholesterol or creatinine is quite different.
This sheet shows blood with sugar molecules drifting in it, converts the diabetes thresholds from one notation to the other and explains what a glucose meter actually measures when a drop touches the strip.
01 · Definition
mole · mass · decilitreThe millimole per litre is a molar concentration: a thousandth of a mole of glucose dissolved in a litre of plasma, or about six hundred quintillion molecules. The milligram per decilitre is a mass concentration: the number of milligrams of glucose in a tenth of a litre. Since a mole of glucose weighs 180.16 grams, one number converts to the other by multiplying by 18.016.
Molar notation suits physiology better, because the body counts molecules, not grams: osmotic pressure, the work of renal transporters and metabolism all depend on the number of particles. Mass notation is older and more familiar to a chemist at the balance, and it was kept where laboratories and doctors had worked with grams for decades, so the switch to SI recommended by the WHO never happened in those countries.
The diagnosis itself does not depend on the choice of unit: the thresholds of the WHO and the American Diabetes Association are the same, just written twice — 7.0 mmol/L fasting is the same as 126 mg/dL, and 11.1 two hours after a glucose load is the same as 200 mg/dL.
How much sugar flows through a capillary
At the top is a capillary in cross-section: crimson red-cell discs float with the plasma flow, and the blue hexagons are glucose molecules — there are more or fewer of them as you move the slider. Below is the reference range for the chosen type of test: the blue zone is hypoglycaemia, light pink is normal, pink is borderline and crimson is the diabetic level; the needle sits at the current value, labelled in mg/dL above the scale and in mmol/L below it.
Why a meter and a lab differ by a tenth
Glucose is dissolved in water, and plasma contains more water than whole blood, where red cells take up a noticeable part of the volume, so the concentration in plasma is about eleven per cent higher. Laboratories and almost all modern meters report a plasma-equivalent value, and the diagnostic thresholds are set for plasma; older devices, however, were calibrated to whole blood, and their readings cannot be compared with reference values without a correction.
02 · Conversion
units · thresholds · other substancesFrom millimoles to milligrams and back
Enter a glucose value in either notation and the sheet will express it in mmol/L, mg/dL and g/L, compare it with the WHO and ADA thresholds for three types of test, and in the third tab show how the same conversion principle gives quite different factors for cholesterol, triglycerides, creatinine and bilirubin.
A rule to remember: multiply millimoles by eighteen, divide milligrams by eighteen, and a hundred mg/dL is just over five and a half mmol/L.
The factor 18 works only for glucose. Each substance has its own, because molar masses differ, and creatinine and bilirubin also use a different prefix — micromole rather than millimole. Convert a whole lab report with a single factor and you can easily be out by a factor of two or more.
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04 · Measuring instruments
strip · meter · sensor · analyserFour ways to count sugar
Colour instead of a number
On the strip pad the enzyme glucose oxidase oxidises glucose, hydrogen peroxide is released, and a dye darkens the more sugar there is. The colour is compared by eye with the chart on the vial, so the result comes in steps rather than as a figure, but that was enough for the first home tests, and the same enzyme still works in most strips today.
Sugar converted into electrons
In a modern strip the enzyme passes electrons from oxidised glucose to an electrode, and the meter measures a tiny current proportional to the concentration, then displays the converted value in the units the device is set to for its country of sale. Under ISO 15197, at values of 100 mg/dL and above, at least 95% of measurements must fall within fifteen per cent of the laboratory value.
A curve instead of a point
A continuous monitor keeps a thin filament with the same enzyme under the skin and records a value every few minutes, but it measures glucose in interstitial fluid rather than in blood, and sugar reaches that fluid with a delay. So its curve follows the blood curve with a lag of several minutes, and when sugar is falling fast the sensor may still show a reassuring number while the blood is already hypoglycaemic.
The reference method in a cuvette
A laboratory analyser uses enzymes to convert the glucose in the sample into a substance that absorbs ultraviolet light at 340 nanometres, and from the absorbance in the cuvette it reads the concentration off the calibration line. This hexokinase method is the reference against which meters are checked, and its result is what gets printed on the lab report — in the units used in the laboratory's country.
05 · Writing rules
unit · sample · type of testThe number, the unit and what was measured
A glucose result is always written with its unit and with what exactly was measured — plasma or whole blood, fasting or after a load — because each of these words changes the reference value.
The first line on the right drops the unit and so makes the number unidentifiable; the second puts millimoles on a number in milligrams and turns a normal value into a lethal one. The third confuses the decilitre with the litre and is out by a factor of ten, the fourth by a factor of a hundred; the fifth does not say which test the threshold refers to, although 11.1 means diabetes only two hours after a load or in a random measurement with symptoms; and the sixth turns the factor upside down.
06 · Neighbouring units
mole per litre · HbA1c · calorieBlood glucose has three neighbours: the mole per litre, the general unit of concentration from which the millimole comes; glycated haemoglobin HbA1c, which shows average glucose over three months; and the calorie, in which the same sugar is counted as fuel.
The chemist's molar concentration; in blood, its thousandth part — the millimole.
The blood's memory of sugar over three months, also in two units.
The same glucose as fuel: about four kilocalories per gram.
07 · Historical section
archive · from test tube to sensorHow sugar became a number, and the number became two
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Why 18 cannot be applied to the whole lab report
Bar height is the factor for converting from mass to molar notation, from zero to ninety: glucose, cholesterol, triglycerides, and creatinine and bilirubin, the last two in micromoles. Glucose is outlined in crimson; a blue outline surrounds the other substances.
The factor is the molar mass of the substance divided by ten, adjusted for the prefix, which is why it changes from line to line on the same lab report.
Where eighteen comes from
A mole of glucose weighs 180.16 grams, so a millimole weighs 180.16 milligrams. If that millimole is dissolved in a litre, each of the ten decilitres holds one tenth of its mass, that is 18.016 milligrams — and that is all there is behind the factor that doctors on both sides of the ocean argue about.
The decilitre comes from clinical tradition, in which concentrations were long expressed “per hundred millilitres”, like percentages, and as late as the mid-twentieth century lab forms said mg%. The move to the litre and the mole was part of medicine's general shift to SI, which the WHO endorsed in 1977; but in a number of countries doctors, pharmacologists and device makers decided that the risk of errors during retraining outweighed the benefit, and stayed with milligrams.
That is why glucose meters for different markets ship with different unit firmware, drug leaflets print thresholds twice, and every person with diabetes who crosses a border is constantly running a simple division by eighteen in their head.
