International abbreviations are written in capitals: LDL-C is low-density lipoprotein cholesterol, HDL-C high-density lipoprotein cholesterol, TG triglycerides, TC total cholesterol. The C after the hyphen matters: LDL is the particle itself, LDL-C is the cholesterol it carries, and that is exactly what the test measures.
The same millimole per litre weighs differently for different substances: for cholesterol it is 38.67 mg/dL, for triglycerides 88.57, for glucose 18.016. So a lipid panel cannot be converted with a single factor — the triglycerides would come out at half their true value.
In the centre is an LDL particle: a golden core of cholesterol esters, a shell of phospholipids slowly rotating, and the apoB-100 protein ribbon wrapped around the particle, gently swaying. Two small blue HDL particles circle it in orbits, on the right a droplet of triglycerides rises and sinks, and the slider of the double ruler reaches 2.6 mmol/L, which reads as 100 mg/dL on the lower scale.
Quantity data sheet
concentration · lipid panel sheet 1/1 · rev. 2026-09Cholesterol and lipids mmol/L · mg/dL
Quantity: molar and mass concentration; mmol/L — SI notation, mg/dL — non-SI
Three substances, three exchange rates
Someone who had a lipid panel done in Toronto brings the report to a cardiologist in Boston; LDL cholesterol is written there as 3.4, and the doctor says this is impossible: no living person has an LDL below thirty. Both are right, because in Canada, as in almost the whole world, lipids are reported in millimoles per litre, whereas in the USA they are reported in milligrams per decilitre, and 3.4 mmol/L of cholesterol is 131 mg/dL.
The catch is that the factor between these notations is different for each substance, because a millimole is a number of molecules and a milligram is their mass: a cholesterol molecule weighs twice as much as a glucose molecule, and a nominal triglyceride molecule twice as much again as cholesterol. So one lab report cannot be converted with one number: cholesterol is multiplied by 38.67, triglycerides by 88.57, glucose by 18.016.
This sheet shows how LDL is calculated from total cholesterol, HDL and triglycerides, how a fatty dinner turns plasma milky and breaks the formula, and why one of the lipids, Lp(a), has no factor at all.
01 · Definition
molar mass · Friedewald · targetsThe millimole per litre is a molar concentration: how many thousandths of a mole of a substance there are in a litre of plasma; the milligram per decilitre is a mass concentration: how many milligrams of it there are in a tenth of a litre. The bridge between them is the molar mass of the substance divided by ten, because a decilitre is ten times smaller than a litre: cholesterol weighs 386.65 g/mol, so the factor comes out at 38.67.
Triglycerides are trickier, because they are not one substance but a mixture of molecules with different fatty acids. For conversion it was agreed to count them all as triolein, a molecule with three oleic acid residues and a mass of 885.7 g/mol, so the factor 88.57 is not a property of the sample but a convention, and for an individual sample it may be off by a few per cent.
In most laboratories LDL cholesterol is not measured but calculated: HDL cholesterol is subtracted from total cholesterol, and so is the share carried by triglyceride-rich particles, which the Friedewald formula estimates as one fifth of the triglycerides in mg/dL. The formula works as long as triglycerides do not exceed 4.5 mmol/L; above that, LDL is measured by a direct method.
What LDL is made of
On the left is a tube of plasma after a night in the fridge: when triglycerides are high it turns cloudy and a creamy layer collects on top. On the right is the same plasma magnified: the large yellow particles are triglyceride-rich VLDL and chylomicrons, the medium pink ones are LDL, the small blue ones HDL; their number is proportional to the cholesterol each class carries. The sliders set the measured total cholesterol, HDL and triglycerides, and the sheet calculates LDL by itself.
3.0
116 4.9
190 6.5 mmol/L
251 mg/dL
Why triglycerides are divided by five, and in millimoles by 2.2
VLDL particles carry roughly five times less cholesterol than triglycerides when counted in milligrams, and in 1972 Friedewald simply took this ratio to be constant. In millimoles the same ratio looks different, since a triglyceride molecule is 2.29 times heavier than a cholesterol molecule, and the European version of the formula divides by 2.2 — a rounded number, slightly different from the exact conversion. So LDL calculated in the two systems can differ by a few hundredths of a millimole, even though the samples are the same.
02 · Conversion
one number · molar masses · ESC targetsOne number — five different tests
Enter a value and choose the notation it is given in, and the sheet will show what it becomes for each substance of the lipid panel and the chemistry panel, how much a mole of each of them weighs, and where the value stands relative to the LDL targets.
A rule to remember: cholesterol is multiplied by almost forty, triglycerides by almost ninety, glucose by eighteen.
Laboratories report lipoprotein(a) sometimes in mg/dL and sometimes in nmol/L, and there is no constant factor between the two: Lp(a) particles in different people carry apo(a) proteins of different lengths and differ in weight, so a conversion with an average coefficient can be off by a factor of one and a half. For Lp(a), use the unit in which the threshold is given in the guideline.
| {k} | {v} | {note} |
{text}
04 · Measuring instruments
photometer · ultracentrifuge · strip · NMRFour ways to weigh the fat in blood
Cholesterol turned into colour
The enzyme cholesterol oxidase oxidises the cholesterol in the sample, the peroxide released stains the reagent pink, and the photometer calculates the concentration from how much the transmitted light has dimmed. Triglycerides and HDL are measured in the same way, each with its own enzymes, and this is the method inside every large analyser that delivers a lipid panel in a few minutes.
Layers by density
Lipoproteins are in fact named after their density, because they were first separated in an ultracentrifuge: at a hundred thousand revolutions per minute, light fat-rich particles float to the top while dense protein-rich ones sink. This method, beta-quantification, underlies the reference of the US Centers for Disease Control and Prevention, to which laboratories around the world are linked through a chain of calibrators.
A lipid panel from a fingertip
Portable analysers for pharmacies and mobile screening put a drop of fingertip blood onto a cassette, where the blood is separated on a membrane and reaches several reagent pads at once, and the device reads their colour by reflected light. The result is usually printed in the notation used in the country of sale, so the same device shows different figures in Europe and in the USA.
Particles instead of millimoles
Nuclear magnetic resonance tells apart the lipid signals from particles of different sizes and makes it possible to count the LDL particles themselves, in nanomoles per litre, rather than just the cholesterol they carry. Such a count, like measuring the apoB protein in grams per litre, sometimes predicts risk more accurately, since two people with the same LDL can have different numbers of particles, small or large.
05 · Writing rules
substance · notation · factorEach substance has its own factor
A concentration is written together with the name of the substance and the unit, and it is converted with the factor of that particular substance, which is not carried over to the neighbouring lines of the report.
The first line on the right leaves the number without a unit, so there is no telling whether it is normal or a deadly low value. The second converts triglycerides with the cholesterol factor and gets half the true value, the third labels millimoles as milligrams, and the fourth converts cholesterol with the glucose factor. The fifth confuses a protein measured in grams with a molar concentration, and the last passes off an approximate Lp(a) conversion as exact, although there is no constant factor for it.
06 · Neighbouring units
glucose · mole per litre · HbA1cOn a lab report, glucose and HbA1c usually sit next to the lipids, and behind all of them stands the same molar concentration, the mole per litre, which is why laboratories switched to the SI in the first place.
The same pair of units with a factor half as large, because the molecule is lighter.
The number of molecules in a litre, which makes all tests comparable.
Another pair of scales on the same report, but linked by a shift rather than a factor.
07 · Historical section
archive · from a gallstone to a target of 1.4How a substance from a stone became a treatment target
{title}
{text}
How many mg/dL in one mmol/L
The height of a bar is the conversion factor from mmol/L to mg/dL, from zero to 88.57; it equals the molar mass of the substance divided by ten. The frame visits the substances in turn, from light uric acid to heavy triolein.
The fivefold difference between the first and the last bar is exactly what trips up anyone who converts a lab report with a single number.
The triglyceride that is not in the sample
Cholesterol is a single substance with an exact formula, and its molar mass, 386.65 g/mol, is known to the hundredth. Triglycerides, by contrast, are a family of hundreds of molecules: glycerol with three fatty acids of any length and saturation attached, and the mass of an “average” triglyceride depends on what the person has eaten over the past few weeks.
The enzymatic assay does not even measure the triglycerides themselves but the glycerol released when they are broken down, so in essence it counts molecules. To give a result in milligrams, the laboratory pretends that all the molecules are triolein and multiplies by its mass. So here mmol/L is closer to what the instrument actually measured, while mg/dL has a convention built into it.
The creamy layer in the tube on the left is chylomicrons after a fatty meal: they are so large that they float to the top overnight in the fridge, and an experienced lab technician spots high triglycerides before the analyser does.
