mg/dL vs mmol/L: Reading Blood Test Units

Pick up two blood test results from two different countries and the same measurement can look completely different. A fasting glucose of 100 in the United States and 5.6 somewhere else describe exactly the same thing. The numbers differ only because of the units, and knowing how to move between them stops a lot of needless worry — and the occasional dangerous mistake.

This article is for general education. It is not medical advice. Always discuss your own results with a doctor or pharmacist.

Two ways to measure the same thing

Most countries report blood chemistry in SI units — millimoles per litre (mmol/L) — which count the number of molecules in a volume of blood. The United States and a few others use conventional units — usually milligrams per decilitre (mg/dL) — which measure the mass of the substance instead.

Neither is more accurate. They are two scales for the same quantity, like Celsius and Fahrenheit. To convert, you multiply by a factor that depends on the molecular weight of the substance being measured. That is the key point: the conversion factor is different for every analyte, so there is no single number that works for everything.

The conversions that come up most

Glucose. 1 mmol/L = 18 mg/dL (18.016 to be exact). So multiply mmol/L by 18 to get mg/dL, or divide mg/dL by 18 to go the other way. A fasting glucose of 5.6 mmol/L is about 100 mg/dL — the top of the normal range. The diabetes threshold of 7.0 mmol/L is 126 mg/dL.

Cholesterol (total, LDL, HDL). 1 mmol/L = 38.67 mg/dL. A total cholesterol of 5.2 mmol/L is about 200 mg/dL.

Triglycerides use a different factor because the molecule is heavier: 1 mmol/L = 88.57 mg/dL. This is exactly why you cannot reuse the cholesterol factor for triglycerides even though they appear on the same lipid panel.

Creatinine flips the size of the units: 1 mg/dL = 88.42 µmol/L, because creatinine is reported in micromoles. A creatinine of 1.0 mg/dL is about 88 µmol/L.

HbA1c is its own special case, covered below.

Why you cannot use one factor for everything

Molecular weight is the reason. A millimole is a fixed number of molecules (strictly, a fixed fraction of Avogadro's number). To turn that count into a mass in milligrams, you multiply by how much one mole weighs — and that weight is different for glucose, cholesterol, creatinine and everything else. Triglyceride molecules are far heavier than glucose molecules, so the same molar concentration corresponds to a much larger mass. That is the whole story behind the different factors.

This is also why converting by hand is risky: it is easy to apply the glucose factor of 18 to a cholesterol value and be off by a factor of two. A unit converter that knows the analyte removes that error.

HbA1c: percent, mmol/mol and average glucose

HbA1c does not use mg/dL at all. It is reported either as a percentage (the older NGSP/DCCT scale) or in mmol/mol (the IFCC scale), and it is often translated into an estimated average glucose so patients can relate it to their day-to-day readings.

• IFCC (mmol/mol) = 10.929 × (NGSP % − 2.15).
• Estimated average glucose (mg/dL) = 28.7 × A1c% − 46.7.

So an HbA1c of 7.0% is 53 mmol/mol and an average glucose of about 154 mg/dL (8.6 mmol/L). A word of caution: HbA1c can mislead in anaemia, pregnancy, recent blood loss or certain haemoglobin variants, because it depends on how long red blood cells survive.

Reference ranges travel with the units

When you convert a value, remember to convert the reference range too, or compare against the range printed on the same report. Normal fasting glucose is "70–99 mg/dL" or "3.9–5.5 mmol/L" — the same range in two scales. A number only means something next to the range for that lab, that method and that population.

The practical takeaway

• The scale is just a unit; the biology is identical.
• Every substance has its own conversion factor, set by its molecular weight.
• Never reuse one analyte's factor for another — glucose and triglycerides are the classic trap.
• Always read the value against its own reference range.

When you need to move between scales quickly, a lab unit converter that already knows the factor for each test — glucose, cholesterol, triglycerides, creatinine, HbA1c and the rest — is faster and far safer than multiplying by a number you half-remember. And if a result looks alarming after you convert it, that is exactly the moment to call your clinician rather than the internet.