Lab units: mg/dL vs mmol/L, and which markers convert exactly
mg/dL weighs a substance per decilitre of blood; mmol/L counts its molecules per litre. Both exist because countries adopted different unit systems. A marker converts exactly only when one fixed factor links the two units for one substance, as with glucose or cholesterol. Where one name covers several forms, as with calcium, no single factor is safe.
Not medical advice. This guide describes what the parts of a lab report are and how they are recorded. It does not say what any result means for a person, and it does not recommend treatment, supplements or targets.
Why two unit systems exist
Mass units, such as mg/dL, say how much a substance weighs in a volume of blood. Molar units, such as mmol/L, say how many molecules are present. The SI system counts amounts of substance in moles, and in the decades after it was adopted many countries moved their laboratories to it. The United States largely kept mass units, so both systems are in everyday use.
For one substance the two are linked by its molar mass. Going from mg/dL to mmol/L divides by the molar mass in grams per mole divided by ten, because a decilitre is a tenth of a litre. That is why glucose, cholesterol and triglycerides each have their own factor, and why a factor must never be borrowed for another marker: 100 mg/dL is 5.6 mmol/L for glucose but 1.13 mmol/L for triglycerides.
Some pairs need no molar mass. g/dL and g/L differ by exactly ten, and for sodium, potassium, chloride and bicarbonate, mEq/L and mmol/L are the same number. A few markers have no single factor at all and should stay in the unit printed.
Choosing a unit for your records
Five steps.
Read the unit on every result
Check the unit beside each number, not only the number. It may sit in its own column, after the result or at the end of the range cell, and some reports print two units for one marker. The same marker can change unit between two labs.
Pick one unit per marker
Choose the unit your sheet will show, often the unit of your latest report, and write it in its own column.
Convert only with an exact rule
Use the table below to see whether a marker has one. For the factors, use the lab units conversion page, and apply them in the right direction.
Convert the range too
A range is printed in the unit of its result. Convert its limits with the same factor, or keep the printed range beside the converted one.
Keep the original
Keep the printed value and unit in their own columns, so any conversion can be checked later.
Which markers convert exactly
Groups as BioPanel treats them. Factors are on the conversion page.
| Marker | Units you may see | One exact factor? |
|---|---|---|
| Glucose | mg/dL, mmol/L | Yes, its own factor from molar mass |
| Total, LDL, HDL and non-HDL cholesterol | mg/dL, mmol/L | Yes, one factor for all four |
| Triglycerides | mg/dL, mmol/L | Yes, a different factor from cholesterol |
| Creatinine | mg/dL, µmol/L | Yes |
| 25-hydroxy vitamin D | ng/mL, µg/L, nmol/L | Yes. ng/mL and µg/L are the same unit |
| Hemoglobin, albumin, total protein | g/dL, g/L | Yes, exactly ten |
| Hematocrit | %, L/L | Yes, exactly 100 |
| Sodium, potassium, chloride, bicarbonate; ALT, AST, GGT, ALP; ferritin, PSA | mEq/L or mmol/L; U/L or IU/L; ng/mL or µg/L | Same number, only the spelling differs |
| HbA1c | %, mmol/mol | No, the relation has an offset |
| Lp(a) | mg/dL, nmol/L | No, particle size varies |
| Calcium, bilirubin, BUN and urea, testosterone | Mass or molar units | Not converted by BioPanel: one name can cover several forms, or the units count different things |
What goes wrong
Most errors come from a factor used in the wrong place.
- Inverting the factor. Multiplying where the factor should divide gives a result wrong by the factor squared. For glucose that is a number more than three hundred times too large.
- Borrowing a factor. Cholesterol and triglycerides are both reported in mg/dL and mmol/L, but their molar masses differ, so their factors differ.
- Converting HbA1c with one multiplier. The two scales are linked by a formula with an offset, so a single factor is wrong almost everywhere.
- Reading g/L as g/dL. Hemoglobin of 135 g/L and 13.5 g/dL is the same amount, but without the unit one looks ten times the other.
- Treating a name as one substance. Calcium and bilirubin each cover several forms, so a factor can be applied to the wrong one.
- Rounding before converting. Rounded input carries into the result. Convert from the printed value and round only for display.
How BioPanel helps
A single lab report comes back in the units it printed, spelling tidied. When reports meet in a merged series, BioPanel converts only where the factor is exact, as in the table above, and shows the factor beside each converted value. Anything else stays in the printed unit and is marked as mixed. The long CSV keeps the printed value and unit in their own columns, and a flag is always decided in the unit the report printed. Free: 1 file per day, up to 10 pages, no account. Files over 10 pages are converted up to page 10 on the free allowance; a page code converts the whole file, up to 150 pages. See pricing.
Questions people ask
Why do some labs use mg/dL and others mmol/L?
Countries adopted different unit systems. SI units count molecules per litre, while mass units weigh a substance per decilitre. Which one a report uses depends on the country, the lab and sometimes the marker.
Is mg/dL the same as mg%?
In practice, yes. mg% is an older way to write milligrams per 100 millilitres, which is the same as mg/dL. BioPanel treats the two spellings as one unit.
Which markers convert with one exact factor?
Glucose, cholesterol fractions, triglycerides, creatinine and 25-hydroxy vitamin D between mass and molar units, plus unit pairs that differ only by a power of ten or a spelling. See the table above.
Why does HbA1c not convert with a factor?
The percent and mmol/mol scales are linked by a formula with an offset, and each is tied to its own reference method. A single multiplier cannot reproduce it.
Does converting a value change its reference range?
Not the range itself, only how it is written. It is printed in the unit of its result, so convert its limits with the same factor to compare in a new unit. The lab's flag still refers to the printed unit.
Is mEq/L the same as mmol/L?
For sodium, potassium, chloride and bicarbonate, yes: each ion carries one charge, so one milliequivalent equals one millimole. For ions with two charges, such as calcium, it does not.