Lab units explained: mg/dL, mmol/L and µmol/L

Two systems for the same quantity

Lab values are reported in two unit systems. Conventional units measure the mass of a substance per volume, for example milligrams per deciliter (mg/dL). SI units count the amount of substance per volume, for example millimoles per liter (mmol/L). Both describe the same concentration; which one appears on your report depends on the country and the laboratory.

How the conversion works

The link is the molar mass of the substance in g/mol: mmol/L = mg/dL × 10 ÷ molar mass. Example glucose (molar mass 180.16): 100 mg/dL × 10 ÷ 180.16 = 5.55 mmol/L. Example creatinine (molar mass 113.12): 1.0 mg/dL equals 88.4 µmol/L.

Why every value has its own factor

Because molar masses differ. Cholesterol (386.65 g/mol) has a factor of 38.67 between mmol/L and mg/dL, triglycerides (about 885 g/mol) have 88.57. A factor can therefore never be carried over from one test to another.

Units that cannot be converted via molar mass

Enzyme activities (U/L and µkat/L), percentages, international units (IU) and equivalents (mEq/L) follow their own rules. For mEq/L the rule is mEq/L = mmol/L × charge, so both numbers are equal for sodium and potassium but not for calcium and magnesium.

Rounding

Calculate with the exact factors and round only the result. Rounding intermediate steps adds small errors that become visible for narrow ranges such as sodium or calcium.