The US uses conventional units developed historically for clinical use, while most of the world uses SI units adopted by the WHO. SI units express concentrations in moles per liter (substance concentration), while conventional units may use weight per volume. The difference can cause dangerous confusion in clinical settings.

Neither is inherently better. SI units are more scientifically consistent (moles relate directly to molecular quantities), while conventional units may be more intuitive for some clinical applications. The important thing is to know which system your lab uses and to convert when referencing literature.

Glucose (mg/dL vs mmol/L) is the most commonly confused conversion. A glucose of 100 mg/dL = 5.6 mmol/L. Creatinine (mg/dL vs µmol/L) is another common source of confusion: 1.0 mg/dL = 88.4 µmol/L.

Most factors are based on the molecular weight of the substance and are scientifically precise. However, some analytes have approximate factors due to variable molecular weights (e.g., proteins). The conversions provided here match standard clinical references.