SI vs Conventional Lab Units
Most of the world reports laboratory results in SI units (Système International d'Unités), while the United States continues to use conventional (non-SI) units for most analytes. This creates confusion when comparing results, interpreting research from different countries, or using clinical decision tools developed in a different unit system.
Key Conversion Examples
| Analyte | SI Unit | Conventional Unit | Conversion Factor |
|---|---|---|---|
| Glucose | mmol/L | mg/dL | × 18.016 |
| Cholesterol (total/LDL/HDL) | mmol/L | mg/dL | × 38.67 |
| Triglycerides | mmol/L | mg/dL | × 88.57 |
| Creatinine | µmol/L | mg/dL | × 0.01131 |
| Urea (BUN) | mmol/L | mg/dL | × 2.801 |
| Haemoglobin | g/L | g/dL | ÷ 10 |
| HbA1c (%) | mmol/mol (IFCC) | % (NGSP/DCCT) | Formula based |
HbA1c — A Special Case
HbA1c is reported in two different ways depending on country. The older NGSP/DCCT method expresses HbA1c as a percentage (%). The newer IFCC method expresses it as mmol/mol. The conversion is: IFCC (mmol/mol) = [NGSP (%) − 2.15] × 10.929. This is not a simple multiplication factor — always specify which system a value is reported in when interpreting results.
FAQs
Why does the US use different lab units than the rest of the world?
The United States adopted conventional laboratory units before the international SI standardisation was established, and the clinical infrastructure (reference ranges, clinical decision tools, and clinical training) became deeply embedded in these units. Despite decades of advocacy from scientific organisations to adopt SI units, the high cost of system transitions and resistance to change have maintained the status quo in US clinical practice.
What is a normal fasting glucose in mmol/L vs mg/dL?
Normal fasting glucose: 3.9–5.5 mmol/L (70–99 mg/dL). Pre-diabetes (impaired fasting glucose): 5.6–6.9 mmol/L (100–125 mg/dL). Diabetes diagnosis: ≥7.0 mmol/L (≥126 mg/dL) on two occasions. These thresholds are used by WHO, ADA, and most international diabetes guidelines.
⚠️ Medical Disclaimer: Reference ranges shown are general adult population norms and may vary by laboratory, age, sex, pregnancy status, and clinical context. Always interpret lab results in the context of the reporting laboratory's own reference ranges and with appropriate clinical assessment.