A lab result has three parts: the value, the unit, and the reference range. Units differ because labs use conventional units such as mg/dL and ng/mL, or SI units such as mmol/L and nmol/L. The measurement has not changed; the scale has. To compare two reports, convert to the same unit first.
Conventional units and SI units
India and the United States mostly use conventional units. The United Kingdom, Canada, Australia, and much of Europe use SI units for many tests. A single report can mix both: cholesterol in mg/dL, HbA1c in percent, and TSH in mIU/L.
Neither system is more accurate. They are conventions, and every conversion below is a change of scale, not a change of result.
How to read unit notation
| Notation | Means |
|---|---|
| mg/dL | Milligrams per decilitre. |
| mmol/L | Millimoles per litre. |
| g/dL | Grams per decilitre. |
| ng/mL | Nanograms per millilitre. |
| pg/mL | Picograms per millilitre. |
| mIU/L and uIU/mL | Milli-international units per litre and micro-international units per millilitre. For TSH, the numeric scale is the same. |
| 10^3/uL | Thousands per microlitre. The same count as 10^9/L. |
| % | Percent, used for HbA1c on the DCCT scale and for some differentials. |
| mmol/mol | Millimoles per mole, the IFCC scale for HbA1c. |
Common conversions
Multiply the value in the left column by the factor to get the value in the right column. The factors are approximate; keep the precision the report gives you.
| Marker | From | To | Multiply by |
|---|---|---|---|
| Glucose | mg/dL | mmol/L | 0.0555 |
| Total cholesterol, LDL, HDL | mg/dL | mmol/L | 0.0259 |
| Triglycerides | mg/dL | mmol/L | 0.0113 |
| Creatinine | mg/dL | umol/L | 88.4 |
| Uric acid | mg/dL | umol/L | 59.5 |
| Bilirubin | mg/dL | umol/L | 17.1 |
| Calcium | mg/dL | mmol/L | 0.25 |
| Vitamin D (25-OH) | ng/mL | nmol/L | 2.496 |
| Vitamin B12 | pg/mL | pmol/L | 0.738 |
| Testosterone | ng/dL | nmol/L | 0.0347 |
| Haemoglobin | g/dL | g/L | 10 |
| C-reactive protein | mg/dL | mg/L | 10 |
| Ferritin | ng/mL | ug/L | 1 (same) |
| TSH | mIU/L | uIU/mL | 1 (same) |
HbA1c uses a different kind of conversion: from the DCCT percentage to the IFCC scale, multiply (value - 2.15) by 10.929. To estimate average glucose in mg/dL from HbA1c percent, multiply by 28.7 and subtract 46.7.
Worked conversions
- Glucose 99 mg/dL is about 5.5 mmol/L.
- Total cholesterol 200 mg/dL is about 5.2 mmol/L.
- Creatinine 1.0 mg/dL is about 88 umol/L.
- Vitamin D 30 ng/mL is about 75 nmol/L.
- HbA1c 6.0 percent is about 42 mmol/mol.
Why converting is not the same as comparing
Units are one difference between two reports. Assays and calibration are another. Two labs can report the same sample with a small difference even after conversion. A conversion makes the numbers comparable in scale; it does not make them identical in method.
That is why a same-lab comparison is cleaner, and why a 5 percent move between labs is often the assay rather than you.
What Doctano does
- Stores each value with the unit that was printed.
- Normalizes names and units so the same marker lines up across reports.
- Keeps the reference range from the report the value came from.
- Shows a marker's history in one consistent unit.
The one rule to remember
Read the unit before the number. If two reports use different units, convert before you compare. If they use the same unit but different labs, read the reference range and the method before you treat a small difference as a change.