What the letters in a unit mean
Most lab units have two parts. The top part is the amount of the substance. The bottom part is the amount of fluid it was measured in.
Take mg/dL. The "mg" is milligrams, which is a mass, a measure of weight. The "dL" is a deciliter, one tenth of a liter. So mg/dL means milligrams of the substance in each tenth of a liter of blood.
Now take mmol/L. The "mmol" is millimoles. A mole counts molecules rather than weighing them. A millimole is one thousandth of a mole. The "L" is a full liter. So mmol/L means thousandths of a mole of the substance in each liter.
The third common unit is ng/mL. A nanogram is one billionth of a gram, and a milliliter is one thousandth of a liter. This unit is used for substances that show up in very small amounts. One ng/mL is the same as one microgram per liter (ug/L), so you may see either form.
Sources: NIST, SI Units (nist.gov/pml/owm/metric-si/si-units), which lists the mole as the SI base unit for amount of substance and the kilogram for mass.
Why two sets of units exist
There are two main habits in the world. In the United States, many labs report in conventional units such as mg/dL. Much of the rest of the world, and many journals, use the International System of Units, called SI. In SI, amounts of substance are counted in moles, so you see mmol/L.
Neither is wrong. They are two ways of writing the same measurement. A lab report from a U.S. lab and a report from a lab in another country can hold the same result and look very different.
Sources: NIST, SI Units; MedlinePlus (U.S. National Library of Medicine), which gives fasting blood glucose in both mg/dL and mmol/L.
How to convert between mg/dL and mmol/L
The factor depends on the substance, because every molecule has its own weight. You cannot use one number for every test.
Glucose is the common example. Its molecular weight is about 180 grams per mole. That makes one mmol/L of glucose equal to about 18 mg/dL.
- To go from mg/dL to mmol/L, divide by 18.
- To go from mmol/L to mg/dL, multiply by 18.
Here is one worked example, shown only to teach the math. A report that says 90 mg/dL becomes 90 divided by 18, which is 5.0 mmol/L. The U.S. National Library of Medicine shows the same pairing in its own examples, where 70 mg/dL is written as 3.9 mmol/L.
For any other substance, look up its molecular weight or use the factor printed by the lab. Do not reuse the glucose factor on a different test.
Sources: MedlinePlus, Blood glucose test (medlineplus.gov/ency/article/003482.htm), for the paired mg/dL and mmol/L examples.
Reading ng/mL and similar small units
Units built on nanograms or micrograms sit at the small end of the scale. The prefixes follow a simple ladder. Each step is a factor of one thousand.
- 1 gram = 1,000 milligrams (mg)
- 1 milligram = 1,000 micrograms (ug)
- 1 microgram = 1,000 nanograms (ng)
Volume works the same way, with one trap. A deciliter is 100 milliliters, not 1,000. So 1 mg/dL equals 10 mg/L. Mixing up dL and L is a common slip when comparing two reports.
Check the full unit every time. A result written as 5 ng/mL is not the same as 5 ng/dL. Since a deciliter holds 100 milliliters, 5 ng/mL equals 500 ng/dL, so the two differ by a factor of 100.
Sources: NIST, Metric (SI) Prefixes (nist.gov/pml/owm/metric-si-prefixes), for the metric prefixes.
Why the same result can sit in different ranges
Most lab reports print a reference range beside each result. The range is built from tests on large groups of healthy people. Different labs can use different methods and different groups, so the printed range can differ from lab to lab. A test can also have separate ranges for children and adults.
That is one more reason to compare like with like. Put two results side by side only when both use the same unit, or after you have converted one with the right factor. The range printed on each report goes with that report's own unit.
Labs in the United States work under the Clinical Laboratory Improvement Amendments, which Congress passed in 1988. The rules aim to make test results accurate, reliable and timely. They set quality standards for testing. They do not set one universal unit or one universal range.
Sources: MedlinePlus, How to understand your lab results (medlineplus.gov/lab-tests/how-to-understand-your-lab-results/); CMS, Clinical Laboratory Improvement Amendments (cms.gov/medicare/quality/clinical-laboratory-improvement-amendments).
A short checklist for comparing reports
- Find the unit printed next to each result.
- Check whether the two reports use the same unit before comparing.
- If the units differ, find the right factor for that exact substance.
- Read the reference range on each report, not from memory.
- Ask the ordering clinician or the lab if anything is unclear.
This article covers units and arithmetic only. Questions about what a result means belong with a licensed clinician who knows the person's full history.