One of the most confusing things when you try to make sense of a blood test is units. You find a "normal range" online, compare it to your result, and the numbers look wildly different, only to realise the two are measured in different units entirely. Here's why that happens.
The Same Quantity, Measured Differently
A measurement of a substance in the blood can be expressed in more than one way, just as a distance can be given in miles or kilometres. The underlying amount is the same; only the unit changes. For blood tests, two families of units are common:
- SI units (the International System of Units), often based on moles per litre, such as mmol/L. These are widely used in the UK and much of the world for many tests.
- Conventional units, often based on mass per volume, such as mg/dL. These are common in some other countries, including in a lot of online material.
Because the two express the same quantity differently, the numbers look different even when the actual result is identical. A value that reads as one number in mmol/L becomes a quite different number in mg/dL.
Why UK Labs Often Differ From What You See Online
A great deal of online health content originates from countries that use conventional (mass-based) units, while UK laboratories frequently report in SI units for the same tests. So a UK reader comparing their mmol/L result to an mg/dL range found online is comparing two different scales. The mismatch isn't an error in either place; it's just two conventions.
Always Read the Units on Your Own Report
Your laboratory report shows the result, the reference range, and the units, together. Because the range and the result are always given in the same units on your own report, they're directly comparable there. Problems only creep in when you take the number off your report and compare it to something measured differently elsewhere.
The Bottom Line
Units differ because the same quantity can be expressed in different ways, and UK labs often use different units from the material you'll find online. The result and range on your own report are in matching units and belong together. The confusion almost always comes from mixing units across sources, so check the units before you compare.