What a workplace blood test can and can't tell you

If someone at work has suggested blood testing for the team, the person signing it off will want to know what you'd actually get out of it. A workplace blood test gives each person a clear picture of a set of their markers at one point in time, checked by our team and explained to them. It also has limits, and knowing those is part of explaining a result properly.

What each person gets

A qualified nurse or phlebotomist takes the samples at your workplace. Every result is checked by our team before it's released, and each person gets their own results explained: what each marker measures and what it can and can't show.

Most of what follows comes from UK guidance written for health professionals, and it shows why one number, read on its own, doesn't always tell the whole story.

Cholesterol is one piece

It's easy to look at an LDL cholesterol result and read it as your heart risk. On its own, it isn't.

NICE asks GPs and other health professionals to use a tool called QRISK3 to estimate a person's risk of heart and circulatory disease over the next 10 years, for most people aged 25 to 84 who don't already have it. [1] The NHS says a GP uses cholesterol levels "along with other factors, such as your age, blood pressure and health conditions". [2] NICE also says risk tools may underestimate risk in some groups of people, including people with severe mental illness. [1]

So that estimate is a job for a GP, with the wider picture in front of them. A cholesterol result is one of the pieces a GP can use.

Iron stores can look fine

Ferritin is the usual blood test for iron stores. UK gastroenterology guidance explains that ferritin rises when the body is inflamed, so a ferritin result can look normal even when someone is short of iron. [3]

That doesn't make a normal ferritin result unreliable for everyone. It means inflammation can hide low iron, so a ferritin result may need reading alongside other information.

Magnesium shows part of it

Most of the body's magnesium is held in bone and other tissues. Less than 2% is in the blood and the fluid around cells. Because of that, a 2021 review of the research says a blood magnesium result doesn't reliably show how much magnesium the body holds, and a normal result doesn't rule out a shortage. [4]

That point comes from one review rather than a UK guideline. It doesn't mean the test is wrong. It means a blood result shows part of the picture for magnesium.

Liver results and size

UK guidance on abnormal liver blood tests says that how far a result is outside the range isn't necessarily a guide to how much it matters. What matters, it says, is which test is out of range and the person's wider clinical picture. [5]

That's about liver tests, not every marker. And "not necessarily" doesn't mean a big result can be ignored.

Red cells and their causes

Red cell size is another number that's easy to read too much into. UK gastroenterology guidance says small red cells can come from low iron, but also from a blood condition called thalassaemia and from some long-term illnesses. [3] So red cell size on its own can't pin down the cause.

The same guidance says red cell size is a less reliable sign of low iron when someone also has a long-term illness, thalassaemia, or low vitamin B12 or folate. [3]

What this means for teams

Each of these examples is about one marker read on its own. That's why each person's explanation covers what each of their markers can and can't show on its own. Reading a result alongside the rest of someone's health is a job for their GP.

What a workplace blood test gives each person is a clear picture of a set of their markers at one point in time, explained to them, and something to take to their GP if a result is outside the range.

If you're the one at work who'd like this, and someone else signs it off, feel free to send them this page.

Common questions