ESR blood test: why you get both ESR and CRP

The test measures the speed at which your red blood cells settle over an hour. Most labs use the Westergren method. The tube stands upright. The clear column at the top is your result, in millimetres per hour.

What happens is stacking. Your red blood cells normally push each other away slightly and float separately. Certain proteins in the blood remove that push. The cells then slide together like a roll of coins and fall faster.

Two proteins do nearly all the work. Fibrinogen gets going in a few days. Immunoglobulins need weeks.

Both belong to your response to inflammation and infection. Neither is the inflammation itself.

The normal values below are upper limits. A rule of thumb that accounts for your age sits beside them.

Those two numbers drift apart as you get older. If you are a woman of 70, the rule of thumb gives 40. Your result still prints 20.

You then get flagged as raised at a value that is entirely ordinary for your age. Every lab also applies its own limits, so read the numbers on your own result.

Almost everyone gets this value together with C-reactive protein (CRP). That is not a duplicate measurement and it is not an oversight. The two look at the same process, but at a different pace.

CRP is fast. It starts rising within hours and settles again within days. The ESR is slow. Fibrinogen needs days to build, immunoglobulins need weeks.

That difference is what makes the pair useful. Two values on two timescales tell you something one value cannot. Is something happening now, or is it already behind you?

Here is how the four possible outcomes read. If both are normal, there is no current sign of inflammation. If only CRP is high, something is recent and the ESR simply has not caught up.

If both are high, something has been running for a while. And if only the ESR is high, it usually concerns something already past.

That last combination causes the most unnecessary worry. A raised ESR with a normal CRP is often the aftermath of an infection weeks ago. Fibrinogen has a half-life of about four days, and immunoglobulins linger far longer.

There are also situations where the ESR is the more useful of the two. Polymyalgia rheumatica and giant cell arteritis are examples. In lupus the CRP sometimes stays normal during a flare while the ESR rises.

Neither value says where something is or what it is. Your symptoms and your GP's judgement are what give them meaning.

Your GP usually orders this value for complaints that have lasted a while. Fever without a clear cause is one. Joint pain that persists, or stiffness that will not ease, are others.

It also belongs to following an inflammatory condition that is already known. Then it is not about one number, but about the line across several measurements.

The draw itself affects the result more than you would expect. The tube needs to stand upright at room temperature and be processed within a few hours. If it tilts, the result reads too high.

Check that before an abnormal number alarms you. An error in the collection is not a medical problem. On paper it just looks exactly the same.

And be patient with the recheck. This value moves slowly, so a new measurement after a week says little. After a few weeks it says a great deal more.