White blood cells (leucocytes): what they are, what they tell you and why we test them

White blood cells, or leucocytes, are the cells of your immune defences. Their number shows how busy those defences are right now, not what they are fighting. You find them in the full blood count, the most common blood test.

Why we test them

White blood cells protect your body from viruses and bacteria. When there is an infection or inflammation, their number usually rises.

Your bone marrow, where blood is made, produces five types, each with its own job. The total adds them all up, so on its own it says little. To see what is going on, doctors look at the differential, which counts them type by type.

A very high or very low count deserves attention. But small changes often just reflect what your body was doing in the days before the blood test.

What they tell you, and what they do not

If they are high, there is often an infection or inflammation. But smoking, stress and a hard workout can raise them too, with no infection at all.

If they are low, a recent virus or a medicine is often behind it. Sometimes it is simply your normal level.

And more white cells does not mean stronger defences: the number shows how hard they are working today, not how well protected you are.

That is why they are read together with other tests:

Normal white blood cell (leucocyte) levels

The lab decides what is normal. It is usually 4 to 11 x10⁹/L, the same for women and men. Your own report may print different limits.

Reports write the unit in different ways, but the number stays the same: x10⁹/L and x10³/µL are equal. Some reports count per mm³: 6.5 thousand per mm³ is the same as 6.5 x10⁹/L.

In pregnancy they rise a little, which is normal. And many people of African descent, and some of Middle Eastern descent, have lower counts because of a common, harmless genetic variant (called Duffy-null).

Not to be confused with:

In the Rosa report you also see your optimal range and your white cells split by type.

What your result means

Find the row for your result, in x10⁹/L. Which values are dangerous? If they are low, your neutrophil count is what matters: below 0.5, see a doctor the same day. If you also have a fever, call 112 or go to the emergency department.

If your white blood cells are high or low

What can skew the result

A few things change the number for a while. It helps to know them before you read it.

When to worry

Talk to your doctor if:

With Rosa, a neutrophil count below 0.5 x10⁹/L is always flagged to a doctor, urgently.

Frequently asked questions

Are leucocytes and white blood cells the same thing? Yes: leucocytes is the medical name for white blood cells. On your report you may also see WBC. White cells in your urine, though, are a different test.

Do normal values change with age? In adults they usually do not, and they are the same for women and men. Children, especially young ones, have higher counts, and they rise a little in pregnancy.

What is the white cell differential? It is the part of the blood count that splits your white cells into five types: neutrophils, lymphocytes, monocytes, eosinophils and basophils. Reports give them as a percentage and as an absolute count, and the count usually matters more.

Do I need to fast for a white blood cell test? Not for white blood cells. They are usually tested with other markers that do need fasting, though, so it is best to arrive fasted. Avoid a hard workout the day before.

How often should they be checked? For people who feel well there is no fixed rule: Rosa measures them twice a year, with the blood count. If they are out of range, show your report to your doctor, who decides whether and when to repeat the test.

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