Leukocyte count explained

The test counts your white blood cells. They appear on your result as leukocytes, and sometimes simply as white blood cells. Both names mean the same thing. The number is given in x10^9/L, which amounts to billions of cells per litre of blood.

At the laboratory the ordinary adult range runs from 4.3 to 10.0 x10^9/L. Sitting just above or just below that is not a problem in itself. Labs use different limits from one another, for example 4.0 to 10.0 or 4.5 to 11.0. So compare your result with the reference column on your own report, not with a number you read somewhere else.

What many people do not know: this single number is really a sum of five kinds of cell. Each kind does something different.

That means a normal total can hide a skewed distribution. Few of one kind alongside many of another simply adds up to a normal number. If you want to know what your distribution looks like, you need the leukocyte differential. You do not need to fast for this test.

The question almost everyone has with a raised number is simple: do I have an infection? The honest answer is that this number alone does not settle it. It moves without infection too, and often substantially.

These are the explanations to run through first, in order of how often they occur. You were recently unwell; the number can linger for weeks afterwards. You had a stressful period or a bad night. You smoke; smokers sit structurally around 1 to 1.5 x10^9/L above. You exercised hard shortly beforehand. Or you had not drunk enough, so your blood was slightly concentrated.

That last one is easy to recognise. In dehydration all cell counts rise together. If your leukocytes, your haemoglobin, your haematocrit and your platelets all sit slightly higher? Then concentration is far more logical than four conditions at once.

There is a daily rhythm too. In the morning your number sits lower than later in the day. Draw once at eight and once at four and that alone can explain a difference.

If you want to know whether inflammation is actually present, CRP says more about that than this number does. And if you want to know where a rise comes from, look at the differential. Plenty of neutrophils points more towards bacterial. Plenty of lymphocytes points more towards viral.

So a single measurement is rarely the whole story. You mainly want to know whether an abnormality is still there on a repeat a few weeks later.

You would usually have this measured as part of a general check. Another reason is symptoms that could point to infection or inflammation. Think of a fever that keeps returning, infections following one another, or fatigue you cannot explain.

Plan the draw so you get an honest picture. Give blood rested, and not within 24 hours of exercising hard. Drink normally in the hours beforehand, otherwise you are mainly measuring your fluid deficit. Fasting is not needed.

As far as possible, draw at the same time of day as last time. Because of the daily rhythm, a morning result and an afternoon result are hard to compare.

Say whether you smoke, are pregnant or take medicines such as prednisone. All three shift the number predictably. Without that information a normal result can look abnormal.

Was your result mildly abnormal last time? Have it measured again after two to four weeks. A temporary shift after an infection has usually gone by then. Request the differential at the same time, because it tells you what the total cannot.