High white blood cell count (leukocytes): when it is your training and when it is not

A high white blood cell count means your immune system is active. That sounds alarming, but in an athlete the most common cause is often the dullest one: you trained hard yesterday. A single intense session can push your leukocytes well above the upper limit of 10.0 x10⁹/l, and it settles again on its own.

That does not make it a meaningless signal. It makes the timing of your draw part of the result.

I find it striking that no Dutch explainer on leukocytes lists training as a cause. Infection, stress and smoking are all there. Last night squat session is not.

What does a high white blood cell count mean?

It means more leukocytes are circulating in your blood than expected, usually above 10.0 x10⁹/l. The medical term is leukocytosis. Your immune system is responding to something: an infection, inflammation, tissue damage, or simply a serious physical stimulus. The value says something is happening, not what.

Your leukocytes are a collective name. The lab can split them into neutrophils (aimed at bacteria) and lymphocytes (aimed at viruses). That split often says more than the total.

A raised total driven mainly by neutrophils looks different from one driven mainly by lymphocytes. That difference is exactly what your GP wants to see.

Why do your white blood cells rise after exercise?

Because exertion chases your immune cells out of hiding. Normally some of them cling to your vessel walls and sit stored in your spleen. Your heart rate, your blood pressure and your adrenaline flush them loose, so your blood sample counts more of them. No new cells are made; they just all show up in the tally at once.

The effect is large and fast. During intense exertion your neutrophils can climb steeply while your lymphocytes dip below your baseline in the hours afterwards (PMID 26477922). That lymphocyte trough was called the open window for years: a period in which you were supposedly more prone to infection.

That picture has been corrected. More recent work argues the cells do not vanish but redistribute to the lungs, gut and skin, where they stand guard (PMID 32139352). Athletes are not falling ill in droves because of their own training.

Imagine two athletes drawing blood on a Monday morning, both with leukocytes at 11.8 x10⁹/l. The first ran a half marathon on Sunday. The second spent the weekend on the sofa with a fever. The same number, and no reason at all to read it the same way.

Which other causes of a high white cell count are there?

Besides training there are a few common explanations that have nothing to do with sport. Infection is the best known, but stress, smoking, pregnancy and some medicines do it too. Corticosteroids, for instance, push your leukocytes up without any inflammation being present.

The last row is the only one that goes to the GP without further thought. The rest is context.

How do you stop your training from clouding your result?

By putting a rest day between your last hard session and your draw. That is the whole trick, and it costs you nothing. A morning after a rest day, drinking normally, no alcohol the night before: then you measure yourself and not your training schedule.

If you do draw right after a hard session, the result is not worthless. You simply know you measured a peak and not a baseline. Note when your last workout was, so the next measurement compares like with like.

My experience is that athletes underrate this. They book the draw on a free morning, and that free morning lands right after the heaviest session of the week.

When should you discuss a high leukocyte value with your GP?

When the value stays high on a calm repeat, or when you have symptoms that stand apart from it. Fever, unexplained weight loss, night sweats, persistent fatigue or repeated infections belong in a conversation, whatever your blood count does. The NHG guidelines describe leukocytosis not as a diagnosis in itself, but as a reason to look further.

A single mildly raised measurement in an otherwise healthy athlete who just trained is a different thing from a value that stays high with no explanation. The difference lies in the repeat.

I would always bring a doctor into that. Not because a single value is frightening, but because the pattern around it means something you cannot read from one number.

Where do you start?

Repeat the measurement on a calm day before drawing conclusions. A leukocyte value is a snapshot, and in athletes that moment is often badly chosen. Our complete blood count test measures your white series alongside your red cells and platelets, with an assessment by a BIG-registered doctor.

How to read all the values together is in complete blood count explained. What a raised inflammation value means beside it is in the ESR value and in hs-CRP and fatigue. If you are unsure whether your tiredness comes from your training, look at spotting overtraining in your blood values.

References

1. Campbell JP, Turner JE. Can exercise affect immune function to increase susceptibility to infection? Exercise Immunology Review. 2020. PMID 32139352.
2. Gleeson M, et al. Exercise and the regulation of immune functions. Progress in Molecular Biology and Translational Science. 2015. PMID 26477922.
3. Fallon KE, Fallon SK, Boston T. Sports haematology. Sports Medicine. 2000. PMID 10688281.
4. NHG. Dutch College of General Practitioners, haematology in primary care. Accessed 2026.
5. RIVM. Blood testing and reference values. Accessed 2026.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. For treatment decisions, discuss your results with your GP.