Leukocyte count in athletes

The assay counts how many white blood cells sit in a fixed volume of blood, expressed in x10^9/L, which is billions of cells per litre. Some reports use the spelling leucocytes or simply white blood cells; it is the same test. White blood cells are made in the bone marrow and travel through the blood to wherever they are needed.

For an athlete the most important property of this number is that it moves. Some of your white blood cells normally rest against the vessel wall and are not counted. During intensive exertion adrenaline releases those cells, so the count rises within minutes; that is called demargination. After strength training that spike is largely gone within half an hour. After long or very hard exertion a second, slower rise follows later, driven by cortisol and made up mostly of neutrophils. After a marathon or a hard interval session the total can therefore sit above the reference range with nothing wrong.

The total is the sum of five kinds of white blood cell: neutrophils, lymphocytes, monocytes, eosinophils and basophils. Exertion can move those kinds in different directions, so a normal total can hide a shifted distribution. The leukocyte differential splits the total out and shows which cell type is moving. Fasting is not part of the preparation.

Athletes like to read this number as a gauge of their resistance, and that is exactly what it is not. The leukocyte count does not say how well your immune system works; it says how many cells were circulating at that moment. On the day of the measurement your training diary often drives that number more than your health does.

Researchers sometimes call the hours after very hard or long exertion an open window: the idea that resistance dips briefly. Whether that window really exists is disputed. What is established is that the cell count can be raised during those same hours. A high number after a race is therefore no proof that your immune system is working well at that moment, and a normal number no proof of the opposite.

What the number can do is show a pattern across a training block. If the value creeps up from block to block while your load rises and your sleep falls, that is a rough sign that recovery is lagging. Rough, because CRP often speaks more directly to inflammation and ferritin to your iron store. Read the leukocyte count as one line in a series, then, and not as a standalone result with a verdict attached.

Two distortions affect athletes more often than others. Dehydration concentrates the blood, so all cell counts rise together; after long endurance work that is the rule rather than the exception. If your leukocytes, haemoglobin, haematocrit and platelets all sit somewhat higher, that is the simplest explanation. Smoking keeps the number higher even in well-trained people, and after quitting it falls within a year.

There is also a normal variant called benign ethnic neutropenia. The neutrophil count, and with it the total, then sits structurally lower without the immune system working any less well. This variant is more common in people of African or Middle Eastern descent and is a reason not to read a low number as overtraining straight away.

Schedule the draw at a moment that reflects your baseline and not your last session. In practice that is the morning after a rest day or a very easy day, and not shortly after a hard workout, a race or a long endurance effort.

Keep the timing constant. Leukocytes sit lower in the morning than later in the day, following the cortisol rhythm. A morning draw in May and an afternoon draw in September therefore compare poorly. If you want to follow a block, draw at its start and its end at the same hour.

Drink normally in the hours beforehand; concentrated blood counts higher. Fasting is not needed. Mention it if you smoke or use corticosteroids, for example for asthma, because both shift the number.

Preferably have the leukocyte count measured together with the differential and with CRP. If you are wondering whether fatigue relates to your iron store, include ferritin as well. If an earlier value was abnormal, a repeat measurement at a comparable rested moment is the first step to see whether the abnormality persists.