Complete blood count: tracking your values across a training year

The panel measures three series of cells from one tube, and for a trend line their tempo is the key distinction.

The red series carries oxygen: the erythrocyte count, the amount of haemoglobin, the haematocrit and the cell volume MCV. These values respond slowly, so a change in your iron intake or training load will not show up within a week. They make up the season trend.

The white series is the mirror image. The total leukocyte count can rise within hours of a hard session, so it is a snapshot. In a differential, the lab splits that total into the five types of white blood cell: neutrophils, lymphocytes, monocytes, eosinophils and basophils. If that share shifts towards young neutrophils, a technician calls it a left shift, and that is a description of the picture and no diagnosis.

As a third series, the platelets handle clotting and move over days to weeks, which makes them context.

Some of these values are not measured but calculated. The haematocrit follows from the erythrocyte count times the MCV, and that is why one artefact can shift several values at once. MCV has a reference interval of 80 to 100 fl in adults according to the NVKC; for the other components the limits differ per laboratory, so always read the limits printed on your own result.

A blood count mainly tells an athlete something when it is measured under the same conditions, because the biggest pitfall is not a deviating value but a value that is correct and measured at the wrong moment.

Plasma volume expansion is the best known example. Endurance training increases plasma volume, and haemoglobin and haematocrit are concentrations, so they fall without you losing a single red blood cell. This is called sports anaemia and it is no anaemia. The reverse happens with dehydration: the concentration rises without any gain. That is why only a fixed measurement condition works: draw at rest, well rested, hydrated and not within 24 hours of a hard session, because if you vary that protocol between measurements you are mostly measuring your own circumstances.

The values also interpret each other. A haemoglobin without an MCV is directionless, and an MCV without an RDW misses combined deficiencies, because a small and a large cell volume cancel out in the average. A percentage from the differential without the total leukocyte count is misleading, because a share is no number. And with a falling red series, reticulocytes are the missing link: they show whether the marrow is producing too little or too much is being lost.

One final point, and it is the most relevant one for athletes. A normal blood count does not rule out iron deficiency, because ferritin falls first, then the RDW rises, then the MCV falls, and only last does haemoglobin fall. A ferritin below 30 µg/l alongside a normal panel is therefore quite possible, and it often explains precisely the dip in performance you came for.

For a trend line, consistency matters more than frequency. Two to four measurements a year is enough, provided they are drawn under the same conditions. Sensible moments are the start of a build period, the end of a heavy block and a fixed point in the rest period, so that you always compare the same kind of week with the same kind of week.

Keep to the pre-analytical checklist, because a substantial share of abnormal results comes from the collection. A long tourniquet or a long time seated causes haemoconcentration, so every cell count reads a little higher. A sample that spent long in transit lets red cells swell in the EDTA, so the MCV reads artificially high and the MCHC low. Platelets can clump in EDTA, giving an artificially low count, and drawing shortly after hard exercise pushes the leukocytes up sharply.

Always work through that list first, because only then is a clinical explanation appropriate. Reverse that order and a collection artefact becomes a medical problem.