Complete blood count: your values by life stage

The panel maps three series of cells, each with its own role.

The red series is sometimes called the red blood picture. It carries the erythrocyte count, the haemoglobin level, the haematocrit and the cell indices. The cell volume MCV is the most important of these, because it tells you whether cells are small, normal or large, and large cells are described as a macrocytic picture.

The white series counts your leukocytes, and a differential splits that total into the five types of white blood cell: neutrophils, lymphocytes, monocytes, eosinophils and basophils. The platelets form the third series.

Not every value is measured. The haematocrit follows from the erythrocyte count times the MCV, and the MCH and MCHC follow from the haemoglobin, so one artefact in the measurement can shift several values at once. According to the NVKC, MCV has a reference interval of 80 to 100 fl in adults; the limits for the other components differ per laboratory, so always read the values printed on your own result.

A blood count tells a woman something different depending on her life stage, because the same value means something different in the third trimester than in a woman of twenty-five outside pregnancy.

During pregnancy the plasma volume increases sharply. Haemoglobin and haematocrit are concentrations, so they fall without any red blood cells being lost; this is called dilutional anaemia and it is a normal phenomenon. At the same time the leukocytes rise and the platelets fall slightly. While you menstruate there is a recurring iron loss in which ferritin falls first; postpartum the values recover gradually towards your baseline; and after menopause the monthly blood loss stops and your iron requirement falls.

The values interpret each other, and in women that is especially visible. A haemoglobin without an MCV is directionless. An MCV without an RDW misses a combined deficiency, for instance iron alongside folate: one makes cells small, the other large, and the average then looks normal. Anaemia without reticulocytes leaves open whether the marrow is making too few or too much is being lost.

And a normal blood count does not rule out a deficiency. With heavy menstrual blood loss, ferritin falls first, then the RDW rises, then the MCV falls, and only last the haemoglobin. A ferritin below 30 µg/l alongside a perfectly tidy blood count is therefore quite common.

A blood count is worthwhile with persistent fatigue, with heavy or prolonged menstrual bleeding, around a wish to conceive and during menopause, and at a periodic check it is a logical basic panel. The point in your cycle matters less for the blood count itself than is often assumed; if you also want to assess your iron status, prefer not to be drawn during or straight after a heavy period.

Pre-analytics matter more, because they explain a substantial share of abnormal results. A tourniquet left on too long concentrates the blood and lifts every count. A sample long in transit lets red cells swell in the EDTA, raising the MCV artificially and lowering the MCHC. In EDTA, platelets may clump together and report too low, and hard exercise shortly before the draw raises the leukocytes. Work through that checklist before an abnormality is treated as clinical.