Erythrocytes

A red blood cell is a disc without a nucleus, packed with haemoglobin, that circulates for about four months and is then broken down in your spleen. Your bone marrow makes millions of new ones every second, driven by a hormone from your kidneys that responds to how much oxygen your tissues get. This value counts how many of those cells there are per litre of blood, in ×10¹²/L, so in trillions per litre. The count is one of three numbers that together describe your oxygen transport. Haemoglobin says how much oxygen carrier there is per litre, haematocrit what share of the blood is cells, and the red cell count how many cells that is spread over. From those three follow the cell size and the filling per cell, and that is where the count gains its meaning: many small, pale cells is iron deficiency, few large cells is a deficiency of B12 or folate. Normal is about 4.6 to 5.5 ×10¹²/L for a man from age 19 and 4.2 to 5.0 ×10¹²/L for a woman from age 19. The count is a concentration, so your fluid balance counts: thickened blood gives a higher number without a cell being added.

On its own the red cell count is rarely the value you read something from; haemoglobin says more directly whether you have anaemia. The count becomes interesting in combination. A low haemoglobin with a normal or high number of cells means the cells are small and pale, and that is the handwriting of iron deficiency, or of thalassaemia trait, in which the count is actually high with a low haemoglobin. A low haemoglobin with a low number of large cells is B12 or folate. So the count helps tell the two most common kinds of anaemia apart before anything further is measured. For anyone who does sport the count is a measure of adaptation. Early in a training block the count looks lower because your blood volume grows, not because there are fewer cells; at altitude it rises after a few weeks because the marrow works harder. Both are normal. In doping control the red cell count, the haematocrit and the reticulocytes are for that reason the fixed values of the blood passport. The other side is usually harmless: a high count is nine times out of ten from drinking too little. Smoking and sleep apnoea are the two reasons that keep a high count up for years, because the body makes extra cells to compensate for oxygen that is missing at night or through carbon monoxide. Both can be reversed.

A low count means fewer cells or more plasma. Usually it is the first and rarely the last: anaemia from a deficiency of iron, B12 or folate; blood loss; a larger plasma volume in endurance athletes and in pregnancy; and reduced kidney function, because the kidney makes the hormone that drives the marrow. Look at haemoglobin and MCV: small cells point at iron and ferritin, large cells at B12 and folate, and a normal haemoglobin with a low count is in an athlete usually the blood volume. A high count means thickened blood or more cells. Dehydration is the most common reason; drink normally and have blood drawn again. If it stays high, time at altitude, smoking and sleep apnoea are the usual explanations. A high number of small cells with a low-normal haemoglobin fits thalassaemia trait, and that is good to know but not a disease. Only a persistently high count with plenty of white cells and platelets too belongs to a rare bone marrow condition. The count belongs beside haemoglobin, haematocrit and MCV, and beyond that beside ferritin and RDW. Have blood drawn normally hydrated, rested and not after hard exertion.