Hematocrit

Blood is cells in fluid. Haematocrit is the share of those cells, in the Netherlands as a fraction: 0.45 means that 45 percent of your blood consists of red blood cells and the rest of plasma. Because the red cells carry the oxygen, haematocrit is a second measure of your oxygen transport, beside haemoglobin. The two almost always run in step: three times the haemoglobin in mmol/L comes out at about the haematocrit in percent. It is a ratio, not a count, and that is the core of reading it. If your plasma shrinks, because you drank little, sweated a lot or just trained hard, haematocrit rises without a single cell being added. If your plasma grows, as in endurance athletes and in pregnancy, it falls while your number of cells stays the same or even grows. More than with any other blood value, how you were doing when you had blood drawn counts here. Your report shows haematocrit in L/L. Normal is about 0.40 to 0.50 L/L for men and 0.35 to 0.45 L/L for women. Thick blood flows more slowly and strains the heart, and that is the reason the upper end counts here as much as the lower end.

The first reason to know your haematocrit is that it confirms and complements haemoglobin. A low haemoglobin with a low haematocrit is anaemia; a low haemoglobin with a normal haematocrit is a sign that the cells are there but hold too little haemoglobin, and that points at iron. Together with the cell size, haematocrit shows what kind of anaemia it is, and so where to look. The second reason is sport. Endurance training enlarges your plasma volume within weeks; haematocrit then drops a few hundredths while your oxygen transport is actually better. That is why well-trained endurance athletes often come out below the band with nothing wrong. Altitude does the opposite: thin air sets the marrow to making more red cells, and after a few weeks at altitude the haematocrit sits higher. In cycling, haematocrit was for decades the line on which blood doping was read for that reason, because thick blood flows more slowly and strains the heart. The third reason is your fluid balance itself. A high haematocrit in someone healthy is nine times out of ten too little drunk, and that is the most common explanation for a blood count that suddenly looks different from last year. Smoking and sleep apnoea are the other two reasons that keep a high value up for years, and both can be reversed.

A low haematocrit means proportionally fewer red cells or more plasma. In order of how often it occurs: anaemia, usually from iron deficiency; a larger plasma volume in endurance athletes and in pregnancy; blood loss; and reduced kidney function, because the kidney makes the hormone that drives the marrow. Look at the haemoglobin beside it and at ferritin: if those fall with it, it is a deficiency; if ferritin is normal and you train a lot, it is your blood volume. A high haematocrit means thickened blood or more cells. Dehydration is the most common reason, and a repeat after drinking normally settles that. If the value stays high, time at altitude, smoking and sleep apnoea are the usual explanations. Only a persistently high haematocrit without one of those reasons, especially with plenty of white cells and platelets too, belongs to a rare bone marrow condition and is investigated further. Always read haematocrit beside haemoglobin and the red cell count, and beyond that beside MCV and ferritin. Have blood drawn normally hydrated, not after hard exertion, and at the same time of day if you follow a trend.