Hematocrit: what does this number mean?

Picture a tube of blood. Part of it is fluid, the plasma, and part consists of cells. Hematocrit tells you what share those cells take up. Nearly all of those cells are red blood cells, and they use their haemoglobin to carry oxygen from your lungs to the rest of your body.

Dutch labs write the result as a decimal, for example 0.42 l/l. Abroad you more often see 42 percent; it is exactly the same number, written differently. If you compare your result with an English site, check which of the two is shown.

Blood used to be spun in a machine, after which you could read off the layer of red cells. That no longer happens: a device now counts your red blood cells and measures their average size, and those two numbers together produce your hematocrit.

The normal value depends on your sex and your age. Star-shl's adult ranges are 0.40 to 0.50 l/l for men and 0.35 to 0.45 l/l for women. For children separate bands apply by age: 0.35 to 0.45 between 1 and 5, 0.36 to 0.47 between 5 and 10 and 0.38 to 0.49 between 10 and 15. Every lab can differ a little here, because devices and methods vary, so always look at the range printed next to your own result; that is the range your result was compared against.

Hematocrit almost never appears alone on a report. It belongs to the complete blood count, together with your haemoglobin and your red blood cells.

The most useful thing to know about this value: it is a ratio and no count. You are not measuring how many red blood cells you have, but how closely packed they are. That sounds like a detail, but it explains almost everything that surprises people about their result: if the amount of fluid in your blood changes, the number changes, even when not a single cell has been added or lost.

Take a high result. The most common reason is simply drinking too little, and a sauna, a hot day or heavy sweating shortly before the test does the same. Your blood thickens, the cells sit closer together and the number rises while nothing about your blood has changed. It works the same way in reverse: if you carry a lot of fluid, the value drops. In pregnancy that happens naturally, and in people who do a lot of endurance sport too.

If your value stays high while you drink normally, other explanations apply. Smoking is one of them, because your body then makes more red blood cells to compensate for the carbon monoxide. Sleep apnoea does the same, and often goes unnoticed for years. A rare cause is a bone marrow disorder in which the body makes too many red cells by itself; that is something a doctor investigates and not something you can read from one number.

With a low value a doctor thinks first of anaemia. That can come from blood loss, or from something missing that is needed to make new cells: iron, vitamin B12 and folate are required for that. A low value right after a heavy bleed can be missed, by the way, because the concentration only falls once the body has replaced fluid, and that takes hours.

Worth knowing: one abnormal number is not a diagnosis. Your GP looks at the whole picture and often at a second measurement. So do not be alarmed by an arrow next to your hematocrit.

You rarely request hematocrit on its own; it is a standard part of the complete blood count, so you usually get it included. The commonest reason is a complaint that fits anaemia, such as being tired despite enough sleep, getting out of breath on the stairs or feeling dizzy when you stand up. In a periodic check the value simply comes along too.

Preparation is simple. Fasting is not needed for this test, but on the morning of the draw drink what you always drink and skip the sauna and a heavy workout that morning. There is one more thing many people do not know: standing or sitting for a long time before the draw thickens your blood slightly, and a tightly fastened band around your arm does the same, so your result can come out a few percent higher.

If you have the test repeated later, go at roughly the same time of day, so that you compare like with like. If you get an abnormal result, a repeat after a few weeks is often the logical next step.