Hematocrit: normal values and what they mean

Hematocrit is the share of your blood volume made up of red blood cells. Blood consists broadly of plasma and cells. Red blood cells account for by far the largest part of the cellular fraction.

A hematocrit of 0.45 l/l therefore means 45 percent of your blood volume is red cells.

The measurement used to be made by spinning blood and reading off the red cell layer. Modern cell counters work differently. They calculate the value from the red cell count multiplied by the mean cell volume, the MCV.

Star-shl lists the method for this test literally as "Berekening", meaning calculation. That explains a property which is often missed. Any measurement artefact that shifts the MCV shifts your hematocrit with it.

Reference values differ per laboratory and per method. Star-shl publishes the values below for hematocrit (code BH01, EDTA blood). The laboratory cites the Handboek medische laboratoriumdiagnostiek as its source.

Children therefore sit in a different band from adults. Newborns run well above that band in the first months. Always use the range printed on your own report.

A useful rule of thumb links hematocrit and haemoglobin. The hematocrit percentage is roughly three times the haemoglobin in g/dl. The Netherlands, however, reports haemoglobin in mmol/l.

Convert first: mmol/l times 1.61 gives g/dl. If the ratio still deviates sharply, that is itself a signal.

Hematocrit measures a concentration, not an amount. That distinction drives almost every interpretation error made with this value. The result is a ratio between cells and plasma.

If the plasma changes, the result changes, even when the red cell count stays the same.

Dehydration is the clearest example. Drinking too little, a sauna, a heavy sweat session or a diuretic all shrink the plasma volume. The cells then sit closer together and the hematocrit rises.

Not one extra red cell has been made. This is the most common reason for an apparently high result.

The reverse holds too. Fluid overload lowers the value. In pregnancy that happens physiologically: plasma volume rises faster than red cell mass.

Plasma volume also expands in well-trained endurance athletes. Both groups see a lower hematocrit with nothing wrong.

If a raised value persists despite good hydration, other explanations apply. Smoking loads the blood chronically with carbon monoxide and lifts production. Sleep apnoea and chronic lung disease do the same through a lower oxygen level.

Living or training at altitude works along the same route. Testosterone therapy, anabolic steroids and EPO raise the value directly. A primary bone marrow cause such as polycythaemia vera is rare and needs medical assessment.

A lowered value shares its differential diagnosis with a low haemoglobin. Blood loss, a shortage of iron, vitamin B12 or folate, red cell breakdown, chronic disease and reduced kidney function all belong there. Dilution is an additional separate cause.

One timing point matters clinically. Immediately after acute bleeding the hematocrit has not yet fallen. You lose plasma and cells in the same proportion, so the concentration is initially unchanged.

Only once the body shifts fluid into the bloodstream does the value drop. That takes hours. A normal result shortly after bleeding therefore does not exclude blood loss.

Hematocrit is rarely requested on its own. It belongs to the blood count and comes from the same EDTA tube as haemoglobin, the red cells and the MCV. That is also how the result is best read.

Common reasons are complaints that fit anaemia. Think of persistent fatigue, breathlessness on exertion, pallor or dizziness. The opposite picture is a reason too: headache, a flushed face, or itching after a hot shower.

The value is also followed in known conditions. Kidney disease, chronic lung disease and sleep apnoea belong in that group. Under testosterone therapy, periodic monitoring of hematocrit is standard practice.

Preparation determines how usable the result is. Drink normally on the morning of the draw. Avoid a heavy sweat session or sauna in the hours beforehand.

Both raise the value artificially.

The draw itself counts as well. Standing or sitting for a long time beforehand causes haemoconcentration. A tightly applied tourniquet does the same.

The result can come out a few percent higher than in blood drawn lying down. If you repeat the measurement, choose a comparable time and the same conditions.