Optimize
← All biomarkers
Oxygen TransportAnemia

Hematocrit

Hematocrit is the percentage of your blood made up of red blood cells, your measure of oxygen-carrying capacity.

What is Hematocrit?

Hematocrit is the percentage of your blood made up of red blood cells. It directly reflects how dense your blood is as an oxygen-transport medium. Hematocrit and haemoglobin move closely together — they tell essentially the same story from two angles. Hematocrit is a concentration measure, not an absolute cell count. With dehydration, plasma volume shrinks and hematocrit rises even though the cell count stays the same. Well-trained endurance athletes often run a relatively lower hematocrit because training expands plasma volume — a physiological adaptation, not anaemia.

Why is Hematocrit relevant?

Hematocrit confirms and quantifies the picture haemoglobin paints. A low hematocrit combined with a low haemoglobin confirms anaemia. Together with the red cell indices (MCV, MCH, MCHC), it helps determine whether the anaemia fits small cells (iron deficiency) or large cells (B12 or folate deficiency). For athletes, hematocrit is a functional marker of adaptation: altitude exposure stimulates red cell production and raises hematocrit. A persistently elevated hematocrit in an otherwise healthy person — beyond dehydration or altitude adaptation — warrants medical evaluation.

Hematocrit high or low — what it means

Read hematocrit alongside haemoglobin, MCV, MCH, MCHC, and RDW. Only together do those values form the pattern that points toward the cause of an abnormality. Hydration is the biggest confounder. After intense exercise, with low fluid intake, or after altitude exposure, hematocrit can look artificially high. Measure when well hydrated and not right after hard training. Track the trend across repeat measurements — that is more reliable than one reading.

Hematocrit reference ranges

Men≈ 41-51%0.41-0.51 L/L
Women≈ 36-47%0.36-0.47 L/L

Cutoffs vary by lab and method; the reference range on your own report is what counts. In the Netherlands hematocrit is usually reported in L/L (0.45 = 45% red blood cells). Always read it alongside hemoglobin and the red cell indices, and account for hydration.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.

Read about our scientific approach →Read the guide: Energy →

Frequently asked questions

What is a normal hematocrit value?

In adults the normal range is roughly 0.41-0.51 L/L (41-51%) for men and 0.36-0.47 L/L (36-47%) for women. Hematocrit reflects the share of your blood made up of red blood cells: 0.45 L/L means 45% of your blood is red cells. Cutoffs differ between labs, so the reference range on your own result is what matters.

What does a low hematocrit mean?

A hematocrit below the lower limit (roughly <0.41 L/L in men or <0.36 L/L in women) usually points to anemia — for example from iron deficiency, a vitamin deficiency (B12 or folate), blood loss, or chronic illness. Well-trained endurance athletes sometimes run a slightly lower hematocrit because training expands plasma volume; that is a normal adaptation, not anemia.

What does a high hematocrit mean?

A raised hematocrit (above ~0.51 L/L in men or ~0.47 L/L in women) is most often caused by dehydration: plasma volume shrinks and the blood concentrates without any extra cells. Drinking enough usually corrects it. Other causes include altitude exposure, smoking, sleep apnea or lung disease, and rarely the bone-marrow disorder polycythemia vera.

When is a hematocrit value a concern?

A single reading says little — hydration and a hard workout beforehand can distort it temporarily. Measure well hydrated and not right after exercise, and follow the trend across several measurements. A persistently elevated hematocrit unrelated to dehydration or altitude, or a low hematocrit together with a low hemoglobin, warrants evaluation by a doctor.

How do you read hematocrit alongside your other blood values?

Hematocrit says the most in context with hemoglobin and the red cell indices (MCV, MCH, MCHC and RDW). A low hematocrit together with a low hemoglobin confirms anemia; the indices then show whether it fits small cells (iron deficiency) or large cells (B12 or folate deficiency). Also account for hydration, since it distorts the reading.

Related biomarkers

Hemoglobin
Hemoglobin is the oxygen-carrying protein in your red blood cells and the marker that defines anemia.
Erythrocytes (RBC Count)
Erythrocytes are your red blood cells, and the RBC count measures them per volume of blood to gauge oxygen transport.
MCV
MCV is the average size of your red blood cells and helps distinguish the type of anemia.
RDW
RDW shows how much your red blood cells vary in size from one another.

Hematocrit is one of the biomarkers in the Optimize blood test. Book a blood draw at any of 238+ partner labs in the Netherlands, or upload your existing results in the app.

See the full test