Haematocrit test explained (HCT)

Haematocrit is a value in the full blood count that shows what part of the blood volume is taken up by red blood cells (erythrocytes) compared with the liquid part of the blood (plasma). It is measured by centrifuging a blood sample, or with an automated analyser together with the other values of a full blood count — haemoglobin and the red cell, white cell and platelet counts. Haematocrit is closely linked to haemoglobin and most often moves in the same direction: when the number of red blood cells, or the haemoglobin they contain, falls, the haematocrit falls too, and that indicates anaemia. A raised haematocrit may mean that the body is producing too many red blood cells (for example because of a long-term lack of oxygen in lung or heart disease, living at high altitude, or smoking), or that the amount of blood plasma has fallen because of dehydration or shock. Together with haemoglobin and the red cell values, haematocrit helps a doctor assess the overall state of blood formation and decide the direction of any further tests.

• As part of a full blood count during a routine health check
• When anaemia is suspected — with tiredness, weakness, breathlessness, dizziness, headache or an irregular heartbeat
• When polycythaemia or an excess of red blood cells is suspected — with headache, dizziness, itching or burning skin, night sweats or vision problems
• To follow blood loss after an injury or an operation, or when there is chronic bleeding
• To monitor blood values during treatment (cancer treatment, for example), in chronic illness (kidney disease) or in other conditions that affect the composition of the blood

A haematocrit is considered raised when it is above the normal range according to MedlinePlus (above 55% in men, above 48% in women). The most common causes are dehydration, which reduces the volume of plasma, and too much production of red blood cells because of a long-term lack of oxygen, kidney disease or polycythaemia. A doctor helps to work out the cause together with the other values of the full blood count.

Causes of a raised haematocrit

• Dehydration or shock — the amount of blood plasma falls
• Chronic lung or heart disease that leaves the tissues short of oxygen over a long period
• Kidney disease that leads to too much erythropoietin being produced
• Polycythaemia (polycythaemia vera) — a bone marrow disease in which too many red blood cells are produced
• Smoking
• Testosterone use

Symptoms with a raised haematocrit

• Headache
• Dizziness
• Breathlessness when lying down
• Tiredness
• Itching or burning skin
• Night sweats
• Vision problems
• Unusual bleeding

What to discuss with a doctor when haematocrit is raised

• Whether there were signs of dehydration before the test (not drinking enough fluid, diarrhoea, vomiting)
• The other full blood count values — haemoglobin, red blood cells — to show whether the change is consistent
• Lung and heart function, if a long-term lack of oxygen is suspected
• Kidney values, if a kidney-related cause is suspected

A haematocrit is considered low when it is below the normal range according to MedlinePlus (below 40% in men, below 36% in women), and it most often indicates anaemia or blood loss. The range of causes is wide — from iron deficiency to bone marrow or kidney disease — so a broader picture of blood test results helps to establish the cause. The tests used to look into anaemia are described separately.

Causes of a low haematocrit

• Blood loss — an injury, an operation or chronic bleeding
• A deficiency of iron, vitamin B12 or folic acid
• Bone marrow failure or disease
• Conditions in which red blood cells are destroyed (haemolysis)
• A side effect of cancer treatment
• Chronic illnesses — kidney disease, ulcerative colitis, rheumatoid arthritis
• Leukaemia or multiple myeloma
• A poor diet lacking iron or B group vitamins

Symptoms with a low haematocrit

• Breathlessness
• Weakness or tiredness
• Headache
• Dizziness
• An irregular heartbeat

What to discuss with a doctor when haematocrit is low

• The other full blood count values (haemoglobin, red cell size) — to show what type of anaemia there might be
• Iron values (ferritin, iron, transferrin saturation) and vitamin B12, folic acid
• A possible source of blood loss, especially if the bleeding goes unnoticed
• When to repeat the test, so that the change over time can be seen

Why the range differs for men and women

According to MedlinePlus, the haematocrit range is 36-48% for women, while the same source gives men a higher and wider interval — 40-55%. MedlinePlus presents the two ranges simply as separate rows in a table but does not explain why they differ by sex: the part of the text describing the haematocrit test gives no reason (menstruation or muscle mass, for example, are not mentioned there), so no reason is claimed here either. In practice this means a result has to be compared with the interval for that sex, and not with a general range or the range for the other sex, because otherwise the same number can wrongly look low or high. It is also worth noting that the laboratory that ran the test usually gives its own range by sex in a separate section of the report, and it may differ slightly from MedlinePlus because of a different method or analyser.

Haematocrit is measured as part of a full blood count. At the Medicina practica laboratory in Lithuania, that test costs from €10 (September 2026 pricing).