Red blood cell count (RBC): normal range and results

Red blood cells, or erythrocytes, are the most abundant cells in the blood — they carry haemoglobin, which binds oxygen in the lungs and delivers it to the tissues, and carries carbon dioxide back out. The red blood cell count (RBC) shows how many of these cells there are in one unit volume of blood, and it is one of the values in a full blood count, alongside haemoglobin, haematocrit, MCV, MCH and MCHC. These values are assessed together: the red blood cell count on its own does not show whether the cells are of normal size or saturated with haemoglobin, so a low or high RBC is usually interpreted together with the other red-cell values.

The red blood cell count can fluctuate because of fluid balance, altitude above sea level, smoking, and chronic lung or heart disease. It is also affected by the state of the kidneys (because the kidneys produce erythropoietin, which stimulates red blood cell production in the bone marrow), as well as by blood loss or disorders of bone marrow function.

• As part of a full blood count during a preventive check-up
• With tiredness, weakness, breathlessness, dizziness or pale skin — when anaemia is suspected
• When bone marrow disorders, kidney disease, haemolysis or leukaemia are suspected
• When there are signs of bleeding, or to monitor the situation after blood loss
• To monitor heart or lung disease that can cause a chronic shortage of oxygen in the body
• To assess signs of polycythaemia vera (excessive red blood cell production)

A raised red blood cell count is treated as an altered result under the MedlinePlus guidance. The most common causes are the body's response to a lower level of oxygen (smoking, lung or heart disease, living at high altitude) or a shortage of fluid because of dehydration. Less often it can point to a bone marrow disorder (polycythaemia vera) or a kidney problem.

• Smoking
• Dehydration (a shortage of fluid)
• Heart failure or congenital heart disease
• A reduced level of oxygen in the blood, scarring of the lungs (fibrosis)
• Kidney disorders that increase erythropoietin production
• Polycythaemia vera — a bone marrow disorder in which too many red blood cells are produced

What to discuss with a doctor: whether there was dehydration or recent smoking at the time of the test; an assessment of lung and heart function if a chronic increase is found; kidney values and the erythropoietin level; and a repeat test to assess the change over time.

A low red blood cell count is treated as an altered result under the MedlinePlus guidance and is most often linked to anaemia or blood loss. Other causes are a disorder of bone marrow function, insufficient erythropoietin production, or a deficiency of iron, copper, folic acid, vitamin B6 or B12.

• Anaemia or bleeding
• Bone marrow failure or multiple myeloma
• Insufficient erythropoietin production, leukaemia
• An unsuitable diet or a deficiency of nutrients (iron, copper, folic acid, vitamins B6, B12)
• Pregnancy
• Haemolysis (the breakdown of red blood cells)

What to discuss with a doctor: a full blood count with haemoglobin, haematocrit, MCV, MCH and MCHC to establish the type of anaemia; iron values, ferritin, vitamin B12 and folic acid levels; a possible cause of blood loss; and a repeat test to assess the change over time. The tests used to look into anaemia are described separately.

Why the range differs for women and men

According to MedlinePlus (the US National Library of Medicine), the red blood cell (RBC) range for women is given as 3.8–5.1×10¹²/l and for men as 4.2–5.7×10¹²/l, so the male interval is both wider and higher. The source gives separate intervals by sex but does not explain why they differ — no specific physiological reason (for example muscle mass, menstrual bleeding or the effect of hormones) is stated in its text, so none is guessed at here. In practice this means a woman's result has to be compared with the interval for women, not with a general range or the one for men. The exact laboratory interval depends on the equipment used and can differ slightly from the one given by MedlinePlus, so the interval printed on your own report matters more. A separate range is used during pregnancy.

According to MedlinePlus (the US National Library of Medicine), the red blood cell (RBC) range for men is given as 4.2–5.7×10¹²/l and for women as 3.8–5.1×10¹²/l, so both the lower and the upper limit of the male interval are higher. The source presents separate intervals by sex, but the reason the male range is higher is not explained in its text — there is no reference to muscle mass, testosterone or other factors, so no such reason is given here as established. In practice this means a man's result should be compared with the interval for men, rather than with the one for women or a general range. The exact laboratory interval can differ from the one given by MedlinePlus depending on the equipment and method used, so the interval printed on your own report is a more accurate point of reference than a general value from the literature.

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