RBC
The red blood cell count (RBC, erythrocyte count) is the number of red blood cells in a given volume of blood. Red cells carry haemoglobin and with it almost all of the oxygen delivered to the tissues. The count is read together with haemoglobin, haematocrit and the red cell indices (MCV, MCH, MCHC, RDW), which tell not only how many red cells there are, but also what kind.
Red blood cells
A red blood cell is a flexible biconcave disc about 7–8 µm in diameter and 2 µm thick. Mature red cells have no nucleus or mitochondria: almost all of their interior is filled with haemoglobin. The disc shape gives a large surface for gas exchange and allows the cell to fold and squeeze through capillaries narrower than itself. The adult body contains about 25 trillion red cells — roughly 80 % of all cells in the body.
Life cycle and regulation
Red cells are made in the bone marrow from stem cells over about 7 days, in a process called erythropoiesis. It is driven by erythropoietin (EPO), a hormone released by the kidneys when they sense low oxygen. Iron, vitamin B12, folate and other nutrients are essential raw materials. Young red cells that have just left the marrow — reticulocytes — make up about 0.5–2.5 % of red cells; their count shows how actively the marrow is working.
After about 120 days the cells become stiff and are removed by macrophages in the spleen and liver. Iron is recycled, and haem is converted to bilirubin. Any condition that shortens red-cell survival (haemolysis) or slows production lowers the count.
Reference values and red cell indices
| Parameter | Typical adult range | What it describes |
|---|---|---|
| RBC, men | 4.5–5.9 ×10¹²/L | Number of red cells |
| RBC, women | 4.1–5.1 ×10¹²/L | Number of red cells |
| MCV | 80–100 fL | Average cell volume (size) |
| MCH | 27–33 pg | Average haemoglobin per cell |
| MCHC | 32–36 g/dL | Haemoglobin concentration inside cells |
| RDW | 11.5–14.5 % | Variation in cell size (anisocytosis) |
| Reticulocytes | 0.5–2.5 % (≈ 25–100 ×10⁹/L) | Bone-marrow output |
Low RBC count
A low red-cell count usually means anaemia. The main causes are:
- Too little production — iron, B12 or folate deficiency; chronic kidney disease (low EPO); chronic inflammation; hypothyroidism; bone-marrow failure, leukaemia or infiltration; chemotherapy.
- Increased loss or destruction — acute or chronic bleeding (gastrointestinal, menstrual); haemolysis due to autoimmune disease, inherited membrane or enzyme defects, sickle-cell disease, mechanical heart valves, malaria or drugs.
- Dilution — pregnancy, intravenous fluids, fluid overload.
Symptoms appear when tissues receive too little oxygen: tiredness, shortness of breath, fast heartbeat, pale skin, dizziness. Haemolysis can also cause jaundice (yellow skin and eyes) and dark urine.
High RBC count
A raised count can be relative (dehydration, low plasma volume) or absolute. Absolute increases are driven by chronic hypoxia (smoking, altitude, lung disease, sleep apnoea), by excess EPO or androgens (some kidney and liver tumours, testosterone therapy, doping) or by a bone-marrow neoplasm — polycythaemia vera. A special case is thalassaemia trait: the count is often high-normal or high while the cells are very small (low MCV), so haemoglobin stays normal or slightly low.
A genuinely increased red-cell mass thickens the blood and raises the risk of thrombosis, headache, dizziness and visual disturbances.
Reading RBC with other indices
When to see a doctor
a low count comes with chest pain, fainting, severe breathlessness, black or bloody stools, vomiting blood, sudden jaundice or very dark urine — or if a high count comes with sudden weakness, speech or vision problems, or a painful swollen leg.
the red-cell count, haemoglobin or indices are repeatedly outside the reference range. Typical next tests are ferritin and iron studies, vitamin B12 and folate, reticulocytes, kidney and liver function, and — depending on the findings — haemoglobin electrophoresis or JAK2 testing.
References
- Sarma PR. Red cell indices. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Boston: Butterworths; 1990. Chapter 152. ncbi.nlm.nih.gov/books/NBK260
- Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015;372(19):1832–1843. doi:10.1056/NEJMra1401038
- Mentzer WC Jr. Differentiation of iron deficiency from thalassaemia trait. Lancet. 1973;1(7808):882. doi:10.1016/S0140-6736(73)91446-3
- Kassebaum NJ, Jasrasaria R, Naghavi M, et al. A systematic analysis of global anemia burden from 1990 to 2010. Blood. 2014;123(5):615–624. doi:10.1182/blood-2013-06-508325
- McMullin MF, Harrison CN, Ali S, et al. A guideline for the diagnosis and management of polycythaemia vera. A British Society for Haematology Guideline. Br J Haematol. 2019;184(2):176–191. doi:10.1111/bjh.15648
This article is for general information and does not replace a consultation with a doctor. Reference ranges vary between laboratories; interpret your result together with a healthcare professional.
Published on 2 June 2019