Red blood cell (RBC) count: high, low and next steps

Red blood cells carry oxygen from the lungs to every tissue and bring carbon dioxide back. The RBC count tells you how many of these cells are present in a given volume of blood. Lab Tests Online UK says the count is used to evaluate any decrease in red blood cells, called anaemia, or any increase, called polycythaemia.

It is only one of three closely linked red cell measures, the others being haemoglobin and haematocrit. A scientist checks that the three agree, and the red cell indices (MCV, MCH, MCHC) are calculated from them.

The count is normally requested as part of a full blood count, which has many reasons: tiredness, pallor, breathlessness, dizziness, heavy bleeding, a routine health check, monitoring of a long-term condition or checking before an operation. Lab Tests Online UK says a person with a low count may feel tired, faint, pale or short of breath, and that a high count may cause headaches, visual disturbances and a flushed appearance.

The NHS notes that a GP checking for iron deficiency anaemia will usually do a full blood count to find out whether the number of red cells is normal, and that no preparation is needed.

No fasting is needed and the blood is taken from a vein in the arm, or by finger prick in children and heel prick in newborns. Lab Tests Online UK explains that pregnancy brings a normal decrease in red cell counts because the increased body fluid dilutes them, and that living at high altitude increases the count because the body responds to lower oxygen.

Hydration also changes the count. If you are dehydrated the plasma shrinks and the count looks higher; intravenous fluids can do the reverse. Tell the clinician if you were unwell with vomiting or diarrhoea before the test.

Lab Tests Online UK lists anaemia, bleeding, kidney disease, bone marrow failure (for instance from radiation or a tumour), malnutrition and deficiencies of iron, folate and vitamin B12. Because the kidneys make the hormone that drives red cell production, long-term kidney disease commonly leads to a lower count.

Treatment depends on the cause. The same source says it may include a vitamin or mineral supplement, a change in nutrition, removal of excess cells or replacement with donor red cells, or a change in current medication. The NHS describes iron tablets for iron deficiency anaemia, usually taken for about six months, and injections or tablets for B12 and folate deficiency. A balanced diet helps prevent nutritional anaemia, but problems caused by disease are not likely to be corrected by diet.

According to Lab Tests Online UK a high count may indicate congenital heart disease, dehydration, obstructive lung disease, sleep apnoea or bone marrow over-production. Smoking and some medicines also play a part, and a persistently high count with a raised haemoglobin and haematocrit is the picture that leads to tests for polycythaemia, including erythropoietin and JAK2 testing.

In many people a mildly high count is explained by dehydration or smoking and falls with adequate fluids or when those factors change.

Your doctor will work out the cause before prescribing treatment. For a low count that usually means looking at the red cell indices and requesting iron studies, B12, folate, kidney function or a reticulocyte count. For a high count it often means reviewing smoking, medicines, sleep symptoms and oxygen levels, then checking whether the result persists.

Seek urgent help rather than waiting for a routine review if a low red cell count comes with chest pain, severe breathlessness or fainting; call NHS 111 or 999 as appropriate.