Low Red Blood Cell Count: Causes and Meaning

Red blood cells carry oxygen from the lungs to the rest of the body. When there are fewer of them, tissues receive less oxygen, which is why a low count is linked with tiredness and breathlessness. Lab Tests Online UK defines the count as a measurement of the number of red cells in the blood, normally requested as part of a full blood count.

The count is closely related to two other results on the same report: haemoglobin and haematocrit. Lab Tests Online UK describes the haemoglobin result as usually mirroring the red cell count, and the haematocrit as also mirroring it. If all three are low, the diagnosis of anaemia is straightforward. If only the count is low while the haemoglobin is acceptable, the clinician looks at cell size and shape before deciding what it means.

It is also worth knowing that a result is flagged only against a laboratory range. Ranges differ by age and sex, and the report you receive prints the range that applies to that laboratory.

Lab Tests Online UK says a low count may indicate anaemia, bleeding, kidney disease, bone marrow failure for instance from radiation or a tumour, malnutrition, or other causes. It adds that a low count may also reflect nutritional deficiencies of iron, folate and vitamin B12.

Lab Tests Online UK lists tiredness, fainting, pallor, shortness of breath and dizziness as symptoms that can accompany low red cells. The NHS lists tiredness, lack of energy, shortness of breath, noticeable heartbeats, paler than usual skin and headaches for iron deficiency anaemia, with a longer list of less common symptoms such as tinnitus, a sore tongue, hair loss, pica and restless legs syndrome.

Many people have a low count without noticing any of this, particularly when it has developed slowly. A result found on a routine test is therefore a prompt to look for the reason, not necessarily a sign that you are unwell.

The first step is the rest of the full blood count. The size of the red cells, reported as MCV, separates small-cell patterns such as iron deficiency from large-cell patterns such as vitamin B12 or folate deficiency. The NHS notes that the blood test for iron deficiency anaemia will also check for B12 or folate deficiency anaemia.

Lab Tests Online UK describes further tests that may be requested when the pattern points to anaemia: iron studies, vitamin B12 and folate levels, and a blood film. For anaemia where the reason is unclear, the NHS says a GP may arrange a faecal immunochemical test (FIT) to look for blood in a stool sample and may refer you to a specialist.

NICE Clinical Knowledge Summaries sets out that, in iron deficiency anaemia, men and postmenopausal women are normally referred to gastroenterology to look for a gut source of bleeding, with lower haemoglobin thresholds prompting more urgent investigation.

Lab Tests Online UK says treatment starts with finding the cause and may include a vitamin or mineral supplement, a change in nutrition, a change to current medication, or a drug that stimulates red cell production in the bone marrow. It also says that eating a well-balanced diet helps prevent iron deficiency anaemia and vitamin B12 or folate deficiency, but that red cell problems caused by disease are not likely to be corrected by diet alone.

For iron deficiency specifically, the NHS says iron tablets are usually needed for about 6 months and that drinking orange juice after a tablet may help absorption. For B12 or folate deficiency, treatment is aimed at replacing the missing vitamin once the cause has been identified.

A low red cell count with fainting, chest pain, severe breathlessness, black or bloody stools or heavy ongoing bleeding should be assessed urgently. These features suggest that the count has fallen quickly or is causing strain on the heart and lungs, and they do not belong in a wait-and-see approach.

A count that is only slightly below range, in someone who feels well, is more often followed by a repeat test and a look at iron and vitamin levels. Lab Tests Online UK notes that many results slightly outside the normal range do not require treatment.