Low Red Blood Cell Count: What Does It Mean and What Comes Next?

A low red blood cell count means your blood sample contained fewer red blood cells than the laboratory's reference range. Red cells carry oxygen, so a low count can be a sign of anaemia, but its meaning depends on your hemoglobin, the size of the cells and your symptoms. A slightly low result on its own is not a diagnosis.

The red blood cell (RBC) count, sometimes called the erythrocyte count, is usually part of a complete blood count (CBC), also called a full blood count. MedlinePlus does not give a single normal range, and reference ranges differ by laboratory, age, sex and pregnancy. Use the range printed on your own report.

Some people with a low count feel well, while others notice symptoms. MedlinePlus lists shortness of breath, weakness or tiredness, headache, dizziness, an irregular heartbeat and pale skin as possible symptoms of a low red cell count. Symptoms help a clinician judge how quickly to act, but they cannot show the cause.

Call your local emergency number for chest pain, severe breathlessness, fainting or collapse. These need immediate assessment whatever your last blood test showed.

Contact a clinician promptly, the same day if possible, if you have any of the following. Bleeding in the stomach or bowel is a common cause of iron deficiency, so do not wait for a routine repeat test.

• Black, tarry or bloody stools, vomiting blood, or very heavy bleeding.
• A racing or irregular heartbeat, or new breathlessness when doing very little.
• Feeling dizzy or light-headed on standing, or rapidly worsening tiredness.
• New numbness, pins and needles, unsteadiness or confusion, which can occur with vitamin B12 deficiency.

Hemoglobin is the oxygen-carrying protein inside red cells. Healthdirect describes anaemia as not having enough red blood cells or hemoglobin, and clinicians usually look most closely at the hemoglobin value when deciding whether anaemia is present.

A low RBC count together with low hemoglobin and hematocrit usually points towards anaemia, and the next step is finding the cause. A slightly low RBC count with a normal hemoglobin is a different situation. The result may sit only just outside the range. Clinicians often compare it with earlier results and the other CBC values before deciding whether anything more is needed. If your hemoglobin is low but iron stores look normal, our guide on low hemoglobin with normal ferritin covers that pattern.

MedlinePlus lists anaemia, blood loss, kidney failure, malnutrition, alcohol use disorder and pregnancy among causes of a low RBC count, along with less common conditions of the bone marrow such as leukaemia or multiple myeloma. Healthdirect adds inherited conditions such as thalassaemia, some medicines, viral infections and chronic conditions.

With kidney disease, NIDDK notes that anaemia is less common in the early stages and often becomes more marked as kidney function falls. It is more likely in people with diabetes and in those over 60, and symptoms may develop slowly or not be noticed at first.

• Iron deficiency: the most common type of anaemia. The NHS notes heavy periods, pregnancy and bleeding from the stomach or bowel as common causes.
• Vitamin B12 or folate deficiency: the body makes abnormally large red cells that do not work properly. Causes include pernicious anaemia, diet and some medicines.
• Kidney disease: damaged kidneys make less erythropoietin, the hormone that tells the bone marrow to make red cells, according to NIDDK.
• Infections and long-term conditions, which Healthdirect lists among other causes of anaemia.

During pregnancy the liquid part of the blood (plasma) increases more than the number of red cells. The MSD Manual describes red cell mass rising by around 15 to 25% while plasma volume rises by around 40 to 50%, which dilutes the blood. A lower RBC count and hemoglobin can therefore be partly expected.

Dilution does not explain every low result. Pregnancy also increases iron needs, and iron deficiency is the main cause of true anaemia in pregnancy. Pregnancy, Birth and Baby in Australia notes that a full blood count is usually checked at the first antenatal visit and again around 28 weeks. Pregnancy uses its own hemoglobin thresholds, so ask your midwife or doctor how your result was interpreted.

The rest of the blood count helps point to a cause. Mean corpuscular volume (MCV) shows average red cell size. Small cells often suggest iron deficiency or thalassaemia trait, explained in our low MCV guide. Large cells can suggest B12 or folate deficiency, alcohol, thyroid problems or some medicines, covered in our high MCV guide.

Depending on the pattern, clinicians may order a reticulocyte count, iron studies including ferritin, vitamin B12 and folate levels, kidney function, or a blood film. If the cause of iron deficiency is unclear, the NHS notes a stool test for hidden blood or a specialist referral may follow. Ask your clinician before starting iron or vitamin supplements to change the numbers.

Bring the full report with the units and reference ranges, plus any earlier blood counts.

• Is my hemoglobin also low, and do I have anaemia by your assessment?
• How far outside the range is my RBC count, and has it changed compared with earlier results?
• What do my MCV and other red cell values suggest about the likely cause?
• Which follow-up tests are planned, such as ferritin, B12, folate or kidney function?
• If I am pregnant, how much of this could be normal dilution?
• Which symptoms should make me seek help before the next test?

On SameCase, patients and verified doctors share de-identified cases. Seeing how others with a low red blood cell count laid out their hemoglobin, MCV, ferritin and B12 results can help you organise your own reports and prepare questions. Similar cases cannot tell you the cause of your result; that needs your own clinician's assessment.