Red blood cell count (RBC)
Number of red blood cells, which use hemoglobin to carry oxygen from the lungs to every cell
Also known as
Definition
Red blood cells, or erythrocytes, are made in the bone marrow and contain hemoglobin, an iron-rich protein that carries oxygen from the lungs to every cell in the body. The red blood cell count states how many of these cells are present in one microlitre of blood. It is almost always part of a complete blood count and is interpreted together with the other values in that panel.
| Parameter | Value |
|---|---|
| Unit | million/µl (= 10¹²/l) |
| Reference Range | Typical for adults: men 4.2–5.7 million/µl, women 3.8–5.1 million/µl. Labs differ slightly; the range printed on your own report is what counts. Discuss any deviation with your doctor. |
Agree on your personal target range with your doctor.
1 million/µl equals 1 × 10¹²/l (also written /pl or T/l). US reports use million cells/mcL.
↓ What a low value means
A lower-than-usual count can be linked to anemia, blood loss, malnutrition or a lack of iron, copper, folic acid, vitamin B6 or B12, and also to kidney disease with resulting erythropoietin deficiency, early breakdown of red blood cells (hemolysis), bone marrow disorders, leukemia or multiple myeloma. Pregnancy, overhydration, alcohol use disorder and some medicines, such as chemotherapy drugs or non-steroidal anti-inflammatory drugs, can also lower the value. Discuss any deviation with your doctor.
↑ What a high value means
A higher-than-usual count can be linked to dehydration, smoking, a low blood oxygen level, lung disease such as pulmonary fibrosis, sleep apnea, heart disease or kidney cysts and tumors, and also to the bone marrow disorder polycythemia vera. After a stay at high altitude the count stays raised for several weeks. Medicines such as testosterone, anabolic steroids or erythropoietin can also increase it. Discuss any deviation with your doctor.
When to test
The red blood cell count is usually measured as part of a blood count at routine check-ups, and also with symptoms such as tiredness, weakness, shortness of breath, dizziness or pale skin, to monitor conditions such as chronic kidney disease, and during treatments that can affect blood cell production, such as chemotherapy. No special preparation is needed.
Frequently asked questions
Can fluid balance affect the value? +
Yes. With dehydration the red blood cell count can come out higher, with too much water in the body lower. An abnormal value is therefore always interpreted together with other findings and your situation on the day of the blood draw.
Which tests follow an abnormal red blood cell count? +
Further values are often measured to narrow down the cause, such as reticulocytes (young red blood cells), which show whether the bone marrow is producing enough new cells, as well as iron tests and B vitamins. Your doctor decides which tests make sense.
Does a stay at altitude change the red blood cell count? +
Yes. After a stay at higher altitude, the red blood cell count is raised for several weeks. If you were in the mountains before your blood draw, mention it when you discuss your results.
Do I need to fast for the red blood cell count? +
No, the red blood cell count needs no special preparation. If other values are measured at the same appointment, they may have their own requirements; check with your practice if unsure.
Red blood cell count (RBC)
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.