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Red blood cells (RBC)
The cells that carry oxygen from the lungs to every tissue. The count is the first line checked when anemia is suspected.
Other names: Red blood cell count, erythrocyte countSection: Blood cells
Updated: August 29, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortRed blood cells transport oxygen around the body. Common adult ranges are 4.2–5.7 million cells/mcL (4.2–5.7 × 10¹²/L) for men and 3.8–5.1 million cells/mcL (3.8–5.1 × 10¹²/L) for women. The count is read together with hemoglobin and hematocrit — the three lines of the CBC that describe the red cell population.
Reference ranges
Group
Reference range
Adult men
4.2–5.7 million cells/mcL (4.2–5.7 × 10¹²/L)
Adult women
3.8–5.1 million cells/mcL (3.8–5.1 × 10¹²/L)
Common reference values from the MedlinePlus Medical Encyclopedia. Intervals differ between laboratories and shift with altitude — the range on the report applies.
Check your value
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
What this marker is
Red blood cells (erythrocytes) are the most numerous cells in the blood. Each one is packed with hemoglobin, the protein that binds oxygen in the lungs and releases it in the tissues. How much oxygen the body receives depends on both the number of red cells and how well each one works.
The cells are made in the bone marrow and live for about four months before being recycled by the spleen. Production is steered by erythropoietin, a hormone released by the kidneys when oxygen levels fall — which is why kidney disease so often leads to anemia.
The RBC count is almost always reported as part of a complete blood count (CBC), alongside hemoglobin, hematocrit, and the red cell indices that describe cell size and hemoglobin content. Those related lines are what turn a bare count into a picture of which kind of anemia or excess is present.
Altitude matters: people living high above sea level carry a naturally higher count, and a stay in the mountains raises the count for several weeks. Laboratories account for this in their reference intervals to different degrees, so the report's own range is the reference that applies.
When it is high
Chronic low oxygenLung disease, congenital heart disease, and long-term hypoxia push the kidneys to release more erythropoietin, and the marrow responds with more red cells.
DehydrationWith less plasma water, the same number of cells sits in a smaller volume — the concentration rises without any extra production. Severe diarrhea is a classic example.
SmokingCarbon monoxide reduces oxygen delivery, and the body compensates by raising the red cell count.
High altitudeA natural adaptation: the count climbs within weeks of moving to the mountains and falls again after returning.
Polycythemia veraA bone marrow disorder that produces red cells without the usual brakes — the reason a persistently high count with no obvious cause is investigated.
Kidney cysts or tumorsSome kidney lesions secrete extra erythropoietin and drive the count up. Certain medicines — anabolic steroids, testosterone, erythropoietin — do the same.
When it is low
AnemiaThe umbrella term: too few red cells or too little hemoglobin. The other CBC lines point to the specific type.
BleedingObvious or hidden blood loss (for example from the gut) drains red cells faster than the marrow replaces them.
Iron, B12, or folate deficiencyWithout these building blocks the marrow cannot finish new cells. Copper and vitamin B6 deficiency do the same more rarely.
Kidney diseaseDamaged kidneys produce less erythropoietin, the signal for red cell production — a standard cause of anemia in chronic kidney disease.
Bone marrow failure or cancerLeukemia, multiple myeloma, radiation, toxins, and some medicines (including chemotherapy and chloramphenicol) suppress production at the source.
Pregnancy and overhydrationExtra plasma volume dilutes the same number of cells — the count reads lower without true cell loss.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
Dehydration or overhydrationAltitude of residence or recent travelSmokingPregnancyTestosterone and steroid medicinesRecent bleeding or donation
How to prepare
No special preparation is needed for an RBC count. It usually arrives as one line of a CBC; if the same draw includes tests that require fasting, the order form or the lab will say so. Heavy dehydration (a hot day, a stomach bug) can shift the concentration, so the circumstances of the draw are worth mentioning to the provider.
When a retest makes sense
A mildly off-range count with an obvious explanation (a stomach bug, a mountain trip) is commonly rechecked after recovery. In treatment of anemia or polycythemia the count is repeated on the schedule set by the treating provider to track the response.
Who usually reviews this result
A result outside the reference range is usually reviewed by: your GP or primary care doctor; a hematologist if a blood disorder is suspected. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: RBCRBC countErythrocyte countRed count
Units and conversion
Measured in: million cells/mcL
million cells/mcL=× 10¹²/L
1 million cells/mcL = 1 × 10¹²/L — the number stays the same, only the unit label changes between report formats. Results are rounded to 2 decimal places.
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
The count measures how many red cells there are; hemoglobin measures how much oxygen-carrying protein they contain in total. They usually move together, and the pair — plus hematocrit — is read as one picture.
Does a low RBC count always mean anemia?
A low count is the definition of one dimension of anemia, but the cause varies — from a missing vitamin to hidden bleeding to marrow disease. The red cell indices in the same report narrow it down.
Why did my count change after a mountain trip?
At altitude the air holds less oxygen, so the kidneys signal the marrow to make more red cells. The rise persists for several weeks and is a normal adaptation, not a disease.
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