Low Red Blood Cell Count (Low RBC): What It Means, Causes & Next Steps

A low red blood cell count (low RBC) means your CBC found fewer red cells than the lab range. It usually moves with low hemoglobin and hematocrit, but a borderline low RBC on its own does not confirm anemia. This page explains what a low RBC means, how to read it with hemoglobin, its causes and symptoms, and when a retest or a clinician workup is the next step.

Published

April 23, 2026

Last updated

September 28, 2026

Read time

12 min read

Medically reviewed by

Rhonda Collins, FNP-C

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A low red blood cell count (low RBC) means your complete blood count (CBC) found fewer red blood cells per microliter of blood than the lab’s reference range. Red blood cells carry oxygen from the lungs to the rest of the body using hemoglobin, an iron-rich protein inside them. A low RBC is read together with hemoglobin and hematocrit on the same CBC. When all three are low, the usual explanation is anemia, which is diagnosed from a low hemoglobin. A slightly low RBC with normal hemoglobin does not by itself confirm anemia. The most common cause of a true drop is iron deficiency, often from blood loss, and other causes include low vitamin B12 or folate, chronic disease, kidney disease, and faster red cell breakdown. See the Red Cell Count biomarker overview for what the RBC count measures.

What a Low Red Blood Cell Count Means

RBC count, hemoglobin and hematocrit are three measures of the same red cells on one CBC. The RBC count is the number of cells. Hemoglobin is the amount of oxygen-carrying protein inside them. Hematocrit is the share of blood volume the cells take up.

• Anemia is defined by hemoglobin. The American Society of Hematology describes anemia as diagnosed when a blood test shows a hemoglobin value below the normal range for your sex, and the World Health Organization’s anemia definitions are set as hemoglobin cutoffs. The RBC count helps describe the anemia, but it is not the number that defines it.
• Usually the three move together. When red cells are lost, broken down, or not made fast enough, the RBC count, hemoglobin and hematocrit usually fall together. A low RBC with low hemoglobin and hematocrit is the typical CBC pattern of anemia.
• Sometimes they split. The RBC count and hemoglobin can point in different directions when the cells are unusually large or small, because each cell then carries more or less hemoglobin. An RBC count alone often cannot explain why the count is low, so the other CBC values and follow-up tests do that work.

A low hemoglobin or low hematocrit has its own page: low hemoglobin and low hematocrit. For which blood tests confirm anemia and its type, see the anemia test guide.

Low Red Blood Cell Count and Cell Size (MCV)

The RBC count adds specificity when it is combined with mean corpuscular volume (MCV), the average size of the red cells on the same CBC:

• Low RBC + low MCV (small cells): iron deficiency anemia or thalassemia. In iron deficiency the cell count is low and cells are small. In thalassemia trait the cell count may be normal to high but cells are small.
• Low RBC + high MCV (large cells): vitamin B12 or folate deficiency (megaloblastic anemia). The bone marrow makes fewer but larger cells because DNA synthesis is impaired.
• Low RBC + normal MCV: acute blood loss, anemia of chronic disease, kidney disease, hemolysis, early iron deficiency, or bone marrow failure such as aplastic anemia.

For more on what the MCV result means on its own, see the MCV blood test explained.

Low Red Blood Cell Count With Normal Hemoglobin

A low RBC count with a normal hemoglobin is not, by that result alone, anemia, because anemia is diagnosed from hemoglobin. This pattern usually has one of three explanations:

• A borderline result. Reference ranges are built from large groups of healthy people, and it is common for healthy people to sometimes land just outside a range. Ranges also differ between labs, so a value flagged low by one lab may sit inside another lab’s range.
• Fewer but larger cells. Hemoglobin is a total amount, while the RBC count is a number of cells. When each cell is larger and carries more hemoglobin, shown as a high MCV or high mean corpuscular hemoglobin (MCH) on the same CBC, fewer cells can still add up to a normal hemoglobin. Large red cells have their own causes, such as low vitamin B12 or folate, which are covered in the MCV blood test explained.
• An early change. Some causes of anemia, such as iron deficiency, develop gradually, so one value can shift before the others do. A repeat CBC shows whether the pattern is stable or moving.

The next step is usually the same: look at the MCV and MCH on the same report and repeat the CBC rather than act on the RBC count alone.

Symptoms of a Low Red Blood Cell Count

A mildly low RBC often causes no symptoms. When symptoms appear, they depend on how low the red cells and hemoglobin are and how quickly they fell. Gradual anemia gives the body time to adjust, while sudden blood loss does not. MedlinePlus lists shortness of breath, weakness or fatigue, headache, dizziness, an irregular heartbeat, and pale skin as symptoms of a low red blood cell count.

Mild anemia:

• Mild fatigue, especially with exertion
• Slightly reduced exercise tolerance
• Pallor (pale inner eyelids and palms), often noticed by others rather than the person
• Often no symptoms, or symptoms attributed to other causes

Moderate anemia:

• Significant fatigue and weakness
• Palpitations (the heart beats faster to compensate)
• Shortness of breath on exertion, such as climbing stairs
• Headache and difficulty concentrating
• Pallor clearly visible in the inner eyelids, palms, nail beds, and mouth

Severe anemia:

• Shortness of breath at rest
• Chest pain, because the heart muscle itself is not getting enough oxygen
• Fainting or near-fainting
• A fast heart rate and low blood pressure, especially after sudden blood loss
• Angina and a higher risk of heart attack in people with existing coronary artery disease

Type-specific symptoms:

• Iron deficiency anemia: pica (craving ice, dirt, or clay), restless legs syndrome, spoon-shaped nails (koilonychia), a smooth, sore tongue (glossitis), and cracks at the corners of the mouth (angular cheilitis)
• B12 deficiency: nerve symptoms, which are the most distinctive. These include symmetric tingling or numbness in the hands and feet, loss of vibration and position sense, unsteady walking, memory or thinking changes, and vision problems from optic nerve damage. Nerve symptoms can come before or alongside the anemia.
• Hemolytic anemia: jaundice (yellow skin and eyes from bilirubin released when red cells break down), dark urine, and an enlarged spleen
• Aplastic anemia: low white cells (repeated infections) and low platelets (easy bruising, tiny red skin spots called petechiae) along with the low red cells

What Causes a Low Red Blood Cell Count

Iron deficiency (the most common cause):

• Most often from blood loss: heavy menstrual bleeding (the most common cause in premenopausal women), gastrointestinal (GI) blood loss (peptic ulcer, colorectal cancer, inflammatory bowel disease, hemorrhoids), or surgery
• Rarely from diet alone (except in infants and toddlers). Absorption is impaired in celiac disease, H. pylori gastritis, and after stomach surgery
• Ferritin, the iron storage protein, is the key blood test for iron stores. Because ferritin also rises with inflammation, it can read normal even when iron is low in someone who is unwell.

Vitamin B12 and folate deficiency (megaloblastic anemia):

• B12 deficiency: most commonly from pernicious anemia, an autoimmune condition that destroys the stomach cells making intrinsic factor, which the body needs to absorb B12. Other causes are a strict vegan diet (B12 comes almost only from animal foods), stomach surgery, and Crohn’s disease of the last part of the small intestine, where B12 is absorbed.
• Folate deficiency: poor intake (alcohol use disorder, eating disorders), malabsorption (celiac disease), increased demand (pregnancy, hemolytic anemia), and methotrexate, which blocks folate metabolism
• B12 and folate are checked together. Treating B12 deficiency with folate alone can improve the blood count while nerve damage from low B12 continues.

Anemia of chronic disease:

• Occurs with long-term inflammation (rheumatoid arthritis, inflammatory bowel disease, cancer, chronic infections). The hormone hepcidin rises and locks iron in storage, so the bone marrow cannot use it. The RBC count is mildly to moderately low, the MCV is usually normal or slightly low, and ferritin is typically normal or high.

Chronic kidney disease:

• The kidneys make erythropoietin (EPO), the hormone that tells the bone marrow to make red cells. Kidney disease lowers EPO, which causes an anemia with normal-sized cells.

Hemolytic anemia (red cells destroyed faster than they are made):

• Autoimmune hemolytic anemia, where antibodies attack red cells
• Mechanical damage, for example from artificial heart valves
• Inherited conditions: hereditary spherocytosis, G6PD deficiency, sickle cell disease, and thalassemia

Bone marrow problems:

• Aplastic anemia: the marrow stops making enough of all three blood cell types
• Myelodysplastic syndrome (MDS): the marrow makes abnormal cells that do not mature properly
• Leukemia, multiple myeloma, and cancer that has spread to the marrow
• Chemotherapy and radiation, which suppress marrow output

Dilution and other factors:

• Pregnancy and overhydration, which increase the fluid part of blood
• Malnutrition and alcohol use disorder

Should I Be Worried About a Low RBC?

Not necessarily. A low RBC can reflect a condition that needs treatment, but a slightly low value often does not. How much it matters depends on the pattern:

• Less concerning: a single, slightly low RBC with normal hemoglobin, hematocrit and MCV, no symptoms, and otherwise unremarkable results.
• Worth a workup: a low RBC that comes with low hemoglobin and hematocrit, that persists or falls further on repeat testing, or that comes with small or large red cells, low white cells or platelets, or symptoms such as fatigue, shortness of breath, dizziness, or pale skin. Signs of blood loss (very heavy periods, black or bloody stools) or of red cell breakdown (yellow skin or eyes, dark urine) also call for a prompt workup. These are the patterns a clinician works up to find the cause.
• Needs emergency care now: new or ongoing chest pain, significant shortness of breath at rest, or fainting. Call 911 or go to an emergency room rather than waiting for a retest or an appointment.

When a Retest Is the Right Step

A repeat CBC is often the most useful next step when the RBC is only slightly low and hemoglobin is normal. Repeating the test under similar conditions shows whether the low RBC persists and whether hemoglobin, hematocrit and MCV are moving with it. Recheck white cells and platelets on the same draw, since a drop in more than one cell line points toward a different workup. If the pattern holds, follow-up tests usually include a reticulocyte count (immature red cells, which shows whether the marrow is keeping up), iron tests including ferritin, and vitamin B12 and folate levels. Mito offers a CBC with differential and platelet count that reports RBC, hemoglobin, hematocrit, the red cell indices, white cells and platelets together.

Normal Red Blood Cell Count Levels

When to See Your Care Team

Book a 1:1 consultation with your care team for an RBC count that stays below the reference range on repeat testing, or that comes with low hemoglobin. The initial workup usually combines the CBC with red cell indices (MCV, MCH, MCHC), ferritin, vitamin B12, folate, and a reticulocyte count, which together point to most common causes. In a premenopausal woman with small red cells, iron deficiency from menstrual blood loss is the usual explanation, but other sources of blood loss still need to be considered. For adult men and postmenopausal women with iron deficiency anemia, the American Gastroenterological Association recommends endoscopy of both the upper and lower GI tract to look for a source of bleeding, including cancer.

Frequently Asked Questions

Is a low red blood cell count the same as anemia?

Not exactly. Anemia is diagnosed from a low hemoglobin. A low RBC count usually comes with low hemoglobin and hematocrit, and together they show anemia. A slightly low RBC with a normal hemoglobin is not anemia on its own, and it is often rechecked before any further testing.

What does it mean when my red blood cell count is low and my cells are small (low MCV)?

Small red cells (microcytic anemia) most often mean iron deficiency or thalassemia. They can look similar on the CBC but are told apart by ferritin (low in iron deficiency, normal in thalassemia), the RBC count (lower in iron deficiency, often normal or high in thalassemia trait despite the small cells), and hemoglobin electrophoresis, which can show the pattern of beta-thalassemia trait. The treatment differs: iron for iron deficiency, and no iron for thalassemia trait, where unneeded iron can build up.

Can pregnancy lower your red blood cell count?

Yes. Pregnancy is a recognized cause of a low RBC count. The body makes more blood during pregnancy, and the fluid part expands more than the red cells, which dilutes them. The need for iron and folate also rises, and iron-deficiency anemia is common in pregnancy, so a low result is still worth checking.

Why does B12 deficiency cause nerve symptoms but folate deficiency does not?

Vitamin B12 has two roles in the body. It works with folate in making DNA, which is why both deficiencies cause the same large-cell anemia. It also helps maintain myelin, the protective coating around nerves, a role folate does not share. Without enough B12, nerve damage can develop, and it may be permanent if treatment is delayed. That is why a large-cell anemia is checked for both B12 and folate.

Can anemia come from cancer itself, or from cancer treatment?

Both. Cancer can cause anemia through inflammation (anemia of chronic disease), by crowding out the bone marrow, through bleeding, and, less often, by triggering antibodies against red cells. Chemotherapy and radiation to bones that hold marrow, such as the pelvis, also reduce red cell production for a time.

References

• MedlinePlus: Red blood cell (RBC) count
• MedlinePlus: RBC indices
• MedlinePlus: How to understand your lab results
• MedlinePlus: Anemia caused by low vitamin B12
• NHLBI: Anemia diagnosis
• NHLBI: Anemia in pregnancy
• American Society of Hematology: Anemia
• World Health Organization: Anaemia
• Cleveland Clinic: Anemia
• Testing.com: Red blood cell count (RBC) test
• American Gastroenterological Association: Gastrointestinal evaluation of iron deficiency anemia (2020)

Related Resources

• Red Cell Count: Biomarker Overview
• High Red Blood Cell Count (High RBC): What It Means, Causes & Next Steps
• Low Hemoglobin Symptoms and What the Result Means
• Anemia Test Guide
• How to Improve Your Red Cell Count Naturally