A low red blood cell count (anemia) happens when the body has too few healthy red blood cells to carry enough oxygen, and it usually comes from one or more of three mechanisms: not making enough red cells, losing blood, or destroying red cells faster than they are made. Each of these has many possible underlying causes that a clinician needs to sort out with history, exam, and lab tests.

Main categories of causes

• Decreased production: This includes iron deficiency (often from poor intake or chronic blood loss), low levels of vitamin B12 or folate, chronic kidney disease (less erythropoietin), bone marrow disorders like aplastic anemia, myelodysplastic syndromes, leukemia, or marrow suppression from chemotherapy, radiation, certain medications, toxins, and chronic inflammatory diseases (such as cancer, autoimmune disease, chronic infection).

• Blood loss: Acute bleeding (injury, surgery, childbirth) and chronic blood loss (heavy menstrual periods, bleeding from the stomach or intestines such as ulcers, polyps, cancers, hemorrhoids, or frequent blood donation) can all lower red blood cell count.

• Increased destruction (hemolysis): Inherited conditions such as sickle cell disease and thalassemia, autoimmune hemolytic anemia, certain infections, reactions to medications, mechanical destruction (for example from some heart valves), and an enlarged spleen can all make red cells break down too quickly.

Other factors and risk situations

• Nutritional and absorption problems: Diets low in iron, B12, or folate, alcohol misuse, and conditions that affect absorption in the small intestine (such as celiac disease or Crohn’s disease) can all contribute to anemia.

• Chronic disease and inflammation: Long‑standing conditions like cancer, HIV/AIDS, rheumatoid arthritis, inflammatory bowel disease, diabetes, and chronic kidney or liver disease commonly cause “anemia of chronic disease” by interfering with red cell production and iron use.

• Life stages, lifestyle, and family history: Pregnancy, endurance training, older age, exposure to certain toxins or drugs, and having a family history of inherited anemias (such as sickle cell disease) all increase the risk of low red blood cell count.

If a lab test shows low red blood cell count, it is important to see a healthcare professional promptly, especially if there is fatigue, shortness of breath, chest pain, dizziness, very pale skin, or rapid heartbeat, so the specific cause can be identified and treated appropriately.