A red blood cell count indicates how many red blood cells a person has, according to MedlinePlus. Doctors use it to diagnose conditions such as anemia, as well as other medical conditions. A normal range of red blood cells for women is 4.2 to 5.4 million cells per microliter, while a normal range for men is 4.7 to 6.1 million cells per microliter.
Some conditions that cause a higher than normal red blood cell count include dehydration, smoking cigarettes, a tumor in the kidney or low blood oxygen levels, states MedlinePlus. Additional causes of abnormally high red blood cells are congenital heart disease, scarring or thickening of the lungs, bone marrow disease and failure of the right side of the heart. Some medications and moving to a higher altitude sometimes cause a temporary increase in red blood cells as well.
Red blood cell counts that are lower than normal often result from medical issues such as bone marrow failure, anemia, bleeding and leukemia, according to MedlinePlus. Malnutrition, over-hydration, multiple myeloma and hemolysis, which is the destruction of red blood cells, also cause a lower than normal red blood cell count. Some medications, such as cancer chemotherapy and some antibiotics, also cause a decrease in red blood cells.