Low oxygen levels, misuse of certain drugs and blood cancers can cause a high red blood cell count.

Low oxygen levels

The body might make more red blood cells as a response to conditions that result in low oxygen levels. These might include:

• Congenital heart disease in adults
• COPD (chronic obstructive pulmonary disease) — the blanket term for a group of diseases that block airflow from the lungs — including emphysema.
• Heart failure
• Hemoglobinopathy, a condition present at birth that reduces red blood cells' ability to carry oxygen.
• Living at high altitudes.
• Pulmonary fibrosis — a disease that happens when lung tissue becomes damaged and scarred.
• Sleep apnea — a condition in which breathing stops and starts many times during sleep.
• Nicotine dependence (smoking)

In some people, cancers or pre-cancers that affect the bone marrow can cause too many red blood cells to form. An example is:

• Polycythemia vera

Misuse of drugs to improve athletic performance

Certain drugs boost the making of red blood cells, including:

• Anabolic steroids.
• Blood doping, also called transfusion.
• Shots of a protein known as erythropoietin.

Higher red blood cell concentration

If the liquid part of the blood, known as plasma, gets too low, the red blood cell count seems to go up. This happens in dehydration. However, the red blood cells are simply more tightly packed. The number of red blood cells stays the same.

• Dehydration

Other diseases

Rarely, in some kidney cancers or after kidney transplant, the kidneys might produce too much of the hormone erythropoietin. This causes the body to make more red blood cells. Red blood cell counts also can be high in nonalcoholic fatty liver disease.

• Nonalcoholic fatty liver disease

A high red blood cell count is an increase in a type of cells made in bone marrow and found in blood. The main job of red blood cells is to move oxygen from the lungs to the rest of the body. A condition that limits oxygen can cause a rise in red blood cell count. Other conditions can cause the body to make more red blood cells than it needs.

What's considered a high red blood cell count is different at different labs. For adults, the usual range is generally 4.35 to 5.65 million red blood cells per microliter (mcL) of blood for men and 3.92 to 5.13 million red blood cells per mcL of blood for women. In children, what's thought of as high depends on age and sex.

A high red blood cell count is most often found when a health care provider is doing tests to find the cause of symptoms or check for changes in certain illnesses. Your provider can talk to you about what test results mean.