Low oxygen levels, the misuse of some medical treatments, 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 can include:

• Congenital heart disease in adults — one or more problems with the heart's structure that are present at birth.
• COPD — a long-term lung disease that causes trouble with breathing.
• Heart failure — a condition in which the heart doesn't pump blood as well as it should.
• Hemoglobinopathy — a condition present at birth that makes red blood cells less able 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 — a condition in which you need the tobacco chemical nicotine and can't stop using it.

Blood cancers

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

• Polycythemia vera — a type of blood cancer.

Misuse of medical treatments to improve athletic performance

Some treatments boost the making of red blood cells, including:

• Anabolic steroid treatment — lab-made forms of the hormone testosterone.
• Blood doping — a term for misuse of techniques such as blood transfusions.
• Erythropoietin treatment — a lab-made version of a hormone produced by the kidneys.

Other diseases

Rarely, in some kidney cancers or after a kidney transplant, the kidneys might make too much of the hormone erythropoietin. This causes the body to make more red blood cells. Red blood cell counts also can be high due to a condition in which too much fat builds up in the liver. This is called nonalcoholic fatty liver disease. You also may hear it called metabolic dysfunction-associated steatotic liver disease.

• Nonalcoholic fatty liver disease

Higher red blood cell concentration

The liquid part of blood is called plasma. If plasma gets too low, the red blood cell count seems to go up in a blood sample. This happens with dehydration. It also can happen as a side effect of medicines called diuretics, a common treatment for high blood pressure. But the red blood cells are simply more concentrated. In other words, the red blood cells are packed more tightly in the same space because there isn't as much plasma. The number of red blood cells stays the same.

• Dehydration
• Diuretics (water retention relievers)

Most often, healthcare professionals find high red blood cell counts when they test for other things. The tests might be to find the cause of symptoms or check for changes in some illnesses. Ask your healthcare professional to talk with you about what your test results mean.