How Colon Cancer Can Impact the Hemoglobin of Your Blood

How is a low hemoglobin level (anemia) linked with colorectal cancer, and what levels of low hemoglobin are considered mild, moderate, or severe?

Nurse drawing blood from a patient
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What Is Hemoglobin?

Hemoglobin refers to a protein, found in red blood cells, that is responsible for carrying oxygen from the lungs to all other tissues of the body. Red blood cells circulate through the lungs and the hemoglobin in these cells pick up (bind with) oxygen. These cells carry the oxygen, in a form called oxyhemoglobin, out to the tissues and cells of the body. Once at the proper destination, the oxyhemoglobin releases the oxygen and becomes hemoglobin again. The red blood cells, carrying hemoglobin (without oxygen), circulate back to the lungs to pick up more oxygen, and the process begins again.

Hemoglobin is often measured by a blood test to help analyze how well the red blood cells are able to carry oxygen to the rest of the body. Hemoglobin is included as part of a complete blood count (CBC) test and used to check for anemia. When it is low, the results of a hemoglobin test can indicate conditions ranging from mild, such as not getting enough iron in the diet, to serious, such as abnormal bleeding due to colon cancer or other health problems.

What If Your Hemoglobin Is Low?

Low hemoglobin is generally diagnosed as anemia, which means the cells of your body are not receiving the optimal oxygen level. Depending on age and sex, and which reference range a lab uses, anemia is generally diagnosed when hemoglobin is less than 12 grams per deciliter. However, people often don't notice symptoms of anemia until hemoglobin is even lower. Also, symptoms of anemia develop slowly, so people often are not aware of them until they worsen. Symptoms of anemia include:

  • A faster heartbeat
  • Shortness of breath
  • Difficulty breathing during everyday activities
  • Dizziness
  • Chest pain
  • Swelling in the hands or feet
  • Pale skin, nail beds, mouth and gums
  • Fatigue

Level of Anemia

When deciding whether and how to treat anemia, doctors often use the following ranges to determine the level of severity. It's important to note that this is all relative and can vary between the sexes. How dangerous low hemoglobin may be also varies depending on what other medical conditions you may have. For example, hemoglobin of 11.0 could be dangerous for someone with coronary artery disease to the point of precipitating a heart attack. In contrast, someone who is young and healthy may tolerate hemoglobin of 7.5 without severe symptoms (for a short period of time).

Level of Anemia Hemoglobin Range
Mild anemia Between 10 and 12 g/dL
Moderate anemia 8 to 10 g/dL
Severe anemia 6.5 to 8 g/dL
Life-threatening anemia

Less than 6.5 g/dL


Low hemoglobin can often be explained by known blood loss, such as from menstruation, or it can be the first signal to a physician that there is occult, or hidden, bleeding somewhere in the body. The gastrointestinal tract is one of the most common sites of occult bleeding and can be due to hemorrhoids, polyp, colon cancer or other conditions. Depending on your age and health history, a low hemoglobin test may indicate the need for a colorectal cancer screening test such as a colonoscopy.

In those with established colorectal cancer, anemia can be caused by a number of different factors, including cancer itself, as well as treatments such as radiation and chemotherapy (especially a group of chemotherapy drugs known as platinum-based chemotherapy).

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Forget BG, Bunn HF. Classification of the disorders of hemoglobin. Cold Spring Harb Perspect Med. 2013;3(2):a011684. doi:10.1101/cshperspect.a011684

  2. NIH National Heart Lung and Blood Institute. Anemia.

  3. NIH National Heart, Lung, and Blood Institute. Iron-deficiency anemia.

  4. American Society of Hematology. Anemia.

  5. Cappellini MD, Comin-colet J, De francisco A, et al. Iron deficiency across chronic inflammatory conditions: International expert opinion on definition, diagnosis, and management. Am J Hematol. 2017;92(10):1068-1078. doi:10.1002/ajh.24820

Additional Reading

By Suzanne Dixon, MPH, RD
Suzanne Dixon, MPH, MS, RDN, is an award-winning registered dietitian and epidemiologist, as well as an expert in cancer prevention and management.