What is the hemoglobin test?

A hemoglobin test measures the amount of hemoglobin in your blood. Hemoglobin is a protein that is the main component of red blood cells (erythrocytes). Red blood cells are produced in the bone marrow. Hemoglobin contains iron, which allows it to bind to oxygen. Red blood cells transport oxygen from the lungs to other tissues and organs. They also carry carbon dioxide to the lungs so it can be removed from the body.

Lower than normal hemoglobin levels, otherwise known as anemia, can indicate that the organs of your body might not be getting an adequate supply of oxygen. This can lead to a lack of energy, fatigue, or other problems.

Low levels of hemoglobin may be a sign that the bone marrow is not producing enough red blood cells, or that cells are being destroyed faster than they are made. Chronic blood loss also leads to a drop in hemoglobin levels and is one of the most common causes of anemia.

Why is a hemoglobin test done?

A hemoglobin test may be performed along with a routine physical examination to assess the amount of hemoglobin in your blood. It is done as part of a complete blood count (CBC) test, which looks for the numbers of red blood cells, white blood cells, and platelets in a blood sample. A CBC is also done when there are symptoms of a low hemoglobin level, such as fatigue, weakness, or dizziness.

There are many disorders that result in low hemoglobin levels or anemia, including:

  • Iron deficiency, which is almost always caused by blood loss
  • Deficiency of other nutrients, such as vitamin B12 or folic acid
  • Internal or external blood loss resulting from surgery, injury, menstrual bleeding, or bleeding from the gastrointestinal tract
  • Cancer
  • Thalassemia, a common inherited blood disorder caused by genetic mutations in the hemoglobin genes. It is usually mild but severe forms can cause symptoms in childhood.
  • Kidney disease
  • Autoimmune diseases

Symptoms indicating low red blood cell levels may include:

  • Fatigue
  • Pale skin (pallor)
  • Weakness
  • Dizziness
  • Shortness of breath
  • Cold hands and feet

The hemoglobin test may be used to screen for anemia and other blood disorders and monitor the progress during treatment. It can only establish whether anemia is present and its degree of severity. The test cannot detect what is causing the abnormal level of hemoglobin. Additional tests are required.

Some conditions result an increase in red blood cells in the body, causing an excess of hemoglobin in the blood. These disorders include:

Too many red blood cells can cause the blood to thicken and become sluggish. Thick blood does not flow as quickly, which can deprive the organs of oxygen.

Symptoms of excess levels of red blood cells may include

  • Headaches
  • Blurred or double vision
  • Dizziness
  • Itching
  • Blood clots

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy