To evaluate the hemoglobin content of your blood as part of a general health checkup; to screen for and help diagnose conditions that affect red blood cells (RBCs); if you have anemia (low hemoglobin) or polycythemia (high hemoglobin), to assess the severity of these conditions and to monitor response to treatment
With a hematocrit or as part of a complete blood count (CBC), which may be ordered as a component of a general health screen; when you have signs and symptoms of anemia (weakness, fatigue) or polycythemia (dizziness, headache); at regular intervals to monitor these conditions or response to treatment
A blood sample drawn from a vein or by a fingerstick (children and adults) or heelstick (newborns)
Hemoglobin is the iron-containing protein found in all red blood cells (RBCs) that gives the cells their characteristic red color. Hemoglobin enables RBCs to bind to oxygen in the lungs and carry it to tissues and organs throughout the body. It also helps transport a small portion of carbon dioxide, a product of cell metabolism, from tissues and organs to the lungs, where it is exhaled.
The hemoglobin test measures the amount of hemoglobin your sample of blood. A hemoglobin level can be performed alone or with a hematocrit, a test that measures the proportion of blood that is made up of RBCs, to evaluate your red blood cells. It is also one component of the complete blood count (CBC), a group of tests that are often used in the general evaluation of your health.
Red blood cells, which make up about 40% (ranging 37-49%) of the blood's volume, are produced in the bone marrow and are released into the bloodstream when they are, or nearly are, mature. The typical lifespan of an RBC is 120 days, and the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding.
Several diseases and conditions can affect RBCs and consequently the level of hemoglobin in the blood. In general, the hemoglobin level and hematocrit rise when the number of red blood cells increases. The hemoglobin level and hematocrit fall to less than normal when there is a decrease in RBCs.
Some conditions affect RBC production in the bone marrow and may cause an increase or decrease in the number of mature RBCs released into the blood circulation. Other conditions may affect the lifespan of RBCs in the circulation. If there is increased destruction of RBCs (hemolysis) or loss of RBCs through bleeding and/or the bone marrow is not able to produce new ones fast enough, then the overall number of RBCs and hemoglobin will drop, resulting in anemia.
Anemia is a condition in which tissues and organs in the body do not get enough oxygen, causing fatigue and weakness. If too many RBCs are produced, polycythemia results and the blood can become thickened, causing sluggish blood flow and related problems.
How is the test used?
The hemoglobin test is often used to check for anemia, usually along with a hematocrit or as part of a complete blood count (CBC). The test may be used to screen for, diagnose, or monitor a number of conditions and diseases that affect red blood cells (RBCs) and/or the amount of hemoglobin in blood.
A hemoglobin test may be used to:
- Screen for, diagnose, and measure the severity of anemia (low RBCs, hemoglobin and hematocrit) or polycythemia (high RBCs, hemoglobin and hematocrit)
- Monitor the response to treatment of anemia or polycythemia
- Help make decisions about blood transfusions or other treatments if the anemia is severe
- Determine eligibility for blood donation
When is it ordered?
Some signs and symptoms of anemia include:
- Weakness or fatigue
- Lack of energy
- Paleness (pallor)
- Shortness of breath
- Fast or irregular heartbeat
- Cold hands or feet
Some signs and symptoms of polycythemia include:
- Disturbed vision
- Enlarged spleen
This test may be performed several times or on a regular basis when you have been diagnosed with ongoing bleeding problems or chronic anemia or polycythemia to determine the effectiveness of treatment. It may also be ordered routinely if you are undergoing treatment for cancer that is known to affect the bone marrow.
What does the test result mean?
Since a hemoglobin level is often performed as part of a complete blood count (CBC), results from other components are taken into consideration. A rise or drop in the hemoglobin level must be interpreted in conjunction with other parameters, such as RBC count, hematocrit, reticulocyte count, and/or red blood cell indices. Age, sex, and race are other factors to be considered. In general, hemoglobin mirrors the results of the RBC count and hematocrit.
Low hemoglobin with low RBC count and low hematocrit indicates anemia. Some examples of causes include:
- Excessive loss of blood from, for example, severe trauma or chronic bleeding from sites such as the digestive tract (e.g., ulcers, polyps, colon cancer), the bladder or uterus (in women, heavy menstrual bleeding, for example)
- Nutritional deficiencies such as iron, folate or B12 deficiency
- Damage to the bone marrow from, for example, a toxin, radiation or chemotherapy, infection or drugs
- Bone marrow disorders such as aplastic anemia, myelodysplastic syndrome, or cancers such as leukemia, lymphoma, multiple myeloma, or other cancers that spread to the marrow
- Kidney failure—severe and chronic kidney diseases lead to decreased production of erythropoietin, a hormone produced by the kidneys that stimulates RBC production by the bone marrow.
- Chronic inflammatory diseases or conditions
- Decreased hemoglobin production (e.g., thalassemia)
- Excessive destruction of red blood cells, for example, hemolytic anemia caused by autoimmunity or defects in the red blood cell itself; the defects could be hemoglobinopathy (e.g., sickle cell anemia), abnormalities in the RBC membrane (e.g., hereditary spherocytosis) or RBC enzyme (e.g., G6PD deficiency).
High hemoglobin with a high RBC count and high hematocrit indicates polycythemia. Some examples of causes include:
- Lung (pulmonary) disease—if someone is unable to breathe in and absorb sufficient oxygen, the body tries to compensate by producing more red blood cells.
- Congenital heart disease—in some forms, there is an abnormal connection between the two sides of the heart, leading to reduced oxygen levels in the blood. The body tries to compensate by producing more red blood cells.
- Kidney tumors that produce excess erythropoietin
- Smoking—heavy smokers have higher hemoglobin levels than nonsmokers.
- Genetic causes (altered oxygen sensing, abnormality in hemoglobin oxygen release)
- Living at high altitudes (a compensation for decreased oxygen in the air)
- Dehydration—as the volume of fluid in the blood drops, the hemoglobin artificially rises.
- Polycythemia vera—a rare disease in which the body produces excess RBCs inappropriately
Is there anything else I should know?
Can I test my hemoglobin at home?
Yes, there are some home tests currently available that have been approved by the U.S. Food and Drug Administration (FDA). Home testing offers many benefits, but it's also important to recognize the potential tradeoffs between quality and convenience and to take steps to protect yourself against the possibility of false results, and your own lack of training. Talk to your healthcare practitioner about this type of testing and discuss any questions or concerns you may have. For more about these tests, see the article With Home Testing, Consumers Take Charge of Their Health.
What other tests may be done in addition to hemoglobin?
A hemoglobin test can indicate if there is a problem with red blood cell production and/or lifespan, but it cannot determine the underlying cause. In addition to the full CBC, some other tests that may be performed at the same time or as follow up to establish a cause include a blood smear, reticulocyte count, iron studies, vitamin B12 and folate levels, and in more severe conditions, a bone marrow examination.
Is anyone at greater risk of abnormal hemoglobin levels?
Women of childbearing age tend to have lower hemoglobin levels than men due to loss of iron and blood during menstrual periods and increased need for iron during pregnancy. Others who are at greater risk of a low hemoglobin level (anemia) include people with poor nutrition and diets low in iron or vitamins, people who have undergone surgery or have been severely injured, people with chronic conditions such as kidney disease, cancer, HIV/AIDS, inflammatory bowel disease, chronic infection or chronic inflammatory conditions (e.g., rheumatoid arthritis). Someone who has family members with a genetic cause of anemia, such as sickle cell or thalassemia, also has a higher risk of having the condition and a higher risk of anemia.
Are there warning signs for abnormally low hemoglobin levels?
Can a healthy diet and nutrition help keep optimal hemoglobin levels?
Yes, to the extent that if you eat a well-balanced diet, you can prevent anemia due to a lack of iron, vitamin B12, or folate in the foods you eat. Sometimes use of a supplement is recommended if you are at risk of a vitamin deficiency. However, the most common cause of vitamin B12 deficiency is malabsorption, and the most common cause of iron deficiency is bleeding. These conditions and other red blood cell problems that are caused by diseases other than nutritional deficiencies cannot be corrected by diet.