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. Hemoglobin is the iron-containing protein found in all red blood cells that enables RBCs to bind to oxygen in the lungs and carry it to tissues and organs throughout the body. (For more, see the "What is being tested?" section.)

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

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.

This 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.

The hemoglobin test may be ordered during a general health examination or when a person has signs and symptoms of a condition affecting red blood cells such as anemia or polycythemia.

Some signs and symptoms of anemia include:

• Weakness or fatigue
• Lack of energy
• Fainting
• Paleness (pallor)
• Shortness of breath

Some signs and symptoms of polycythemia include:

• Disturbed vision
• Dizziness
• Headache
• Flushing
• Enlarged spleen

This test may be performed several times or on a regular basis when someone has been diagnosed with ongoing bleeding problems or chronic anemias or polycythemia to determine the effectiveness of treatment. It may also be ordered routinely for people undergoing treatment for cancer that is known to affect the bone marrow.

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 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 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

A recent blood transfusion can affect a person's hemoglobin level.

Hemoglobin decreases slightly during normal pregnancy.