Hemoglobin (Hgb) is the main component of red blood cells (RBCs). This protein serves to carry oxygen and carbon dioxide in the body. Hgb levels are measured as part of the complete blood count (CBC), which is drawn during routine bloodwork and during evaluation of illness. High Hgb is caused by increased RBCs or by decreased blood volume.
Normal Hemoglobin Levels
Normal Hgb levels are generally defined as 14 to 18 grams per deciliter (g/dl) for men and 12 to 16 g/dl for women. Pregnant women should have at least 11 g/dl. Critical values for Hgb are less than 5 g/dl and greater than 20 g/dl. Reference values may vary slightly from lab to lab.
Function of Hemoglobin
Hgb carries oxygen and carbon dioxide; therefore, the Hgb concentration determines the oxygen-carrying capacity of the blood. Increased Hgb can be a compensatory measure in illnesses where oxygen intake is reduced.
Increased RBCs as Compensatory Mechanism
The body can increase production of RBCs to increase the amount of oxygen carried by the blood. This response is seen in congestive heart disease, chronic obstructive pulmonary disease, and other diseases that reduce respiratory efficiency. This also occurs as a response to external situations, such as living at high altitude.
Polycythemia vera is a disease in which the bone marrow produces an overabundance of RBCs. Blood tests on these patients will show increased Hgb, increased hematocrit, and of course increased RBCs. Symptoms include a flushed face, systemic itching, shortness of breath, chest pain and fatigue. This is a potentially life-threatening condition.
Erythropoietin and Other Drugs That Increase Hemoglobin
Erythropoietin (EPO), a substance produced by the kidneys, triggers production of RBCs by the bone marrow. EPO can be given as a drug, usually to counteract the side effects of chemotherapy. Injections of EPO are sometimes given in hopes of increasing athletic performance, a practice known as "blood doping." Any increase in EPO will cause an increased Hgb. Hgb levels can also be increased by the drugs gentamicin and methyldopa.
The Hgb level is a measure of the concentration of Hgb in the blood. Therefore, if the volume of the blood is decreased, the Hgb level will rise, even when the number of RBCs has not changed. Severe dehydration decreases the blood volume. An increased Hgb is often seen with severe diarrhea or burns.