A hemoglobin reading measures the amount of hemoglobin, the oxygen-carrying protein in red blood cells, per deciliter of whole blood. Hemoglobin levels decrease with age, and men tend to have higher hemoglobin counts than women. For adults, the normal range for hemoglobin is 14 to 18 grams per deciliter for men and 12 to 16 grams per deciliter for women.
A low hemoglobin level, also known as anemia, can be the result of blood loss. It may also be a sign of deficiencies in key nutrients, particularly iron, vitamin B12 and folate. More rarely, low hemoglobin levels can be a symptom of bone marrow disorders, kidney failure or hemoglobin disorders, such as sickle cell anemia or thalassemia.
Abnormally high hemoglobin counts are caused by increased red blood cell production, the body's natural compensation mechanism in response to low blood oxygen levels. Low blood oxygen levels are common among people living at high altitudes, as well as smokers. Dehydration at the time of the blood test can also lead to falsely elevated levels of hemoglobin. However, a high hemoglobin count may also be a sign of more serious health issues, such as heart disease, emphysema or lung cancer. Among athletes, a high hemoglobin count may also be evidence of the abuse of erythropoietin, a performance-enhancing drug that artificially stimulates red blood cell production.