Hemoglobin is a protein found in red blood cells that carries oxygen from the lungs to the rest of the body. A hemoglobin test measures the amount of hemoglobin in the blood, and it is often used to check for anemia, a condition in which the body has fewer red blood cells than normal. The test is usually done as part of a complete blood count (CBC) and involves taking a blood sample, which is then sent to a lab for analysis. The normal range for hemoglobin levels varies slightly among different laboratories, but generally, for adult men, the normal range is 13.5 to 17.5 grams per deciliter (g/dL) of blood, and for adult women, it is 12.0 to 15.5 g/dL of blood. Low hemoglobin levels may be a sign of anemia, bleeding, or other medical conditions, while high hemoglobin levels may indicate polycythemia vera, living at a high altitude, smoking, or dehydration. If a hemoglobin test reveals abnormal results, further testing may be needed to determine the underlying cause.