Hemoglobin is the red blood cell protein that carries oxygen to the tissues and is responsible for the red colour of blood. A deficiency in hemoglobin and in red blood cells (anemia) is manifested by weakness or fatigue, lack of energy, fainting, paleness and shortness of breath. Conversely, excess hemoglobin and red blood cells (polyglobulia) can manifest as vision problems, dizziness, headache, flushing, etc. People who live in higher altitudes generally have higher hemoglobin levels, whereas levels tend to be slightly lower in older men, women, children and pregnant women.
Hemoglobin results are interpreted in light of other blood count parameters (see sections on red blood cells, platelets, MCV).