A hemoglobin derivative that is incapable of reversibly associating with oxygen, and so is unable to carry oxygen from the lungs to the cells. The primary defect in dyshemoglobins is a chemical (or stereochemical) alteration of the heme prosthetic group. Two common dyshemoglobins are carboxyhemoglobin (COHb) in which carbon monoxide is covalently bonded to the hemoglobin molecule, and methemoglobin (metHb) in which the ferrous iron is oxidized to the ferric form. Other, indeterminate dyshemoglobins exist in minute amounts in circulating blood.