Hemoglobin levels and transfusion

During steady state, when patients are clinically well, hemoglobin levels vary within a narrow range characteristic of the individual patient and the genotype. Most patients with homozygous sickle cell disease have steady state levels of 6 to 9 g/dl, although levels may be as low as 5 g/dl or as high as 12 g/dl. HbS within the red cell manifests a low oxygen affinity, with more oxygen being unloaded per gram of hemoglobin than is the case with HbA. The oxygen affinity varies between patients, but those with the lowest steady state hemoglobin levels have the most marked shifts in their oxygen dissociation curve. A patient with a steady state hemoglobin level of 6 g/dl has a low oxygen affinity and maintains that level because oxygen delivery is near normal; thus a higher level is unnecessary. Transfusing a patient up from his or her steady state level does not necessarily improve oxygen delivery since, although hematocrit increases, the oxygen dissociation curve returns towards normal. Therefore a knowledge of the patient's steady state hemoglobin level is important for management, and transfusion up from that level achieves little in terms of oxygen delivery.

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