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Hemoglobin levels might be subject to postural effects — lower when supine and higher when upright — because of shifts in plasma volume to the vascular space with recumbent posture. In this single-center U.S. study, investigators evaluated 35 medical inpatients, measuring hemoglobin levels after patients laid supine for at least 6 hours and again after patients sat upright for at least 1 hour. Patients receiving intravenous diuretics and those with hematologic malignancies, risk for blood loss, or transfusion requirements were excluded.
Median supine hemoglobin concentration was 11.7 g/dL, which increased significantly, by a median 0.6 g/dL, after sitting upright. Nearly one third of patients had hemoglobin increases of 1.0 g/dL or more afte…