Background: Hemoglobin and hematocrit, routinely measured in hospitalized patients, are likely affected by shifts in plasma volume related to posture. This could have important ramifications on hemoglobin trends overnight, resulting in over diagnosis of anemia and potentially unnecessary anemia work-up and treatment. We hypothesized that hemoglobin measured in early morning (supine) samples, after plasma shifts resulting in increased intravascular volume, are significantly lower than values measured in patients sitting upright.
Methods: This study included patients hospitalized on the general medicine ward who were 18 years or older, scheduled for morning complete blood count (CBC) as part of their care, and had a baseline hemoglobin above 8 g/dL. Patients with hematologic malignancy, undergoing surgery, unable to change posture, or receiving red blood cell transfusion or intra-vascular modifying agents (such as diuretic doses between blood samples) were excluded. The first CBC was collected in the morning (after a night rest) while the patient remained in the supine posture. The patient was then transferred to a chair and approximately 1-hour later had a repeat CBC collected. We sought to avoid any drops in hemoglobin by phlebotomy by ensuring that the supine sample was always drawn first. Changes in hemoglobin, that is, supine versus upright, were compared via paired t-tests.
Results: Patients had a mean increase in Hgb of 0.59 g/dL with sitting (95% CI 0.42 to 0.76), a 5.0% relative change (95% CI, 3.6% to 6.5%). Similar changes were seen with hematocrit (data not shown). 10 of 35 subjects (29%) exhibited an increase in hemoglobin of 1.0 g/dL or more.
Conclusions: On average there was a 0.59 g/dL increase in hemoglobin from supine to a seated posture in hospitalized patients. Over ¼ of patients had a change of at least 1.0g/dL. Hospitalists should be aware that posture significantly influences hemoglobin levels in hospitalized patients on the general medicine wards.
