Q & A Column, 9/14

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Editor: Frederick L. Kiechle, MD, PhD

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Effect of diurnal variation, postural changes on CBC

Using “transference” to standardize reference intervals throughout a regional health system

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Q. Occasionally on certain patients, when we draw for a CBC in the early morning, we get a low Hgb of 6 or 7 g/dL. We draw the same patient for a CBC in the afternoon and we get a higher Hgb by at least 1–1.5 g/dL. Can you explain the reason for this difference?

A. The CBC, and specifically hemoglobin, is affected by a number of different parameters. These include diurnal variation and posture-related changes, for example. Diurnal variation has been reported with many of the measurements of the complete blood count, including hemoglobin, from as early as 1947.1 Specifically, the RBC count, hemoglobin, and hematocrit show diurnal variation with gradually falling mean levels throughout the day, with the nadir at midnight. In contrast, the WBC count and individual leukocyte counts (neutrophil, eosinophil, monocyte, lymphocyte) gradually increase mean levels throughout the day, peaking at midnight. In one study2 hemoglobin and leukocytes showed the highest amount of variation when measured in healthy young men. In another study of athletes, hemoglobin decreased over the day by approximately 0.55 g/dL.3

In contrast, postural changes (supine to standing) increase hydrostatic pressure in dependent regions, resulting in a loss of plasma volume and subsequent hemoconcentration. In one study of hematocrit, a relative increase of 11 percent ± 3.6 percent was seen in individuals within 30 minutes of standing.4 Given the situation described in the question, one can wonder if the individual who has a higher hemoglobin later in the day has had some loss of plasma volume and subsequent hemoconcentration (e.g. postural, diuresis).