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Hemoglobin (Hgb) and hematocrit (Hct) continue to be the hallmark laboratory parameters used to monitor and manage anemia in dialysis patients. Nephrology clinicians generally assume that these two tests are equally useful and therefore use them interchangeably. This practice has been perpetuated by the seemingly close correlation between Hgb and Hct, and the clinical practice of multiplying the Hgb times three to calculate an approximate Hct level (Hillman & Finch, 1994).
Despite the frequency of this practice, clinical evidence indicates that Hct readings are more likely to be affected by outside influences, with the ensuing potential for variability in serial readings. This article examines the comparative accuracy of Hct and Hgb, including a retrospective analysis of same-patient laboratory data. Potential implications for clinicians who care for anemic dialysis patients are explored as well.
Laboratory Measurement of Hgb and Hct
Hgb, a blood-borne protein that facilitates the transport of oxygen to, and the removal of carbon dioxide from tissue, constitutes over 90% of a normal red blood cell (each cell contains almost 250 million Hgb molecules). Hgb is composed of two pairs of globin chains and four heme groups containing ferrous iron. It is commonly measured by automated spectrophotometry in which the red cell is lysed and the total Hgb count is determined by measuring the iron-containing pigment. Hgb thus provides a direct measurement of the oxygen carrying capacity of the blood (McClatchey, 1994).
By contrast, Hct provides an indirect measurement of the body's oxygen-carrying ability. Hct can be determined either by automated blood-counters or by microhematocrit (or spun) readings. In the case of automated complete blood count equipment, the Hct is a calculated percentage derived from the product of two red cell indices: the red blood cell count and the average size of red blood cells (also referred to as the mean corpuscular volume, or MCV). This autoanalyzed method uses an electric field, a highly focused light source, or a radiofrequency wave to analyze the size and number of red blood cells in solution. Measurements are derived by determining the change in electrical resistance (the Coulter principle), and Hct is then calculated by multiplying the MCV by the red cell count (Hillman & Finch, 1994; Ryan, 1995).
In contrast, the microhematocrit method reflects the separation of red blood cells from the plasma: The blood sample is collected and spun in a microhematocrit capillary tube, and the distance from the clay/red cell interface to the meniscus of the red cell/white cell interface is then noted. These results are then matched with a well-defined size distribution curve to determine the percentage of red blood cells (England, Walford, & Waters, 1972; Hillman & Finch, 1994; Williams, Morris, & Nelson, 1995; Wintrobe, Lee, Boggs, Bithell, Foerster, Athens, & Lukens, 1981).
Comparative Accuracy …