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Clinical Chemistry 48: 567-570, 2002;
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(Clinical Chemistry. 2002;48:567-570.)
© 2002 American Association for Clinical Chemistry, Inc.

Technical Briefs

Minimizing Error in the Determination of P50

Robert W. Burnett1

1 Department of Pathology and Laboratory Medicine, Hartford Hospital, Hartford, CT 06102; fax 860-545-3733, e-mail rburnett@attbi.com

The measurement of P50 in whole blood, defined as the oxygen tension corresponding to 50% oxygen saturation, gives a quantitative measure of hemoglobin oxygen affinity. The determination of P50 is useful for screening for hemoglobin variants in cases of unexplained anemia or erythrocytosis, both because it is easily performed and because many hemoglobin variants with abnormal oxygen affinity are "silent", i.e., not detectable with conventional electrophoretic techniques (1).

The determination of P50 previously required obtaining several points on the oxyhemoglobin dissociation curve. The procedure was seldom performed because of the labor and specialized apparatus required. However, it was suggested >25 years ago that P50 could be accurately calculated from single measurements of oxygen tension (Po2) and the corresponding oxygen saturation (So2) (2). Since then, an improved mathematical model of the oxyhemoglobin dissociation curve (3) has been incorporated into a guideline for the so-called "single-point" determination of P50 (4) that has been tested (5)(6) and subsequently approved by the IFCC. The single-point approach is based on the fact that a plot of log Po2 vs log[So2/(1 - So2)], the so-called Hill plot, is linear over a fairly wide interval. Therefore, if one knows both the slope and one point on the line, any other point on the line can be determined.

There have been different recommendations for the interval within which the single-point method is valid. Some studies have suggested that it can be used for oxygen saturations as low as 20% (5). The original IFCC guideline (4) specified saturations in the interval between 40% and 80%, and a subsequent IFCC document on definitions of quantities used in blood pH and gas analysis (7) extended the . . . [Full Text of this Article]


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Copyright © 2002 by the American Association for Clinical Chemistry.