Hb and HCT do not match in oncology patients.

Q Our laboratory has a large oncology population. We have noticed recently that some patients have hemoglobin/hematocrit results that do not match and require spun hematocrit for report. Can you advise how to report these specimens or direct me to literature addressing this problem? We know one patient is a known polycythemic, but the others are being treated with a new red-cell-boosting drug.

A Nearly all current procedures for measuring hemoglobin involve red-blood-cell (RBC) lysis/dilution and formation of a cyanomethemoglobin compound, which is read by a spectrophotometer at 540 nm. Hematocrit values may be determined manually or by an automated method. Automated hematocrit values are derived from a calculation in which the hematocrit equals the RBC count multiplied by mean corpuscular volume (MCV), then divided by 10. The hematology analyzer directly measures both the RBC count and MCV. Manual methods are determined by microhematocrit centrifugation and can be 2% to 3% higher than automated hematocrits due to plasma trapping. To ensure that the hemoglobin and hematocrit are accurate, a quick mathematical check referred to as the "rule of three" has been described. The hemoglobin is multiplied by three, and this value should agree within +/-3% of hematocrit. This rule can only be applied, however, to normal erythrocytes and does not apply when the MCV, hemoglobin, or RBC count is abnormal.

The disagreement of "rule of three" could be seen in following situations using automated hematology analyzer:

* true abnormal erythrocytes, such as reticulocytosis, microcytosis, hypochromic RBCs, erythrocytosis, and others;

* false elevations in hematocrit, hemoglobin, or RBC counts. These may be seen in patients with cryoproteins, lipemia, high white-blood-cell counts, and giant platelets; and

* instrument problem, random error may have occurred.

The function of erythropoietin is to stimulate erythropoiesis in the bone marrow by stimulating committed stem cells to proliferate and differentiate, decreasing erythroid precursor (normoblast) maturation time, increasing the rate of hemoglobin synthesis, and stimulating early release of bone marrow reticulocytes. It is likely that treatment with recombinant erythropoietin in oncology patient population is leading to reticulocytosis, which could cause increased hematocrit and, thus, mismatch in your hemoglobin and hematocrit values.

--Sarah Henry, MD

Guang Fan, MD PhD

Department of Pathology

Oregon Health and Science University

Portland, OR

References

1. Jacobs S, Oxley DK, eds. Laboratory Test Handbook. Cleveland, OH: Lexi-Comp Inc.; 2001:441-443.

2. McKenzie SB, ed. Clinical Laboratory Hematology. Upper Saddle River, NJ: Pearson Prentice Hall; 2004:63,133.

3. Wennecke G. Hematocrit--a review of different analytical methods. Available at: http://www.heartpumper.com/hematocrit.htm. Accessed September 2004.