How Reliable Are Red Cell Indices?

Abstract

Rudolph M. Widmark, M.D., Ph.D., is a staff consultant, Life Sciences, Technicon Corporation, Tarrytown, N. Y. In 1952, Wintrobe proposed that erythrocytes be characterized in terms through the calculated indices known as MCV (mean corpuscular volume), MCH (mean corpuscular hemoglobin), and MCHC (mean corpuscular hemoglobin concentration). Obviously, these indices are only as reliable as the data—RBC count, Hb, and PCV (PCV stands for packed cell volume, and should replace the old term hematocr i t )— upon which they are based. In view of the known inaccuracy of RBC chamber counts, it comes as no surprise that the widely accepted of the red cell indices is the MCHC, the only index not derived from an RBC count. However, the MCHC alone is quite insufficient to characterize the red blood cells—like indication of only the height of a three-dimensional object in order to describe it. Furthermore, the belief that the RBC count can be unfailingly estimated from the Hb concentration and the PCV is erroneous; a correlation between these three parameters exists only under normal physiologic condit ions. Therefore, the three primary data and the three indices derived from them are equally important. The accurate estimation of the absolute red cell indices has become possible through automated cell counters such as Technicon's SMA 4A/7A, which should yield accurate results if properly operated and calibrated. However, malfunctions of the instrument or unexpected behavior of some blood specimens in the instrument should arouse the concern of the clinical pathologist. We have found the red cell indices to be useful in the quality control of RBC counts, Hb, and PCV values. As a general rule, the MCV should parallel the MCH; that is, the larger the cell, the more hemoglobin it might contain, and vice versa. Furthermore, it is physically impossible for any MCHC value to exceed 38 percent. Such a value would be indicative of other erroneous data. If values of MCV and MCH are discrepant, the blood smear should be examined. Macrocytic hypochromic cells will confirm the correctness of PCV, Hb, and RBC count. If the blood smear does not reflect such discrepancy, it is recommended that examination of the three parameters be repeated by a different method in the automated instrument, in order of their known precision. Thus, for the SMA 4A/7A, the order is: first, PCV; next, Hb; and, finally, if necessary, the RBC count. As soon as the result of the repeated examinations is obtained, the indices should be recalculated and interpreted as described above. In this manner, incorrect data can be discarded and the most l ike ly result decided upon. Though primarily designed for quality control of the output of automated hematologic instruments, the above-outlined ground rules are equally applicable to results obtained manually.