New erythrocyte diagnostic parameters in neonatal medicine

Introduction

In 1929 Wintrobe published a method for measuring the percent of blood comprised of red cells—the hematocrit. Soon thereafter he reported a method for measuring the volume of the average circulating red blood cell (RBC), in femtoliters (fL, 10 ^-15 L), as a way to judge whether a patient’s RBCs are normal size or smaller or larger than normal. In that publication Wintrobe also reported a way to determine the average hemoglobin content of a RBC, in picograms (pg, 10 ^-12 g), as a way to judge whether the amount of hemoglobin within the RBCs is normal. Lastly, he described the concentration of hemoglobin in RBCs (g/dL blood) as the mean corpuscular hemoglobin (MCH) concentration. These basic measurements formulated the basis of his classification of anemias as microcytic, normocytic, or macrocytic and for judging whether erythrocytes were hypochromic. Although the methods for performing these measurements have markedly improved through advancing technology, Wintrobe’s classification has remained for the past 90 years the standard approach to anemia evaluation.

Some of the recent additions to the automated complete blood cell count (CBC) reveal yet more useful information about a patient’s erythrocytes. These additions include the four parameters discussed in this chapter: specifically the reticulocyte hemoglobin count (RET-He), fragmented red blood cell count (FRC), percentage of microcytic erythrocytes (Micro-R), and percentage of hypochromic erythrocytes (HYPO-He). In most parts of the world these four additional RBC parameters are readily available to clinicians as part of each automated CBC. In the United States the RET-He is similarly available to all who order the test as part of a CBC with no added phlebotomy volume from the patient or laboratory run time. However, in the United States the FRC, Micro-R, and HYPO-He are currently designated as research parameters. They can be obtained on virtually every CBC, but the values are only released from the clinical laboratory under certain regulatory conditions. One reason for this has been the lack of reference ranges for these tests, particularly in neonatal subjects, by which to judge whether the values are low, normal, or elevated.

Our group has sought to remedy this by creating rigorous reference intervals for each of these four new erythrocyte parameters. We have done this with three goals in mind: (1) We hope that the neonatologists in Europe and Asia who currently have all four of these tests available with each CBC run will take advantage of the reference intervals shown in this chapter and thereby will learn more about each patient’s erythrocytes with these tests. (2) We hope that the neonatologists in the Americas will use the RET-He routinely when considering iron sufficiency in their patients and will work with their clinical pathology laboratories to, under proper regulatory guidance, further assess the value of the three other new tests in clinical practice. (3) We hope that this body of work will facilitate the process of eventually having all of these new erythrocyte parameters released by the clinical laboratory to the clinicians as an integral part of every CBC run on a neonate, along with the hemoglobin, hematocrit, RBC indices, and red cell distribution width (RDW), and that so doing will help neonatologists recognize various erythrocyte abnormalities in their patients.

Flow-cytometric evaluation as an integral part of a modern CBC

Adding the technology of flow cytometry to automated CBC analysis created the possibility of several new diagnostic parameters. The technology involves the blood sample being diluted and moving through a tube thin enough that the cells pass by one at a time. Characteristics about the cell are measured using lasers and electrical impedance. To focus on specific aspects of the blood, for instance on leukocytes or reticulocytes or nucleated RBCs, the sample is separated into a number of different channels depending somewhat on the model of counter. Such analyzers typically use EDTA (lavender-top) tubes or microtainer tubes. Evolving technology is making it possible for an automated CBC, with expanded parameter reporting, to be done on a blood sample as small as 15 µL (0.015 mL).