CE Update Submitted 4.30.10 | Revision Received 9.27.10 | Accepted 9.27.10 The Red Cell Histogram and The Dimorphic Red Cell Population Benie T. Constantino, SH, I; ART, MLT(CSMLS) (CML Healthcare Inc., Mississauga, Ontario, Canada) DOI: 10.1309/LMF1UY85HEKBMIWO Abstract conditions and may provide major clues in some morphological features of dimorphism The RBC histogram is an integral part of the diagnosis and management of significant and the ensuing characteristic changes in their automated hematology analysis and is now red cell disorders. In addition, it is frequently RBC histograms. routinely available on all automated cell used, along with the peripheral blood film, as counters. This histogram and other associated an aid in monitoring and interpreting abnormal Keywords: histogram, dimorphic red cells, red complete blood count (CBC) parameters have morphological changes, particularly dimorphic blood cell distribution width been found abnormal in various hematological red cell populations. This article discusses After reading this article, readers should be able to correlate a RBC Hematology exam 51102 questions and corresponding answer form histogram and red blood cell distribution width with microscopical are located after this CE Update on page 309. findings. Table 1_Conditions Associated With Dimorphic To paraphrase an adage, 1 histogram graph is worth 5-12 1000 numbers. A large collection of data, displayed as a visual Red Cells image, can convey information with far more impact than the numbers alone. In hematology, these data take on several Early iron developing microcytic population forms, 1 of which is the RBC histogram. Visual scanning Folate/vitamin B12 developing macrocytic population Post-iron treatment of iron deficiency anemia of the histogram gives a good initial sense of the range, size, Post-iron treatment of iron deficiency with megaloblastic anemia shape, and other salient features of the red cell morphology. Post-iron treatment of megaloblastic anemia The RBC histogram, a graphic representation of particle Post-iron treatment of megaloblastic anemia with iron deficiency size distribution, is now routinely available on automated cell Post-iron transfusion macrocytic anemia analyzers as a standard part of automated complete blood Post-iron transfusion microcytic anemia count (CBC) analysis. This histogram in association with Iron deficiency anemia with either folate or vitamin B12 deficiency other CBC parameters, such as RBC distribution width Sideroblastic anemia (myelodysplasia) (RDW) and mean corpuscular volume (MCV), has been Hemolytic anemia (reticulocytosis, spherocytosis, fragmentation, pyropoikilocytosis) found abnormal in various hematological conditions and may Cold/warm auto agglutination provide major clues in the diagnosis and management of sig- Erythropoietin-induced erythropoiesis nificant red cell disorders.1-4 In addition, it is frequently used, Delayed transfusion reaction along with the peripheral blood film, as an aid in monitoring Homozygous hemoglobinopathies (admixture of many RBC forms) and interpreting abnormal morphological changes, particu- Myelofibrosis (admixture of extramedullary hematopoiesis) larly dimorphic red cells. Constitutional chromosomal translocation t(11;22)(p15.5;q11.21) Table 1 lists conditions associated with dimorphic red cell changes. Before reporting CBC results, a clear distinc- tion between dimorphic and dual populations must be clari- fied as they are sometimes interchangeable and confusing. In a dimorphic picture, the histogram may have 2 or more (multiple) red cell populations, whereas in dual populations the histogram has 2 distinct red cell populations (eg, hypo- chromic-microcytic and normochromic-normocytic red cells). Corresponding Author These 2 distinct populations may be comprised of either a Benie T. Constantino, SH, I; ART, MLT(CSMLS) patient’s own red cells (post-iron treatment) or a mixture of [email protected] patient and donor red cells (post-iron transfusion). Although this is an arbitrary categorization, the term dimorphic is less restrictive and therefore more widely used, as it can be applied Abbreviations to either the dual or the multiple red cell populations. Thus, CBC, complete blood count; RDW, red blood cell distribution width; the dimorphic blood picture will look like a dual popula- Image 1A MCV, mean corpuscular volume; MCHC, mean corpuscular hemo- tion of microcytic and normocytic ( ) or normocytic globin concentration; IDA, iron deficiency anemia; RDW-CV, red and macrocytic red cells, or an admixture of small, normal, Image 1B blood cell distribution width coefficient of variation; thal, thalas- and large cells of different sizes and forms ( ). Some semia; SD, standard deviation; HPP, hereditary pyropoikilocytosis morphological features of dimorphism and the ensuing char- acteristic changes in their RBC histograms are discussed in this article. 300 LABMEDICINE ■ Volume 42 Number 5 ■ May 2011 labmedicine.com CE Update A B Image 1_(A) Wright-stained peripheral blood film from post-iron treated patient showing dual population of hypochromic-microcytic (short arrow) and normochromic-normocytic red cells (long arrow). (B) Peripheral blood film from post-splenectomized patient with hereditary pyropoikilocytosis showing dimorphic picture of extreme variability of the RBCs such as red cell fragments (short arrow), microspherocytes (regular arrow), membrane budding (long arrow), elliptocytes, oval macrocytes, and polychromasia. RBC Histogram: An Overview data.5,15 For example, in megaloblastic anemia with developing The well-known Coulter principle of counting and sizing iron deficiency, some of the hypochromic and microcytic red red cells provides the basis for generating the histogram. This cells can be identified in the histogram but may not be reflected method relies on the change in conductance as each cell passes in the numerical data, and this may well be the first clue to a through an aperture. The change in conductance results in not uncommon occurrence. Likewise, iron deficiency may de- an electrical pulse, the amplitude of which is proportional to velop insidiously during the process of some other disease (eg, the cell volume. The 256-channel pulse-height analyzer uses a essential polycythemia), with normal cell counts and indices, number of thresholds to sort the cells into several size (volume) except the RDW, which is elevated. The RBC histogram is also channels from which the histogram is formed. Each channel abnormal, showing a shoulder of microcytic red cells.16 on the X-axis represents a specific size (volume) in femtoliter Furthermore, a histogram can provide useful information (24-360 fL), increasing from left to right. The Y-axis represents for laboratorians in 1) monitoring the reliability of the re- the number of cells per channel, with each cell being stored in sults generated by the analyzers; 2) investigating the potential the channel representing its size, so that after data accumula- cause(s) of the erroneous automated results; and 3) arriving at tion is completed the relative number of cells (frequency) is the presumptive diagnosis.7 For example, certain conditions provided. This data is further processed by the computer, and like the presence of fragmented red cells or red cell agglutina- the RBC curve is smoothed by a moving average technique and tion that could not have been identified earlier without blood displayed on a data management system.13 Figure 1A shows a film examination can now be presumably detected on the red typical normal RBC histogram. For a detailed explanation on cell histogram.3,16 Likewise, in patients with iron deficiency the construction of histogram, please see reference 14. anemia (IDA) or megaloblastic anemia in treatment, a sequen- tial histogram can clearly show the progressive appearance of a new erythrocyte population well in advance of other numeri- cal parameters.3 Value of Histogram The presence of a right-sided shoulder usually corresponds A histogram, as shown in Figure 1 and Figure 2, is a to reticulocytosis (Figure 1J), and a trailer of erythrocyte popu- graphic representation of a collection of data based on cell lations on the far right of the histogram correlates to red cell size and/or cell number depicting variations in the process. It agglutination (Figure 1F). A leftward shift of the RBC histo- is sometimes referred to as a frequency of distribution curve. gram signifies microcytosis (Figure 1B and Figure 1C), and a Because graphics can show data in ways that are meaningful rightward shift suggests macrocytosis (Figure 1D and Figure and quickly understood, the histogram is a very powerful tool 1E). Bimodal red cell histograms (Figure 1F, Figure 1G, Figure in red cell morphological analysis. It enables one to visualize, 1K, Figure 1L and Figure 2B to Figure 2E) are usually associated analyze, and interpret empirical data that displays morpholog- with therapeutic transfusion and/or hematinic agent response to ical changes graphically as points, peaks or valleys, or as a line microcytic and macrocytic anemia, but they may also indicate of frequency curve. It allows the users to intuitively see the other hematological disorders as shown in Table 1. visual comparison of the center and spread of data and data Although the size ranges for RBC histograms are between containing 2 or more variables: dimorphic red cells, subpopu- 24 fL and 360 fL, the instrument counts only those cells with lations, and skewed data, leading to quick and cost-effective