Quantitative reticulocyte counting: Clinical applications

Erythropoietic activity within the marrow is a dynamic process. Red cells emerge into the circulation containing remnants of the erythroblast RNA that was responsible for the synthesis of haemoglobin in the developing cell. Latterly, it has been possible to identify newly emerged cells by their RNA content. The earliest applications related to monitoring the erythropoietic response to recombinant human erythropoietin. Quantitative reticulocyte counters also allow other features of the reticulocyte population to be characterized. Quantitative reticulocyte counting has only lately become available. All the received haematological wisdom on this topic must be re- evaluated in the light of such real data. Marrow erythropoietic activity is a dynamic process. It results in the production of 2-3 million new red cells per second in a normal human subject. The red cells that emerge into the circulation contain remnants of the erythroblast RNA that was responsible for the synthesis of haemoglobin in the developing cell. Originally, these cells were recognized by the appearance of a reticulum when they were exposed to supravital stains such as new methylene blue. Hence, the eponym, reticulocytes. This reticuhun was related to the polymnal remnants within the newly emerged cells. As these cells became adult erythrocytes over the course of 1-2 days the reticulum disappeared. The process by which this takes place is thought to involve removal of these cytoplasmic remnants by the reticuloendothelial cells of the spleen. This also appeared to result in the loss of some cell volume and other cytoplasmic contents. Latterly, it has been possible to identifjl newly emerged cells by their RNA content. This involves using specific RNADNA stains to identi@ the cells (I). Originally reticulocytes were identified and estimated by visual inspection of a stained blood smear. Now it is possible to identifjl cells containing RNA and to count them in large numbers by flow cytometry. In practice the visual estimate of the proportion of retidocytes in the smear is at best semiquantitative. A number of studies have pointed to the high coefficient and variation of the estimate which may range up to 100% (2). The advent of quantitative flow cytometric techniques has brought the reticulocyte count in line with other blood cell counts with coefficient variation closer to 5%. The ability to reliably quantitate the number of reticulocytes in circulation has allowed a direct estimate of current erythropoietic activity. The best estimates of the reticulocyte life span in wo suggest that this is of the order of 0.5 - 2.5 days. Changes in the reticulocyte count therefore reflect current changes in erythropoietic activity. Anecdotal studies in normal subjects have demonstrated that the mechanisms controlling erythropoiesis respond rapidly to changes in the subject's circumstances. Modest changes of altitude and decreased oxygen tension will result in an increase in the erythropoietic activity and reticulocyte count in a matter of days. Conversely, mild infections may have a pronounced affect on suppressing erythropoietic activity. In the absence of such changes grthropoietic activity remains constant and there is no evidence of any diurnal variation.