Microscopic method for assaying F cell production: Illustrative changes during infancy and in aplastic anemia

Fetal hemoglobin (HbF)-bearing reticulocytes (F reticulocytes) can be detected in peripheral blood by a modificcation of the microscopic single-cell radial immunodiffusion method. Thereby otherwise inappreciable changes in HbF production can readily be recognized. F reticulocyte frequencies are reproducibly measurable whenever the product of whole blood HbF-bearing red cell (F cell) frequency and reticulocyte frequency is ~5 x 10^-4 or greater, Serial analyses of F reticulocytes and nonreticulocyte F cells (F erythrocytes) illustrate that (1) levels of F reticulocytes and F erythrocytes are persistently similar in normal adults with more than 6% F cells and thus cell survival times of F and non-F cells must be essentially the same, (2) changing levels of F reticulocytes can be sensitive predictors of later changes in mature F cell frequencies during infancy and in adults recovering from aplastic anemia, and (3) alterations in F reticulocyte frequency and the amount of HbF per F reticulocyte are discordant in some settings but concordant in others.