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Howell-Jolly body interference in reticulocyte counts

With interest we have read the recent paper by van Berkel and colleagues, who reported on pseudoreticulocytosis in a blood sample from a splenectomized patient with high number of Howell-Jolly bodies [1]. These authors observed spuriously high reticulocyte counts when the sample was measured in a CELL-DYN Sapphire (Abbott Diagnostics, Santa Clara, CA, USA) and a XE-5000 (Sysmex, Kobe, Japan) hematology analyzer, but not in an LH750 instrument (Beckman Coulter, Hialeah, FL, USA). They correctly explained the phenomenon by a combination of different dyes and different gating strategies [1]. Interference by Howell-Jolly bodies in automated reticulocyte counts is well known [2]. With respect to CELLDYN Sapphire, this phenomenon has not been previously described in the scientific literature, but is mentioned as a potential interference in the Operators Manual [3]. Along with any reticulocyte count, CELL-DYN Sapphire analyzer always reports the Immature Reticulocyte Fraction (IRF). The IRF is a measure of reticulocyte maturity and is directly derived from the fluorescence intensity of red blood cells (RBC) after staining with the proprietary CD4K530 dye [4], as is depicted in Figure 1A. In a healthy population without reticulocytosis, the IRF ranges between 0.07 and 0.37 (unpublished results). The presence of erythrocyte inclusions that react with the fluorescent dye may cause a shift of the fluorescence signal, which translates into an abnormally low or abnormally high IRF value, dependent on the type of inclusion. In case of Howell-Jolly bodies the IRF is generally decreased (Figure 1B), whereas malaria infection often causes strongly increased IRF values (Figure 1C) [5]. We therefore recommend assessing the IRF value and RBC fluorescence histogram as part of the validation procedure for high reticulocyte counts obtained from CELL-DYN Sapphire analyzer. This can be done