Analytic variability in the enumeration of neutrophil subpopulations in canine blood

Background. Conventional differential leukocyte counts do not enumerate hyposegmented neutrophils (Hypo-PMN), i.e. immature neutrophils that already lost the band morphology but are not yet completely segmented. They may be early indicators of acute inflammation. Objectives. The purpose of the study was to evaluate the analytic variability of counts of Bands, Hypo-PMN, Young-PMN (Bands + Hypo-PMN), mature neutrophils (Seg-PMN), non-Bands (Seg-PMN + Hypo-PMN); to assess if Hypo- or Young-PMN identify inflammation better than Bands. Methods. Neutrophil subpopulations were counted by 2 observers on 2 sets of 100 cells in 267 samples from dogs with changes potentially consistent with inflammation, to calculate the intra- and inter-observer variability. Results. Median intra-observer CV were <5.0% for Seg-PMN and non-Bands, 20.0% - 28.0% for Hypo-PMN and Young-PMN; Median inter-observer CVs for Seg-PMN, non-Bands, Hypo-PMN, and Young-PMN were 4.6%, 5.0%, 60.0%, and 47.1% respectively. Median CVs of Band counts in blood smears with bands were 141%. Conclusions. The analytic variability of Hypo- and Young-PMN is lower than that of Bands. This retrospective study did not allow us to investigate the diagnostic potential or the clinical relevance of these cells. However, the low inter- and intra-observer variability with these cell populations suggest that the count of Hypo- or Young-PMN may better identify acute inflammation than the count of Bands.