Comparison between Nageotte and flow cytometric counting of residual leucocytes in freshly prepared leucocyte-reduced red blood cell components

Background: Flow cytometry (FC) and Nageotte hemocytometry represent the most widely accepted methods for counting residual white blood cells (rWBCs) in leucocyte-reduced (LR) blood components. Our aim was to study the agreement between the two methods, under real working blood bank conditions. Materials and methods: 94 freshly produced LR red blood cell (RBC) units were tested for rWBC concentrations by FC and Nageotte. To assess the precision of each method, we calculated the intra-assay coefficients of variation (CV), and followed the Bland-Altman methodology to study the agreement between the two methods. Results: CV was 18.5% and 26.2% for the Nageotte and the FC, respectively. However, the agreement between the duplicate observations, using the binary cut-off threshold of 1 x 10(6) WBCs per unit to define the results as "pass/fail", was 71.9% for the Nageotte and 93.3% for the FC. Linear regression analysis did not show any correlation (R-squared = 0.01, p = 0.35) between the two methods, while the Bland-Altman analysis for the measuring agreement showed a bias toward a higher Nageotte count of 0.77 x 10(6) leucocytes per unit (p < 0.001) with the 95% limits of agreement (d +/- sd) ranging from -0.40 x 10(6) to 1.94 x 10(6) leucocytes per unit. Conclusion: The absence of agreement between Nageotte and FC method, with the differences within d +/- sd being of high clinical importance, suggests that the two methods cannot be used for clinical purposes interchangeably. The Nageotte seems unsuitable for quality control even with a pass-fail criterion, under real working blood bank conditions.