Comparison of different methods for cell counting in bronchoalveolar lavage fluid

The differential cell pattern in bronchoalveolar lavage (BAL) fluid is used in the diagnostic approach and narrowing of differential diagnoses in diffuse parenchymal lung diseases. The aim of this study was to obtain the cellular profile of BAL fluid via flow cytometry (FCM), a faster and more objective method than optical microscopy, and to evaluate its potential clinical utility by comparing the results with optical microscopy. This retrospective study included 54 patients who underwent bronchoalveolar lavage for the diagnosis of diffuse parenchymal lung disease in 2017 and 2018. BAL fluid from these patients was analyzed via both optical microscopy (OM) and FCM. According to the Wilcoxon signed-rank test, differences in lymphocyte and eosinophil counts between FCM and optical microscopy were found to be significant (p = 0.005 and p < 0.001, respectively). However, the differences between FCM and optical microscopy were not significant for alveolar macrophages or neutrophils (p = 0.820 and p = 0.280, respectively). Bland-Altman analysis showed that, compared with OM, FCM underestimated lymphocyte counts by 34.4%, with a wide 95% CI (− 50.5 to 18.3), and neutrophil counts by 14%, whereas alveolar macrophage measurements showed close agreement. FCM generally detected higher eosinophil percentages than OM. Our study suggests that FCM provides a rapid and objective approach to differential cell analysis of BAL fluid. However, despite comparable results for macrophages and neutrophils, discrepancies in lymphocyte and eosinophil counts should be considered in clinical interpretation. Larger studies are needed to validate these findings and support broader clinical use.