Implementation barriers of standardizing flow cytometric immune reconstitution monitoring post-allogeneic hematopoietic cell transplantation.

INTRODUCTION: Immune reconstitution (IR) after hematopoietic cell transplant (HCT) impacts on patients' outcome. However, assessment of IR is highly heterogeneous, affecting the possibility to produce solid evidence and hampering the potential use of IR as clinical endpoint. Inter-center standardization of IR monitoring is therefore essential to advance the field and ultimately improve patient outcomes. While standardization in flow cytometry is feasible, it requires coordinated efforts across centers, including harmonization of pre-analytical variables (e.g. sample collection and handling) and analytical procedures. AREAS COVERED: we review and discuss which steps should be taken to achieve standardization, from sample management to analytical process. The literature search was conducted using PubMed and updated through June 2026, with a preferential selection for international collaborative works. EXPERT OPINION: The lesson learned from international consortia that have been working on standardization in flow cytometry should be directly applied in IR monitoring. This would require international collaborative efforts that could be achieved through existing working groups. Furthermore, in a moment when artificial intelligence is gaining more space in medicine, a standardized approach would imply a dramatic increase in biological knowledge that can be derived from post-HCT IR and this would translate in a clinical benefit to the patients.