<scp>UK</scp> recommendations for chimerism testing and monitoring following allogeneic haematopoietic stem cell transplantation <scp>(HSCT</scp> ): Best practice consensus guidelines from the British Society for Blood and Marrow Transplant and Cellu

In allogeneic haematopoietic stem cell transplantation (HSCT), important clinical decisions depend upon assessment of chimerism, including immunosuppressant dosing and donor lymphocyte infusions (DLI), which in turn can have major impacts on disease control, graft-versus-host disease (GVHD), immunity and ultimately patient survival. There is a complex range of clinical and laboratory procedural considerations including methodology of testing, types of cell subset selection, frequency of testing, urgency of turnaround times (TATs), interplay with measurable residual disease (MRD) monitoring and duration of testing post-transplant. These aspects are routinely adapted according to disease indication, patient characteristics, donor source and intensity of transplant technique. To encourage the harmonisation of clinical and laboratory practice in the United Kingdom, we held a national workshop meeting to bring together key stakeholders to review the current literature with a view to producing a state-of-the-art position paper. Here, we present best practice consensus recommendations and identify key areas for future audit and research from the UK Cancer Genetics Group (UKCGG), NHS England Genomic Laboratory Hub (GLH) Haematological Oncology Malignancies Working Group, UK National External Quality Assessment Service for Leucocyte Immunophenotyping (UK NEQAS LI) and the British Society of Blood and Marrow Transplantation and Cellular Therapy (BSBMTCT).