The Marrow Is Not the Blood: A Spatially Anchored Immunomics Protocol to Test Haemodilution-Induced Immune-Phenotype Misclassification in Multiple Myeloma

High-dimensional bone marrow immunomics increasingly informs the biological characterisation of multiple myeloma, yet the robustness of aspirate-derived immune phenotypes to peripheral-blood admixture, differential cellular recovery and loss of tissue architecture remains incompletely established. Existing haemodilution indices assess specimen quality but have not been comprehensively evaluated against multidimensional immune-state classification. We propose MARROW-ANCHOR, a spatially anchored validation protocol integrating patient-matched early- and later-pull marrow aspirates, peripheral blood, controlled autologous admixture, multiparameter flow cytometry, selected single-cell transcriptomic and protein profiling, and trephine-based spatial immunophenotyping. The framework distinguishes experimentally introduced blood from unmeasured pre-existing admixture and separates compositional changes from recovery-associated and within-population variation. Prespecified sham controls, participant-level replication, established marrow-quality indices and computational mixture modelling support assessment of classification stability, while independent spatial and functional measurements provide complementary biological corroboration. The primary endpoint quantifies experimentally induced classification reversal; biological misclassification requires additional independent evidence. Explicit validation criteria address technical reproducibility, analytical identifiability, model calibration, overcorrection and external generalisability. Stable classifications, contradictory measurements and failure of adjustment are retained as informative outcomes. MARROW-ANCHOR provides a falsifiable methodological framework for determining when specimen-associated uncertainty warrants qualification of marrow-derived immune interpretations before their translation into precision-medicine applications.