Peripheral blood 8 colour flow cytometry monitoring of hairy cell leukaemia allows detection of high‐risk patients

Although purine analogues have significantly improved the outcome of hairy cell leukaemia (HCL) patients, 30–40% relapse, illustrating the need for minimal residual disease (MRD) markers that can aid personalized therapeutic management. Diagnostic samples from 34 HCL patients were used to design an 8‐colour flow cytometry (8‐FC) tube for blood MRD (B/RD) analysis (188 samples) which was compared to quantitative IGH polymerase chain reaction (Q‐PCR) on 83 samples and to qualitative consensus IGH PCR clonality analysis on 165 samples. Despite heterogeneous HCL phenotypes at diagnosis, discrimination from normal B lymphocytes was possible in all cases using a single 8‐FC tube, with a robust sensitivity of detection of 10−4, comparable to Q‐PCR at this level, but preferable in terms of informativeness, simplicity and cost. B/RD assessment of 15 patients achieving haematological complete remission after purine analogues was predictive of a clinically significant relapse risk: with a median follow‐up of 95 months; only one of the nine patients with reproducible 8‐FC B/RD levels below 10−4 (B/RDneg) relapsed, compared to 5/6 in the B/RDpos group (P = 0·003). These data demonstrate the clinical interest of a robust 8‐FC HCL B/RD strategy that could become a surrogate biomarker for therapeutic stratification and new drug assessment, which should be evaluated prospectively.