Analysis of residual disease in chronic lymphocytic leukemia by flow cytometry

We have investigated the value of both conventional and quantitative flow cytometry to detect minimal residual disease in 21 CLL patients in remission including bone marrow histology: eight in complete remission (CR), 11 in nodular partial remission (nPR) and two in PR. The techniques used were double immunostaining with CD5 and CD19 and quantitative estimation of the number of both antigens with standard microbeads. Reference values were established on normal peripheral blood and bone marrow controls. Patients were considered in 'immunological' remission when the percentage of CD5+ CD19+/total CD19+ cells was <25% in PB and <15% in BM. In six of the eight patients in CR, CLL cells were still detectable by flow cytometry. Only two patients, that underwent allogeneic bone marrow transplant, achieved immunological remission. CLL samples showed significantly higher CD5 and lower CD19 antigen density than normal controls (P < 0.001). Persistence of residual disease was a predictor of time to progression. None of the two patients in immunological remission relapsed within a period of 13 and 33 months, whilst two of the six patients in CR with positive flow cytometry relapsed 3 and 6 months after achieving CR. This study demonstrates that flow cytometry contributes to increase the sensitivity of the clinicohematological criteria to detect residual malignant cells in CLL patients and may be useful to monitor disease status following treatment.