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aRatio of malignant cell and normal nonmalignant cells. bMolecules per cell.

aRatio of malignant cell and normal nonmalignant cells. bMolecules per cell.

residual disease.6-15 However, the sensitivity and reliability of these tests depend on the number of cells to be analyzed (Table 14.1). Flow cytometry allows detection of one abnormal cell in 106 cells, if at least 107 cells are analyzed. Such large numbers are rarely available during remission, and therefore a more realistic sensitivity to detect minimal residual disease would be one abnormal cell in 104-105 cells.16 This level of sensitivity is achievable only if the antibodies can clearly distinguish the leukemic blasts from normal cells. In addition, antibodies with nonspecific light scattering will lower the sensitivity of the test. It is therefore recommended to use serially diluted leukemic blasts with normal cells to confirm the level of sensitivity. Finally, clonal evolution may cause disappearance of one or more antigens detected at diagnosis.17-23 Therefore, after taking into consideration all the caveats mentioned above, lack of minimal residual disease by quantitative flow cytometry at the end of remission induction therapy was associated with better clinical outcome in some studies of adult ALL patients.1013-15 However, because of a multitude of methods and lack of consensus, it is still not possible to make a clear recommendation of how to use flow cytometry to detect minimal residual disease.

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