What is meant by microresidual lesions? Leukemia microresidual lesions (MRD) Regular testing of microresidual lesions is required after achieving complete remission. After leukemia, especially acute leukemia, is treated to achieve complete remission, no obvious leukemia cells can be detected by conventional morphological methods, but a small number of leukemia cells still exist in the body at this time, and generally speaking, even if leukemia achieves complete remission, treatment should continue and MRD should be tested regularly to In general, even if the leukemia is in complete remission, treatment should be continued and MRD should be tested periodically to adjust the treatment method, program and treatment time according to the level of MRD so as to achieve the goal of cure. In acute leukemia, for example, the total number of leukemia cells in the body reaches 1012 or more at the initial onset, and more than 60% of adult patients can achieve complete remission (CR) after chemotherapy, but just after CR, even though no obvious leukemia cells can be detected in the bone marrow and blood, and the bone marrow primitive cells are <5%, the total number of leukemia cells in the body is still 107-9; continuing treatment, if these MRDs cannot be removed and eventually MRD is the root cause of leukemia relapse. the level of MRD is a key indicator to judge the effect of treatment, and elevated MRD can predict the full relapse of malignant hematologic disease in advance, therefore, MRD should be tested regularly to help choose the treatment modality, treatment cycle and protocol. Currently, the main methods used to detect MRD are immunological, chromosomal, and genetic analysis. There are many methods to detect MRD in childhood leukemia, among which quantitative PCR and FCM analysis are the most important methods, and they have good concordance, and their combined use can detect MRD in almost all children.FCM multiparametric analysis of the immunophenotype of leukemic cells has wide coverage and is suitable for MRD follow-up in most children with ALL.Borowitz et al. using FCM analysis of MRD concluded that bone marrow MRD at 29 d at the end of induction was closely related to prognosis, and 5a DFS was worse in patients with MRD in the range of 0.01% to 0.1%. There are many methods for MRD detection by PCR, among which real-time quantitative PCR is the most sensitive, with a sensitivity of 10-5~10-6, and can be used to analyze MRD after the end of induction remission treatment and predict ALL relapse in children. Genetic analysis is the most sensitive method, and one malignant cell in 100,000 cells can be detected. If a malignant cell is detected at first onset as having some (or some) genetic abnormality, MRD can later be determined by detecting the amount of these abnormal genes. chromosomal analysis methods are the most specific methods, but are only sensitive to one percent, and as with genetic analysis, if a malignant cell is detected at first onset as having a chromosomal abnormality, MRD can later be determined by detecting the chromosomal abnormality. since most malignant hematologic cells have a different combination or expression of abnormal antigens than normal cells, and the pattern of antigen abnormality is not identical from patient to patient, flow cytometry can be used to detect these abnormal antigens and thus MRD.