Unlike solid tumors, blood cancers such as acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML) may be monitored by measuring the presence and frequency of cancerous blood cells in the peripheral blood or bone marrow. Minimal residual disease (MRD) assessment uses different techniques to measure the presence of malignant cells, even at low frequency, and has been used as a key prognostic for guiding treatment decisions in pediatric and adult leukemia patients and assigning patients into different MRD-based risk groups. 
Flow cytometry is a powerful tool for assessing MRD, and the standardization and validation of flow-based MRD assays is being applied to multiple types of blood cancers. Flow cytometry-based MRD is now being considered as an endpoint for assessing the effectiveness of therapeutics in many clinical trials.
If you are a researcher thinking about developing a flow cytometry-based MRD assay, consider these questions to guide your decision.
1. Can the malignant cells in a disease type be detected in peripheral circulation or bone marrow?
Flow cytometry-based MRD assays have been developed successfully for blood cancers, and one reason for this success is because a sufficient volume of blood can be used to measure even low-frequency malignant cells. Bone marrow aspirates can also be monitored for MRD but can be more difficult to assess if fewer total cells are collected in the aspirate.
2. Do malignant cells display a unique immunophenotype that distinguishes them as different from different normal cells?
Flow cytometry-based MRD assays typically use 6-10 color staining panels, and these panels are optimized to discriminate normal immune cells from their malignant counterparts, such as B cells in different stages of development versus malignant pre-B ALL cells. A pre-treatment phenotype to identify the unique signature of an individual malignancy can increase the success of later MRD detection.
3. Can a flow cytometry-based MRD assay be sensitive and reproducible?
A novel flow cytometry assay to detect MRD must be tested and validated to confirm that the detection of malignant cells is reproducible and sensitive enough to identify clinically relevant levels of disease and to satisfy important regulatory parameters. This may require comparison to existing molecular diagnostics such as PCR-based MRD assays. In some instances, a combination flow/molecular approach may bring more sensitivity than either alone.
Consider working with organizations with flow cytometry and regulatory expertise in MRD detection to enhance your chances of success in this rare event detection. Flow cytometry MRD measurements are at the forefront of immuno-oncology research, as well as in the clinical arena, and are worth your consideration.
 van Dongen JJ, van der Velden VH, Brüggemann M, Orfao A. Minimal residual disease diagnostics in acute lymphoblastic leukemia: need for sensitive, fast, and standardized technologies. Blood. 2015 Jun 25;125(26):3996-4009.