Table 2.

ELN recommendations for MRD testing

	Recommendations
Flow cytometry
1	Use the following markers in a MRD panel:
	CD7, CD11b, CD13, CD15, CD19, CD33, CD34, CD45, CD56, CD117, HLA-DR (backbone: CD45, CD34, CD117, CD13, CD33, FSC/SSC).
	If necessary, add a “monocytic tube” containing:
	CD64/CD11b/CD14/CD4/CD34/HLA-DR/CD33/CD45.
2	Integrate the classic LAIP approach with the DfN approach. To trace all aberrancies (at and beyond diagnosis, including newly formed postdiagnosis aberrancies), apply a full panel both at diagnosis and follow-up.
3	Aspirate 5-10 mL BM and use the first pull for MRD assessment. At present PB, with its lower MRD content, should not be used for MRD assessment.
	Pull as low as desirable BM volume because contamination with PB increases with BM volume.
4	Estimate the contamination with PB, especially when a first pool of BM was impossible.
5	Use 500 000 to 1 000 000 white blood cells, use the best aberrancy available and relate it to CD45+ white blood cells.
6	To define “MRD-negative” and “MRD-positive” patient groups, a cutoff of 0.1% is recommended.
7	If true MRD <0.1% is found, report this as “MRD-positive <0.1%, may be consistent with residual leukemia.” If applicable, the comment “this level has not been clinically validated” should be added.
8	In a multicenter setting, transport and storage of full BM at room temperature for a period of 3 d is acceptable.
9	Single-center studies with no extensive experience on MFC MRD are strongly discouraged.
Molecular biology
1	Molecular MRD analysis is indifferent to the anticoagulant used during cell sampling; thus, heparin or EDTA can be used as anticoagulant.
2	Aspirate 5-10 mL BM and use the first pull for molecular MRD assessment.
3	WT1 expression should not be used as an MRD marker unless no other MRD marker is available in the patient.
4	Do not use mutations in FLT3-ITD, FLT3-TKD, NRAS, KRAS, DNMT3A, ASXL1, IDH1, IDH2, or MLL-PTD and expression levels of EVI1 as single MRD markers. However, these markers may be useful when used in combination with a second MRD marker.
5	We define molecular progression in patients with molecular persistence as an increase of MRD copy numbers ≥1 log10 between any 2 positive samples. Absolute copy numbers should be reported in addition to the fold increase to enable the clinician to make his or her own judgments.
6	We define molecular relapse as an increase of the MRD level ≥1 log10 between 2 positive samples in a patient who previously tested negative. The conversion of negative to positive MRD in PB or BM should be confirmed 4 wk after the initial sample collection in a second sample from both BM and PB. If MRD increases in the follow-up samples ≥1 log10, molecular relapse should be diagnosed.
Clinical
1	Refine morphology-based CR by assessment of MRD, because CRMRD− is a new response criterion according to the AML ELN recommendation 2017.
	Use MRD to refine risk assessment before consolidation treatment, the postinduction time point closest to consolidation treatment is recommended.
2	MRD monitoring should be considered part of the standard of care for AML patients.
	Monitoring beyond 2 years of follow-up should be based on the relapse risk of the patient and decided individually.
	Patients with mutant NPM1, RUNX1-RUNX1T1, CBFB-MYH11, or PML-RARA should have molecular assessment of residual disease at informative clinical time points.
3	Not to assess molecular MRD in subtypes other than APL, CBF AML, and NPM1-mutated AML.
4	For AML patients not included in the molecularly defined subgroups here, MRD should be assessed using MFC.
	During the treatment phase, we recommend molecular MRD assessment at minimum at diagnosis, after 2 cycles of standard induction/consolidation chemotherapy, and after the end of treatment in PB and BM.
	During follow-up of patients with PML-RARA, RUNX1-RUNX1T1, CBFB- MYH11, mutated NPM1, and other molecular markers we recommend molecular MRD assessment every 3 mo for 24 mo after the end of treatment in BM and in PB. Alternatively, PB may be assessed every 4-6 wk.
5	Failure to achieve an MRD-negative CR or rising MRD levels during or after therapy are associated with disease relapse and inferior outcomes and should prompt consideration of changes in therapy.
6	In APL, the most important MRD end point is achievement of PCR-negativity for PML-RARA at the end of consolidation treatment.
	For patients with PML-RARA fusion and low-/intermediate-risk Sanz score who are treated with ATO and ATRA, MRD analysis should be continued until the patient is in CRMRD− in BM and then should be terminated.
7	Detectable levels of PML-RARA by PCR during active treatment of APL should not change the treatment plan for an individual patient.
8	A change in status of PML-RARA by PCR from undetectable to detectable, and confirmed by a repeat sample, should be regarded as an imminent disease relapse in APL.
9	Patients with CBF AML should have an initial assessment of MRD after 2 cycles of chemotherapy, followed by serial measurements every 3 mo for at least the first 2 y after the end of treatment.
10	MRD should be assessed pretransplant.
11	MRD should be performed posttransplant.
12	All clinical trials should require molecular and/or MFC assessment of MRD at all times of evaluation of response.

Reprinted from Schuurhuis et al.²⁴

ATO, arsenic trioxide; ATRA, all-trans retinoic acid; BM, bone marrow; HLA-DR, HLA–antigen D related; PB, peripheral blood.