If a blood-cancer patient has been told their disease is "in remission," the natural assumption is that the cancer is gone. The truth is more interesting, and ultimately more useful: remission, in the traditional sense, just means the cancer is no longer detectable by ordinary tests. Modern tests — measuring what's called measurable residual disease, or MRD — can see thousands or millions of times more sensitively. Two patients in "complete remission" can have very different prognoses depending on what their MRD test shows.
This is one of the most important conceptual shifts in haematology over the last decade. It explains why some leukaemias relapse months after a clean bone marrow biopsy. It explains why some multiple myeloma patients on maintenance therapy appear to do "fine" but progress within a year. And it explains why treatment decisions in 2026 are increasingly being made not just on the basis of remission, but on the basis of how deep that remission goes.
What "remission" used to mean
Until about ten years ago, the standard for declaring a patient in remission was a clean bone marrow biopsy under the microscope. A pathologist looking at slides would report "less than 5% blasts" — meaning fewer than 5 in every 100 bone marrow cells were leukaemia cells, which is the threshold for normal. Combined with normal blood counts and recovery of healthy cell production, this was called complete remission.
The trouble with that definition is that it relies on light microscopy, which has a detection limit of roughly 1 in 100 cells. Below that, the pathologist simply can't see the cancer cells. But "can't see" is not "isn't there." In a patient with billions of bone marrow cells, even one in ten thousand being a leukaemia cell still adds up to a lot of cancer. And those cells, if not eliminated, are usually the seed of relapse.
What MRD testing actually measures
MRD testing uses techniques far more sensitive than light microscopy. The major methods:
- Flow cytometry — uses fluorescent antibodies that bind specific markers on leukaemia cells. Modern multi-parameter flow can detect 1 abnormal cell in 100,000 normal cells (a sensitivity of 10⁻⁵).
- Next-generation sequencing (NGS) — sequences DNA at high depth to detect leukaemia-specific mutations or rearrangements at sensitivities of 10⁻⁶ or better.
- Quantitative PCR (qPCR) — particularly powerful when the leukaemia carries a known fusion gene like BCR-ABL (in chronic myeloid leukaemia) or PML-RARA (in acute promyelocytic leukaemia). qPCR can quantify residual disease over many orders of magnitude.
- Digital droplet PCR (ddPCR) — newer, even more sensitive, increasingly used in research and clinical contexts.
Each method has its strengths. For patient-specific decisions, the choice depends on the cancer type, the markers known to be present at diagnosis, and what's available locally. In hemato-oncology centres in India, multi-parameter flow cytometry is the most widely available; NGS-based MRD is offered in larger reference labs and is becoming standard for myeloma and ALL.
Where MRD matters most
Acute lymphoblastic leukaemia (ALL)
This is the disease where MRD has been most thoroughly validated. After induction chemotherapy, MRD status is the single most powerful prognostic factor — more powerful than age, white-cell count at diagnosis, or genetic risk group. Patients who achieve MRD-negativity after induction have dramatically better outcomes than those who don't, and treatment is now intensified — including transplant — for MRD-positive patients to try to convert them to MRD-negative.
Acute myeloid leukaemia (AML)
MRD is similarly important in AML. NPM1-mutated AML is monitored with sensitive PCR, and rising MRD typically precedes overt relapse by months — long enough to intervene with pre-emptive treatment.
Chronic lymphocytic leukaemia (CLL)
MRD-driven, time-limited therapy with combinations like venetoclax plus obinutuzumab has changed the CLL treatment paradigm. Patients who achieve MRD-negativity can stop treatment after a defined duration, with very low relapse rates over the next few years.
Multiple myeloma
MRD-negativity (typically by NGS or next-generation flow) after induction and consolidation therapy is the strongest predictor of progression-free and overall survival. The IMWG criteria for "stringent complete response" now include MRD-negativity. Maintenance therapy decisions are increasingly being shaped by MRD status.
Two patients in 'complete remission' can have very different prognoses, depending on what their MRD test shows.
What MRD-negative actually means
MRD-negativity at the 10⁻⁵ sensitivity level (the most common standard) means the test detected fewer than 1 abnormal cell in every 100,000 normal cells. It does not mean zero cancer cells. It means below the detection limit of the assay.
Despite that caveat, MRD-negativity is a powerful prognostic marker. Across most blood cancers, MRD-negative patients have dramatically better outcomes than MRD-positive patients in the same risk group. In some cancers, sustained MRD-negativity over multiple time points is associated with the possibility of treatment-free remission — a goal that was unthinkable a decade ago.
What MRD-positive means
MRD-positivity after expected treatment is a signal — not a verdict. It means residual disease is detectable, which raises the risk of clinical relapse. Depending on the cancer type and the level of positivity, the response can include:
- Intensifying or switching to a different regimen.
- Adding a targeted therapy (e.g., blinatumomab for MRD+ B-cell ALL).
- Proceeding to stem cell transplant earlier than otherwise planned.
- Closer monitoring to catch overt relapse early.
The point is that MRD-positivity is actionable, not just informational. Treatment changes meaningfully based on the result.
How often MRD is tested
Frequency depends on the disease and the phase of treatment:
- ALL: typically at the end of induction (day 28-35), then at defined points during consolidation, before transplant if applicable, and during maintenance.
- AML: end of induction, end of consolidation, and during follow-up at 3-6 month intervals.
- CLL: at defined intervals during fixed-duration therapy and during follow-up after stopping.
- Multiple myeloma: typically at end of induction, end of consolidation/transplant, and annually during maintenance.
Practical takeaways for patients and families
If you or a family member is being treated for a blood cancer, three things are worth knowing about MRD:
One, ask whether MRD testing is being done. In some centres it's routine; in others it's only done if specifically requested. For ALL, AML, CLL, and myeloma, MRD should be on the test list at appropriate intervals.
Two, understand what your result means. MRD-negative is good news but not a guarantee. MRD-positive is concerning but actionable. The number is one input into a treatment decision, not a verdict on prognosis.
Three, MRD trends matter more than single values. A single MRD-positive result doesn't always mean treatment failure; rising MRD over multiple time points usually does. Conversely, a single MRD-negative result is encouraging but sustained MRD-negativity is what matters for long-term outcomes.
The summary
MRD has changed haematology over the last decade. It is now the single most powerful prognostic test in most blood cancers, and treatment decisions — including the question of when to do transplant, when to stop maintenance, when to escalate — are increasingly being driven by it. If your blood cancer treatment plan doesn't include MRD testing at appropriate intervals, that is a question worth asking. If it does, the result is a number worth understanding — and a starting point for shared decisions about what comes next.