Minimal residual disease (MRD) testing looks for cancer cells or tumor-derived fragments that linger after treatment but are too scarce for standard imaging or bloodwork to pick up. The term “minimal” is relative: a patient who appears cancer-free on a CT scan or PET scan can still harbor thousands or millions of malignant cells scattered through bone marrow, blood, or tissue. MRD testing uses molecular and cellular techniques sensitive enough to find those hidden remnants, and the results increasingly shape whether a person gets more treatment, less treatment, or closer surveillance after their initial therapy.
What MRD Testing Actually Measures
Standard tools for evaluating cancer after treatment, such as imaging and conventional blood markers, hit a detection floor. A tumor has to reach a certain size or shed enough protein for those methods to register it. MRD testing works below that floor, searching for molecular fingerprints of cancer at extremely low concentrations. In blood cancers, those fingerprints are often abnormal immune-cell markers or specific gene rearrangements unique to the malignant clone. In solid tumors, the target is usually circulating tumor DNA (ctDNA), tiny fragments of DNA shed by cancer cells into the bloodstream.1PubMed Central. Minimal residual disease in solid tumors: Clinical applications and future directions The practical question MRD testing answers is simple: after surgery, chemotherapy, or other curative-intent treatment, is there still measurable molecular evidence of cancer?
How the Tests Work in Blood Cancers
MRD testing first became routine in blood cancers like leukemia and multiple myeloma, where cancer cells circulate in accessible compartments: the blood and bone marrow. Two families of techniques dominate.
Flow cytometry examines individual cells by passing them through a laser beam after tagging them with fluorescent antibodies. The goal is to spot cells with an abnormal combination of surface markers that distinguishes them from healthy blood cells. Some labs establish the specific abnormal pattern at diagnosis and then hunt for that same pattern in follow-up samples. Others apply a broad panel of antibodies and flag anything that deviates from normal maturation patterns.2PubMed Central. Minimal residual disease detection using flow cytometry: Applications in acute leukemia One practical limitation is that leukemia cells can change their surface markers after treatment, so a pattern identified at diagnosis may not match what persists later. The detected phenotypes are also limited to whatever antibodies are included in the testing panel.3PubMed. Flow cytometry evaluation of acute myeloid leukemia minimal residual disease based on an understanding of the normal maturation patterns in the blast compartments
Molecular methods take a different approach: instead of looking at the cell surface, they look at DNA. In acute lymphoblastic leukemia, for example, each patient’s cancer carries unique rearrangements in immune-receptor genes. Sequencing those rearrangements at diagnosis creates a molecular barcode for that patient’s disease. Follow-up samples are then searched for that barcode. Next-generation sequencing (NGS) has improved this process compared to older sequencing methods. A validation study across five European reference laboratories found that NGS identified slightly more clonal sequences per sample, covered a wider range of possible gene rearrangements, and could sequence rearrangements that older methods struggled with.4Leukemia. Standardized next-generation sequencing of immunoglobulin and T-cell receptor gene recombinations for MRD marker identification in acute lymphoblastic leukaemia; a EuroClonality-NGS validation study
Why MRD Results Matter in Multiple Myeloma
Multiple myeloma is where MRD testing has gained the most clinical traction, largely because the data linking test results to patient outcomes is unusually strong. A meta-analysis pooling data from thousands of myeloma patients found that those who achieved MRD-negative status (no detectable disease at the molecular level) had roughly double the progression-free survival compared to those who remained MRD-positive: a median of 54 months versus 26 months. They also lived longer overall, with a median overall survival of 98 months compared to 82 months for MRD-positive patients.5JAMA Oncology. Association of Minimal Residual Disease With Superior Survival Outcomes in Patients With Multiple Myeloma: A Meta-analysis
Those numbers have pushed MRD toward a role that goes beyond just prognosis. If MRD negativity reliably predicts that a treatment is working, then drug developers and regulators can use it as an earlier endpoint in clinical trials rather than waiting years to see whether patients actually live longer. A large meta-analysis conducted under FDA guidance evaluated exactly this idea and found a strong association: patients who were MRD-negative at 12 months had about four times the odds of remaining progression-free compared to those who were MRD-positive. In the relapsed or refractory setting, the odds were even more lopsided, roughly eight to one.6Blood. EVIDENCE meta-analysis: evaluating minimal residual disease as an intermediate clinical end point for multiple myeloma This evidence culminated in a unanimous vote by the FDA’s Oncologic Drugs Advisory Committee in April 2024 supporting MRD-negative complete response as an early endpoint that could be used to support accelerated approval of new myeloma drugs.7PubMed Central. Minimal Residual Disease as an Early Endpoint for Accelerated Drug Approval in Myeloma: A Roadmap Establishing MRD Negativity as a Surrogate Endpoint In plain terms, that means new myeloma therapies could reach patients faster because regulators now accept MRD negativity as a reasonable stand-in for long-term benefit.
MRD Testing in Solid Tumors
Applying MRD testing to solid cancers is a newer and harder problem. Unlike blood cancers, solid tumors do not conveniently circulate their cells through the bloodstream in large numbers. Instead, researchers rely on ctDNA: tiny fragments of tumor DNA that break off and enter the blood. A simple blood draw, sometimes called a liquid biopsy, can capture these fragments, making it possible to check for residual disease without an invasive tissue biopsy.8PubMed Central. Liquid Biopsy to Detect Minimal Residual Disease: Methodology and Impact The promise is significant: ctDNA analysis could detect residual disease and predict relapse after curative treatment for solid cancers, potentially transforming how doctors decide whether a patient needs additional therapy.9Cancer Discovery. Detecting Liquid Remnants of Solid Tumors: Circulating Tumor DNA Minimal Residual Disease
Some of the most compelling data comes from breast cancer monitoring. In one study of early breast cancer patients, detection of ctDNA during follow-up surveillance was associated with a dramatically higher risk of relapse, and the ctDNA signal appeared a median of nearly 12 months before cancer showed up on clinical scans.10PubMed Central. Longitudinal monitoring of circulating tumor DNA to detect relapse early and predict outcome in early breast cancer That lead time could, in principle, open a window for intervening before a full relapse develops, though clinical trials are still working out whether acting on an early ctDNA signal actually improves survival.
Tumor-Informed Versus Tumor-Agnostic Approaches
Not all ctDNA tests are built the same way, and the distinction matters for how well they work. There are two broad strategies. Tumor-informed assays first analyze the patient’s tumor tissue to identify the specific mutations present, then design a custom test to look for those exact mutations in the blood. Tumor-agnostic assays skip the tissue analysis and instead scan the blood for a preset panel of commonly mutated genes or other molecular markers.
Tumor-informed testing tends to be more sensitive, particularly for ongoing surveillance. In colorectal cancer, a systematic comparison found that the tumor-informed approach had significantly higher sensitivity than the tumor-agnostic approach in serial monitoring, roughly 88% versus 59%.11Cancer Treatment Reviews. Minimal residual disease in colorectal cancer. Tumor-informed versus tumor-agnostic approaches: unraveling the optimal strategy The reason is straightforward: ctDNA from early-stage or residual disease is present in vanishingly small amounts, and knowing exactly what mutations to look for makes it easier to pick a faint signal out of the noise. One colorectal cancer study found that most ctDNA mutations detected during surveillance had variant allele frequencies below 0.1%, a level below the detection threshold of standard tumor-agnostic assays.12PubMed Central. Tumor-informed or tumor-agnostic circulating tumor DNA as a biomarker for risk of recurrence in resected colorectal cancer patients
Tumor-agnostic tests have their own advantages: they do not require a tumor tissue sample, which makes them easier to deploy when tissue is unavailable or degraded. Newer methylation-based assays are trying to bridge the gap. These tests look for cancer-specific patterns in DNA methylation rather than mutations, and some are designed to work without any prior knowledge of the patient’s tumor. Assays of this kind have been validated in colorectal cancer and are under development for breast cancer.13npj Precision Oncology. Validation of a methylation-based, tissue-free MRD assay in colorectal cancer patients from the GALAXY study14npj precision oncology. MammaTrace – a tumor naïve cell-free DNA methylation test for minimal residual disease in breast cancer
How MRD Results Might Change Treatment Decisions
The real clinical promise of MRD testing is its potential to personalize what happens after a patient’s primary treatment. Right now, decisions about post-surgical chemotherapy (adjuvant therapy) are made largely on the basis of staging: how big was the tumor, how many lymph nodes were involved, what did the pathology show? That approach helps on average, but it means some patients receive months of chemotherapy they did not actually need, while others who would have benefited skip it.
MRD testing could refine those decisions in both directions. For patients who test MRD-positive after surgery, that molecular evidence of residual disease is a signal that the cancer might come back, and intensifying or adding treatment could be warranted. For patients who test MRD-negative, the question becomes whether it is safe to withhold or reduce chemotherapy. MRD status is increasingly being evaluated as a tool for exactly this kind of tailoring.15PubMed Central. Circulating tumor DNA-based minimal residual disease-guided adjuvant therapy in solid tumors: Current evidence, clinical trial frameworks, and future directions
The DYNAMIC trial in stage II colon cancer provided some of the first high-quality evidence for the de-escalation side. That trial showed that omitting adjuvant chemotherapy in patients whose ctDNA tests came back negative after surgery was non-inferior to standard management, while substantially reducing the number of people exposed to unnecessary treatment toxicity.16Cancer Pathogenesis and Therapy. Targeting minimal residual disease (MRD) in colorectal cancer: Therapeutic strategies, clinical trials, and the path toward molecular cure – Section: De-escalation in minimal residual disease (MRD)-negative patients That is a big deal: chemotherapy for colon cancer carries real side effects, including nerve damage, fatigue, and infection risk, and many patients with stage II disease are already at low risk of recurrence. Sparing them treatment they do not need, with molecular evidence backing the decision, is exactly the kind of clinical gain MRD testing was designed to deliver. Multiple additional trials are underway across cancer types to confirm and extend this principle.
Technical Hurdles That Complicate Results
MRD testing is not a simple binary readout, and several technical issues can muddy the picture. The most fundamental is sampling. When cancer cells are extremely rare, the blood or bone marrow sample you happen to draw might not contain any, even though they exist elsewhere in the body. Modeling work has shown that at very low cancer cell prevalences, the major obstacle to accuracy is not the sensitivity of the test itself but whether the sample captured any of the rare cells in the first place. Making the assay more sensitive does not fix a sampling problem.17PubMed. Modeling minimal residual disease (MRD)-testing
For ctDNA-based tests in solid tumors, a particular source of false positives is a phenomenon called clonal hematopoiesis. As people age, blood-forming stem cells can acquire mutations that have nothing to do with cancer but look like tumor-derived mutations to a ctDNA assay. If the test picks up one of these mutations and mistakenly attributes it to residual cancer, the patient could be directed toward unnecessary treatment. Clonal hematopoiesis can significantly confound ctDNA detection if not carefully addressed.18Journal for ImmunoTherapy of Cancer. Genomic approaches to cancer and minimal residual disease detection using circulating tumor DNA19PubMed Central. Detection of Solid Tumor Molecular Residual Disease (MRD) Using Circulating Tumor DNA (ctDNA) Tumor-informed approaches partly mitigate this by looking for mutations already known to be present in the patient’s tumor, which makes it less likely that an unrelated age-associated mutation will register as a positive result.
Timing also matters more than people might expect. Surgery itself releases large amounts of normal (non-tumor) cell-free DNA into the bloodstream as tissue heals, and this flood of background DNA can drown out the faint ctDNA signal from any lingering cancer cells. Research has shown that this effect can persist for up to four weeks after surgery, so drawing blood too soon can produce unreliable results.20PubMed Central. The effect of surgical trauma on circulating free DNA levels in cancer patients—implications for studies of circulating tumor DNA Most protocols now recommend waiting at least several weeks post-surgery before the first MRD blood draw.
Even the way samples are handled before reaching the lab introduces variability. How blood is collected, stored, transported, and processed can all influence how much cfDNA is recovered and how reliable downstream measurements are. Delays in processing, repeated freeze-thaw cycles, and underfilled collection tubes have all been associated with lower nucleic acid yield, more contamination from white blood cell DNA, and greater measurement variability.21Scientific Reports. Pre-analytical considerations of the recovery and downstream analysis of circulating nucleic acids These are the kinds of mundane laboratory logistics that rarely make headlines but directly affect whether a result is trustworthy.
Where Commercial Tests Stand
Several companies now offer ctDNA-based MRD assays for solid tumors, and you may hear brand names mentioned by your oncologist. While some commercial liquid biopsy platforms have been approved for guiding treatment in advanced cancers (where tumor DNA is more abundant and easier to detect), their clinical utility for detecting minimal residual disease after curative treatment of early-stage disease is currently more limited.22SpringerLink. Commercial ctDNA Assays for Minimal Residual Disease Detection of Solid Tumors The distinction is important: a platform validated for finding mutations in a patient with widespread metastatic disease is not automatically reliable for finding a needle-in-a-haystack ctDNA signal after surgery when tumor burden is at its lowest. Concordance between different commercial platforms has also been an issue, meaning two tests from different companies might not always agree on the same patient’s sample.
For blood cancers, the landscape is more mature. Flow cytometry and NGS-based MRD testing are integrated into treatment guidelines for conditions like acute lymphoblastic leukemia and multiple myeloma, and the FDA’s 2024 decision on MRD as an accelerated-approval endpoint reflects that maturity. The solid tumor side is catching up but is not there yet for most cancer types.
What It Feels Like to Get MRD Results
From the patient’s perspective, MRD testing adds a layer of information that can be both reassuring and anxiety-producing. A negative result after treatment offers molecular-level reassurance that standard imaging cannot match. But a positive result introduces a specific, quantified form of uncertainty that some patients find difficult to sit with, particularly if the clinical implications are not yet clear-cut.
Research into colorectal cancer survivors’ experiences with ctDNA testing found that negative results were generally welcomed and calming, while positive results produced more varied reactions, from distress and heightened fear of recurrence to a sense of validation when the result led to additional treatment that eventually cleared their cancer.23Wiley Online Library. How Survivors of Colorectal Cancer Experience ctDNA Testing and Fear of Recurrence The emotional weight of these results is something clinicians are still learning to manage. A positive MRD result does not mean the cancer will definitely return, just as a negative result does not guarantee it will not. Communicating that nuance, especially to someone already anxious about recurrence, is its own clinical skill.
Emerging Technologies and Multi-Omics Approaches
The current generation of MRD tests is built mostly on detecting mutations in DNA or abnormal cell-surface proteins. Newer approaches are looking to combine multiple types of molecular data, sometimes called multi-omics, to push sensitivity and specificity higher. One research program is integrating whole-genome sequencing, whole-exome sequencing, transcriptomics, proteomics, metabolomics, and even microbiome profiling from stool and saliva samples, using artificial intelligence to combine these data streams into a single MRD readout.24SpringerLink. Clinical development of molecular residual disease (MRD) and multi-cancer early detection (MCED) using liquid biopsy multiomics with artificial intelligence (AI) The idea is that a cancer signal too faint to detect in DNA alone might become visible when you combine DNA changes with shifts in protein levels, metabolic byproducts, or RNA expression.
Other emerging platforms are working at the single-cell level. Rather than analyzing pooled DNA from millions of cells in a blood sample, single-cell techniques aim to examine individual cells for abnormalities, potentially catching rare cancer cells that would be diluted out in a bulk analysis.25PubMed Central. Advancements in minimal residual disease detection: a practical approach using single-cell droplet PCR for comprehensive monitoring in hematological malignancy These approaches are still largely in the validation phase, but they point toward a future where MRD testing is both more sensitive and more accessible across cancer types. Whether that added sensitivity translates into better patient outcomes, or just earlier detection of disease that would have been found anyway, is the question the next generation of clinical trials will need to answer.