ctDNA for Minimal Residual Disease (MRD) in Colon Cancer: A Prognostic Biomarker

What the Test Measures

Circulating tumor DNA (ctDNA) refers to fragments of tumor-derived DNA that are shed into the bloodstream as malignant cells turn over and die. In the setting of minimal residual disease (MRD) testing, the question is deliberately narrow: after a patient has undergone what appears to be curative surgery for colon cancer, does any measurable tumor DNA remain in the circulation? A positive result implies that tumor cells survived the operation somewhere in the body, even when imaging and standard pathology show no residual disease. A negative result means that, at the limit of the assay's sensitivity, no such DNA was detected.

This is fundamentally a molecular surveillance concept rather than a tissue diagnosis. The ctDNA signal integrates biology from wherever viable tumor persists, which is why it can serve as an early, blood-based readout of recurrence risk before radiographic disease becomes apparent.

How ctDNA MRD Is Tested

The specimen is plasma, obtained from a peripheral blood draw. Two broad assay strategies exist. Tumor-informed approaches first sequence the patient's resected tumor to identify a personalized set of somatic mutations, then design a bespoke panel to hunt for those specific variants in plasma. Because the search is anchored to known tumor mutations, tumor-informed assays generally achieve very high specificity and can detect ctDNA at low fractional abundance. Tumor-agnostic approaches instead apply a fixed next-generation sequencing (NGS) panel to plasma without prior knowledge of the tumor genotype, which offers logistical simplicity at some cost to sensitivity for the individual patient.

Preanalytic handling is not a footnote here — it materially affects whether a result can be trusted. ctDNA is a small fraction of total cell-free DNA, and lysis of white blood cells after collection floods the sample with germline DNA that dilutes the tumor signal and can obscure a true positive. For this reason, blood is typically collected into cell-stabilizing tubes (such as Streck or equivalent formulations) that preserve leukocyte integrity during transport, and plasma is separated within a defined time window. Deviations from these constraints — delayed processing, inappropriate tubes, hemolysis — degrade assay performance and can convert a genuinely detectable case into a falsely negative one.

Scoring in this context is binary at the clinical level: post-operative ctDNA is reported as detectable (MRD-positive) or undetectable (MRD-negative), usually at a defined post-surgical time point once perioperative DNA release has resolved.

What Each Result State Means

The two result states are not mirror images, and understanding that asymmetry is central to interpretation.

An MRD-positive result — detectable ctDNA after surgery — is a strong, adverse prognostic signal. In the DYNAMIC trial, post-operative ctDNA positivity identified patients at substantially elevated risk of recurrence [1]. Detection of tumor DNA where none should remain is difficult to explain by anything other than residual disease, so a positive result carries considerable interpretive weight.

An MRD-negative result is more nuanced. Undetectable ctDNA is a favorable prognostic sign, but "undetectable" is not the same as "absent." Sensitivity is bounded by tumor shedding biology, the amount of plasma sampled, and platform-specific limits of detection. Some tumors shed little DNA into the blood, and a negative test in that setting may reflect assay limits rather than true absence of disease. The interpretive weight of a negative result therefore depends heavily on which platform generated it and on the individual tumor's shedding behavior.

How ctDNA MRD Results Inform Treatment Eligibility

The evidence suggests that post-operative ctDNA status can help stratify which patients derive benefit from adjuvant chemotherapy — the drug class conventionally offered after colon cancer surgery to reduce recurrence risk. In DYNAMIC, a ctDNA-guided management strategy was used to inform whether patients were directed toward adjuvant chemotherapy, with the aim of escalating treatment for MRD-positive patients while sparing MRD-negative patients from therapy unlikely to benefit them [1].

The appropriate framing is one of eligibility, not prescription. An MRD-positive result may inform eligibility for, and intensity of, adjuvant systemic therapy; an MRD-negative result may support consideration of de-escalation. These are decisions made within a clinical team, integrating stage, pathology, patient factors, and assay characteristics. Nothing about a ctDNA result dictates that any individual patient should or should not receive a specific drug.

Caveats and What Is Evolving

Several important limitations temper enthusiasm. First, cutoffs and reporting thresholds are not fully standardized across platforms, so a "negative" from one assay is not directly interchangeable with a "negative" from another. Second, the negative predictive value is constrained by tumor shedding, meaning a reassuring result can coexist with occult disease in low-shedding tumors. Third, optimal sampling timing — how long after surgery to draw blood, and how frequently to repeat testing during surveillance — remains an area of active investigation rather than settled practice.

The evidence base has shifted markedly in the few years since DYNAMIC reported, moving ctDNA MRD from a research concept toward a candidate decision tool, but important questions remain genuinely open [1,2]. A central unresolved issue is whether it is safe to de-escalate — that is, to withhold adjuvant chemotherapy from MRD-negative patients — and whether escalation in MRD-positive patients meaningfully improves outcomes. These questions are being addressed by ongoing prospective trials, including successor studies in the DYNAMIC program designed specifically to test de-escalation safety and treatment intensification in ctDNA-defined subgroups. Until those results mature, ctDNA MRD is best understood as a powerful prognostic stratifier whose predictive role in guiding therapy is still being defined [2]. A thoughtful contemporary review frames the current status precisely as a work in progress rather than a fully validated standard of care [2].

For pathologists and trainees, the practical takeaway is to report ctDNA MRD results with explicit attention to the assay used, the preanalytic conditions, and the asymmetry of interpretation between positive and negative states — and to communicate that a negative result narrows, but does not eliminate, residual-disease risk.

References

1. Tie J, et al. Circulating Tumor DNA Analysis Guiding Adjuvant Therapy in Stage II Colon Cancer (DYNAMIC). New England Journal of Medicine. 2022. doi:10.1056/NEJMoa2200075.
2. ctDNA/MRD Testing for Colon Cancer: A Work in Progress or Ready for Prime-Time Standard of Care? PMID: 39681074. 2024.