Minimal Residual Disease: A Practical Guide

When your doctor talks about "minimal residual disease" (MRD) they’re referring to the tiny number of cancer cells that can hide after treatment. Even if scans look clean, those hidden cells can spark a relapse. Knowing whether MRD is present helps doctors decide if you need more therapy or can stay on watch.

Why MRD matters

MRD is a strong predictor of outcome in blood cancers like acute lymphoblastic leukemia (ALL) and chronic lymphocytic leukemia (CLL). Studies show patients with undetectable MRD after induction therapy have longer remission periods and better overall survival. In solid tumors, MRD is emerging as a way to catch microscopic spread before it becomes visible on imaging.

Because MRD gives early warning, doctors can personalize treatment. If MRD is still positive, they might add targeted drugs, switch to a different chemo regimen, or consider a stem‑cell transplant. If MRD is negative, patients often avoid unnecessary side effects from extra cycles.

How MRD is measured

The most common methods are flow cytometry, real‑time PCR, and next‑generation sequencing (NGS). Flow cytometry looks for abnormal cell surface markers and can detect one cancer cell among 10,000 normal cells. PCR targets specific genetic rearrangements, reaching sensitivity of one in 100,000. NGS digs deeper, spotting mutations at a frequency as low as 0.01%, making it the most sensitive option for many leukemias.

Choosing a method depends on the disease, available labs, and cost. Flow cytometry is quick and widely available, while NGS offers the highest sensitivity but may take longer and cost more. Your treatment center will pick the test that fits your cancer type and the clinical question.

Samples usually come from bone‑marrow aspirates for blood cancers, but peripheral blood can work for some NGS panels. For solid tumors, circulating tumor DNA (ctDNA) in a blood draw is becoming a reliable MRD source.

Interpretation is straightforward: a result reported as "MRD‑negative" means the assay didn’t find disease above its detection threshold. "MRD‑positive" means disease is still present at a low level. Doctors will combine this info with other factors—age, risk scores, and overall health—to shape the next steps.

Keep in mind that MRD testing isn’t a one‑time thing. Most protocols recommend checking at key milestones: after induction, after consolidation, and before any planned transplant. Regular monitoring helps catch a rise early, when treatment changes are most effective.

In short, MRD is a game‑changer for modern oncology. It turns vague imaging results into concrete numbers, letting you and your doctor make smarter, safer choices. Ask your care team if MRD testing is right for you and what method they use—knowing the answer could shape the rest of your treatment journey.