What Does Circulating Tumor DNA Mean?

This page is for anyone whose care team has ordered a blood test for circulating tumor DNA, sometimes called a liquid biopsy. It explains what the test measures, what a negative result does and does not mean, and which tests the FDA lists for choosing drugs.

Everyone's blood holds cell-free DNA, or cfDNA. These are short broken pieces of DNA that float in plasma (the liquid part of blood) after cells die and spill their contents. In a person with cancer, some of those pieces came from tumor cells. That fraction is circulating tumor DNA, or ctDNA.

The FDA describes it as tumor-derived DNA fragments shed into the bloodstream. The same guidance notes that ctDNA levels vary by tumor type, location, stage, and tumor burden.

That variability is one reason these tests can help in some cases and miss disease in others.

There are many kinds of ctDNA test. The FDA-approved Guardant360 CDx shows the scale involved.

Its FDA documents describe a 5 mL specimen. The test reads single-letter changes in 55 genes, extra copies of 2 genes, and fusions in 4 genes.

Its listed limit of detection is 0.2% to 1.5% mutant allele fraction. The limit of detection is the smallest amount the test can reliably find. Mutant allele fraction, also called variant allele frequency, is the share of DNA pieces at that spot that carry the change. At 0.2%, the test is looking for about two changed pieces among a thousand.

If your report lists a variant with a percentage next to it, that percentage is this number. A very low fraction is not automatically less serious, but it is closer to the edge of what the assay (test) can see.

The Guardant360 CDx documents carry an important warning. A negative plasma result does not assure that no disease or tumor change is present.

The FDA documents recommend tissue testing as a follow-up in that situation. Tissue testing means testing a sample of the tumor itself.

How much DNA a tumor puts into the blood depends on factors such as tumor type, location, stage, and burden. So a negative blood test means the test did not find a change. It does not mean nothing is there. Ask your team whether tissue testing is still needed.

Three different jobs get bundled under the same phrase, and they carry different levels of evidence.

Choosing a drug right now. This is the established use. Some plasma tests are FDA-authorized companion diagnostics, which means the FDA authorized them to help pick patients for a named drug. The FDA keeps a public list of these tests.

Finding molecular residual disease. MRD means ctDNA that is still found after treatment meant to clear the cancer, such as surgery. FDA guidance on early-stage solid tumors looks at how MRD tests can be used in drug trials.

Tracking response over time. Watching ctDNA rise or fall during treatment is under study. The FDA says randomized trial data are still needed before ctDNA can be relied on as a trial endpoint (a result used to judge whether a treatment works).

The FDA keeps a public list of companion diagnostics, which are tests approved to help choose a specific drug. The list changes often, so check it or ask your team. Plasma-based tests on it include these.

Guardant360 CDx is listed for non-small cell lung cancer (EGFR, KRAS G12C and ERBB2/HER2 changes), breast cancer (ESR1 changes) and colorectal cancer (BRAF V600E). Paired drugs include osimertinib, amivantamab, sotorasib, fam-trastuzumab deruxtecan, elacestrant and encorafenib with cetuximab.

FoundationOne Liquid CDx is listed for lung, breast, prostate and colorectal cancer, and for solid tumors with NTRK fusions. It covers many gene changes, such as EGFR, MET exon 14 skipping, ROS1, ALK, PIK3CA, BRCA1 and BRCA2.

cobas EGFR Mutation Test v2 is listed for EGFR changes in non-small cell lung cancer, with osimertinib and erlotinib.

These are tests to guide drug choice. They are not listed as tests to tell you whether you have cancer. If you wonder whether a test fits your situation, ask your team.

ctDNA assays come in two main designs, and the difference can show up in your results.

A tumor-informed test first sequences your tumor tissue to find changes specific to you. It then looks for those changes in blood. A tumor-naive test uses a fixed panel of genes and does not need a prior look at your tumor. The FDA guidance describes both designs, and each has trade-offs in sensitivity and specificity.

If you are told the ctDNA test was negative, ask which type it was and which genes it covered.

For MRD use, the FDA sets two different bars:

• Cutting back treatment on a negative result needs good sensitivity and negative predictive value. In plain terms, the test must rarely miss disease that is there.
• Adding treatment on a positive result needs good specificity and positive predictive value. In plain terms, the test must rarely flag disease that is not there.

Using ctDNA to add treatment is not the same as using it to hold treatment back. One test may be good enough for one use and not the other.

The guidance also stresses standard blood collection and handling, because these affect the results.

• Which test was run, by name, and was it tumor-informed or tumor-naive?
• Is this test FDA-approved for this purpose, or is it being used off-label (for a use the FDA has not approved)?
• What was the mutant allele fraction for each variant reported?
• If it was negative, is tissue testing the next step?
• What decision changes based on this result, and what happens if it is wrong?
• When would a repeat draw be meaningful?

https://www.fda.gov/regulatory-information/search-fda-guidance-documents/use-circulating-tumor-deoxyribonucleic-acid-early-stage-solid-tumor-drug-development-guidance https://www.fda.gov/media/183874/download https://www.accessdata.fda.gov/cdrh_docs/pdf20/P200010B.pdf https://www.fda.gov/medical-devices/in-vitro-diagnostics/list-fda-authorized-companion-diagnostic-devices-in-vitro-and-imaging-tools