Liquid biopsy
Also known as: liquid biopsies, ctDNA test, circulating tumour DNA test
A liquid biopsy is a test that looks for signs of cancer, such as fragments of tumour DNA, in a blood sample rather than in tissue removed by surgery or a needle.
It's already used to choose targeted treatments and track how cancer responds. While blood tests that screen healthy people for many cancers at once have generated enormous excitement, the evidence that they save lives is still being gathered, and early trial results have been mixed.
Liquid biopsy, explained at three levels
At home
Tumours shed tiny bits of their DNA into the blood. A liquid biopsy is a blood test that searches for those bits, which can reveal whether cancer is present or how it's changing, without surgery.
At work
Liquid biopsies help oncologists choose treatments matched to a tumour's mutations and monitor response with a simple blood draw, avoiding repeated invasive biopsies.
At home
Most tests look for circulating tumour DNA (ctDNA). Others look at DNA chemical patterns called methylation, which can hint at which organ a cancer came from. Results usually come back in one to two weeks.
At work
Clinical uses include genotyping advanced cancers, detecting minimal residual disease after surgery, monitoring for recurrence, and identifying resistance mutations. Multi-cancer early detection (MCED) tests aim to screen asymptomatic people, but their sensitivity for early-stage cancers is lower than for advanced ones.
At home
Screening tests can cause harm through false positives, which lead to anxiety and more tests, and overdiagnosis of cancers that would never have caused problems. In my opinion, any screening test should prove it reduces deaths, not just that it finds more cancers.
At work
Evaluate tests on sensitivity by stage, specificity, positive predictive value in the intended population, accuracy of tissue-of-origin prediction, and evidence of clinical utility from randomized trials. Consider follow-up pathways for positive results, cost-effectiveness, regulatory status in Canada, and equity of access when tests are sold privately.
Real-world examples
At home
Lena's mother has advanced lung cancer in Ottawa. Instead of another needle biopsy, her oncologist ordered a liquid biopsy that found a mutation matching a targeted pill, which worked better than her previous chemotherapy.
At work
Marcus is a policy analyst at a provincial cancer agency. His team is reviewing multi-cancer blood tests sold privately to Canadians and recommending that public screening wait for stronger trial evidence on mortality and false positives.
How it compares
| feature | Liquid biopsy | Tissue biopsy |
|---|---|---|
| Sample | Blood | Tumour tissue |
| Invasiveness | Simple blood draw | Needle or surgery |
| Repeatability | Easy to repeat | Limited |
| Diagnostic certainty | Lower for early disease | Gold standard for diagnosis |
Liquid biopsy is easier and repeatable. Tissue biopsy remains the definitive diagnosis.
For analysts
Multi-cancer tests are increasingly marketed directly to consumers and employers. I'd ask what happens after a positive result, who pays for follow-up scans, and what trial evidence shows about outcomes before offering one as a benefit.
History & origin
Scientists discovered in the 1940s that DNA circulates freely in blood, and in 1997 Dennis Lo found fetal DNA in pregnant women's blood, leading to non-invasive prenatal testing. Researchers then learned tumours shed DNA into the bloodstream too. The FDA approved the first liquid biopsy companion diagnostic, for a lung cancer mutation, in 2016, and broader genomic tests such as Guardant360 followed. Grail, founded in 2016, developed the Galleri test to screen for more than 50 cancer types. In July 2024, the FDA approved Guardant's Shield blood test for colorectal cancer screening. In 2026, results from the NHS-Galleri trial of about 142,000 people in England showed more early-stage detection on some measures but missed its primary goal of significantly reducing later-stage diagnoses. In my view, liquid biopsy is already invaluable for treating cancer, while its role in screening still has to earn its place.