Circulating Tumour DNA Testing

Circulating tumour DNA testing looks for small fragments of tumour-derived DNA that may be present in the bloodstream.

Cancer cells can release DNA fragments as they break down. A laboratory analyses a blood sample for selected changes or patterns, then reports what was detected within the limits of the test used.

The test may help an oncology team look for tumour-related genetic changes, follow a known change over time, or add information when planning care. It is usually considered alongside pathology, scans, symptoms, examination findings, and the type of cancer involved.

A clinician arranges a blood draw and explains which result is being sought. The laboratory method and the questions it can answer vary between tests.

Not every tumour releases enough DNA into the blood to be detected. A result showing no detectable tumour DNA does not by itself rule out cancer or prove that no cancer remains.

A detected change also needs interpretation: it may not answer where cancer is located, how much is present, or which treatment is suitable without other information. Tissue testing may still be needed.

A tissue biopsy examines cells taken directly from an area of concern. Imaging shows the location and appearance of structures in the body. These approaches can complement a blood-based test rather than replace it.

A blood draw is straightforward, but the meaning of the report can be complex. Ask your oncology team what the test was intended to answer, whether a result could change the next decision, and what additional test may be needed if the result is unclear.

The physical part of the test is a blood sample. You may notice brief discomfort, bruising, or lightheadedness around the draw. Tell the person taking blood if you have fainted during blood tests before or have a condition that affects bleeding.