Doc, Can I have the new DNA test to see if I have cancer?

A new test has been widely reported in the media following publication of the initial results of the NHS Symplify Trial recently. It all sounds a bit too good to be true, so what is going on with cell free cfDNA and cancer diagnosis?

DNA from cancer cells can be detected in the blood giving potential for earlier diagnosis and treatment. The blood test looks at methylation patterns of DNA, and crucially different cancers have different patterns of methylation. The test can use these methylation patterns of cfDNA to detect multiple different types of cancers, in what is called a Multi Cancer Early Detection Test (MCED).

That’s the theory, but ‘how does it perform in practice’ is the million dollar question.

The initial results (not the full publication) were presented at the American Society of Clinical Oncology conference on the 15th June 2023 and hit the headlines.

So, what did the study do? The trial was evaluating the test in patients with symptoms suggestive of cancer and enrolled 6,000 patients who had seen their GP with symptoms suggestive of possible cancer (e.g. change in bowel habit, unexplained weight loss, rectal bleeding etc). They were then investigated in the usual way and also had the Galleri MCED blood test.

Results? The Galleri test detected a cancer signal in 323 people, 244 in whom cancer was actually detected. The positive predictive value of the test was 75.5%, the negative predictive value was 97.5%, and specificity 98.4%

The sensitivity of the test was however only 66% meaning they missed 1/3 of cancers and the sensitivity was dependant of the stage of the cancer. For Stage 1 cancers (i.e. the ones we most want to diagnose) the sensitivity was only 24%, but it was 95% in stage 4.

Importantly though, with a positive test, it was 85% accurate in detecting the source of the cancer. This is perhaps the most significant finding, as it shows real potential for being able to make cancer investigation in suspected cases more targeted, streamlined and quicker thereby reducing treatment delays.

In conclusion, these are promising early trial results, but from this initial data it seems that the most likely use of the test will be as a diagnostic aid in hospital to pinpoint the source of the cancer earlier and streamline investigations in patients that GP’s refer with suspected cancer symptoms. Unfortunately, the test is currently insufficiently sensitive to be used to ‘rule in or rule out’ early stage cancer.

In the meantime, we can advise our curious and hopeful patients that GPs being able to use a cfDNA blood test to diagnose cancer at an early stage in primary care still seems a long way off.

by Dr Simon Curtis, 21st June 2023

Source: Extract taken from the NB medical blog – a cpd source for GP’s,