Galleri test: is this blood test for prostate cancer effective?

What is the Galleri test? 

Galleri is a blood test that looks for DNA fragments in the blood that may have come from cancer cells. There are differences between the DNA of healthy cells and the DNA of cancerous cells, and the test aims to use these differences as signals that might indicate that a patient has cancer. 

Researchers hope this test will pick up cancers at an early stage. Diagnosing cancer early means there is a greater chance of treating it successfully.  

In earlier studies, the Galleri test was able to able to detect a signal shared by more than 50 different types of cancer, including prostate cancer, but the accuracy of the test varied considerably depending on the location and type of the cancer. 

How well does the Galleri test perform in prostate cancer? 

Findings from the Circulating Cell-free Genome Atlas study show that the test was highly accurate at ruling out cancer, correctly identifying 99.5% of the people without the disease. 

The test also picked up just over half of the known cancer cases. This varied considerably across different cancer types, with promising results seen for cancers of the liver, head and neck, oesophagus, and pancreas (among others).  

However, when specifically looking at prostate cancer, the Galleri test correctly identified just 11% of the men with prostate cancer. This means 89% of men with prostate cancer were missed.  

Results also showed that the test’s ability to pick up cases of prostate cancer varied widely depending on the stage of the cancer. While the test found most of the most aggressive, stage IV cancers (correctly identifying 25 out of the 30 men with stage IV prostate cancer), the test missed the vast majority of men with localised disease (finding just 3% of men with stage I cancer, and 5% of men with stage II).  

These findings are extremely important because for a diagnostic test to be effective, it needs to find not only the advanced cancers (stage IV), but also the cancers that are still localised and contained within the prostate (stages I and II), since these cancers are much more treatable and potentially curable. 

In addition, the test’s performance in the PATHFINDER study and SYMPLIFY trial has not improved the case for using the Galleri test in prostate cancer. 

What do we think about the Galleri test?

While we look forward to seeing further results from these multi-cancer early detection tests, the data for prostate cancer specifically remains uncompelling and so researchers will need to look at other approaches to accurately detect prostate cancer.  

This must also not come at the additional cost of men undergoing invasive biopsies only to find out they do not have cancer, or diagnosing men with the disease, and subjecting them to harsh treatments, when their cancer is unlikely to harm them.  

That’s why we recently announced a £42 million trial to find the best way to screen men for prostate cancer, so one day all men at risk are invited for regular tests to find aggressive cancers in time for a cure. 

In the meantime, the UK’s National Screening Committee is currently reviewing the existing evidence for different screening programmes for prostate cancer, and we eagerly await the outcome of this review.