Multicancer Detection Promise and Peril: New Galleri Blood Test Shows High Accuracy, But Low Sensitivity Raises Red Flags
- 4 days ago
- 2 min read
Progress in reducing the global toll of cancer remains challenging, but a new technology—the Galleri multicancer early detection (MCED) blood test, developed by US firm Grail—has generated "unusual levels of hope" among researchers. Medical experts view cancer screening as a cornerstone of preventive medicine, noting that the earlier cancer is found, the more likely it is to be cured. Unlike traditional methods that screen for only one cancer type (such as mammograms for breast cancer or colonoscopies for colorectal cancer), the Galleri test is designed to detect signals from over 50 types of cancer from a single blood sample. It works by identifying cell-free DNA released by cancer cells into the blood and can even suggest the cancer's location in the body. Crucially, about three-quarters of the cancers detected by the test currently have no recommended screening program, including ovarian, pancreatic, and liver cancers.
The Pathfinder 2 trial, which involved 23,161 people aged over 50 in the US and Canada with no prior cancer diagnosis, provided the headline figures now circulating widely. The test correctly identified cancer in 62 percent of people who received a positive result. This metric, known as the Positive Predictive Value (PPV), answers the vital question: "If I test positive, what's the chance I actually have cancer?". For those without cancer, the test demonstrated exceptional specificity, correctly ruling out the disease in a reported 99.6 percent of cases.
Despite these encouraging figures, experts caution that early promise must be scrutinized. Less widely reported is the measure of sensitivity—how many true cancer cases the test actually detects. On this measure, the result was 40.4 percent. This means the test missed approximately three in every five cancers that appeared over the following year.
This shortfall raises the risk that patients receiving a negative result might be falsely reassured, potentially delaying a diagnosis. Furthermore, while the specificity rate of 99.6 percent appears strong, applying this rate to a large population—such as the over 26 million people aged 50+ in the UK—could still generate over 100,000 false positive results. A false positive triggers anxiety and requires further medical follow-up.
The Galleri test is expensive, costing US749toUS949 out-of-pocket, and currently, most insurance plans do not cover it.
Experts emphasize that the test remains a "work in progress" and does not replace current standard screening protocols like mammograms or colonoscopies. While the early data is encouraging, researchers are divided on its immediate use for mass population screening. Some experts describe the low sensitivity and resulting low "pick-up rate" as making the approach "unsuitable for population screening" based on current data. Statisticians also caution that these reported values are estimates, and real-world accuracy often performs less well than results observed in carefully controlled trials. Ultimately, more research is required, specifically looking at the test's impact on mortality reduction, before it can be widely adopted as a definitive screening tool.
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Keywords: Multicancer Detection
Multicancer Detection
