The Limitations of Blood Tests in Cancer Detection

The American company Grail has developed a blood test called Galleri, designed to detect signals from over 50 types of cancer through circulating DNA fragments in the bloodstream. Marketed as a screening tool, the test promises results indicating whether a cancer signal is present, with a cost of US$949. However, the test has not yet received approval from the U.S. Food and Drug Administration (FDA), raising questions about its efficacy and safety.

The Guardian

In partnership with the National Health Service (NHS) in the UK, Grail launched a large-scale randomized controlled trial in 2021, enrolling 142,000 asymptomatic participants aged 50 to 77. The trial aimed to determine if the Galleri test could reduce late-stage cancer diagnoses. Participants were divided into two groups: one received the blood test, while the other continued with standard healthcare practices. Despite the scale of the study, it failed to meet its primary endpoint of reducing late-stage cancer diagnoses, a significant setback for the company.

The Guardian

Following the trial's results, Grail claimed that the study demonstrated a substantial reduction in stage 4 cancer diagnoses and an increased detection rate of early-stage cancers. However, these assertions did not appease investors, leading to a nearly 50% drop in the company's share price and prompting scrutiny, including potential class action lawsuits.

The Guardian

The failure of the Galleri test to demonstrate a population-level benefit highlights a critical distinction in cancer detection: finding more cancers does not equate to saving lives. For instance, detecting a cancer at stage 3 instead of stage 4 may not necessarily lead to better treatment outcomes or prolonged survival. This complexity is often difficult for the public to grasp, emphasizing the need for a more nuanced understanding of cancer screening.

The Guardian

The issue of overdiagnosis is particularly relevant in cases where incidental findings, such as tiny cancers discovered during scans for unrelated issues, lead to unnecessary interventions. Many patients may opt for treatment despite the low likelihood of the cancer becoming life-threatening. This raises ethical questions about the implications of early detection and the potential for causing more harm than good.

The Guardian

While the Galleri test and similar innovations represent promising advancements in cancer detection, the reality remains that no blood test or total body scan currently exists that guarantees life-saving outcomes. Experts recommend focusing on lifestyle modifications—such as reducing processed foods, limiting alcohol intake, avoiding smoking, and maintaining regular exercise—as proven strategies to lower cancer risk. These recommendations may not generate excitement in the stock market, but they are grounded in substantial evidence regarding their impact on public health.

The Guardian

• “Naturally, for a cancer test that has not yet been approved by the FDA but made it into a Super Bowl ad, there were quite a few disclaimers – but I was drawn to one particularly bold claim: “The Galleri test is intended to detect cancer signals and predict where in the body the cancer signal is located.” — Grail

The Guardian

• “This tells us there is a difference between finding more cancers and saving lives – a very difficult thing for most people to wrap their head around but important from a public health perspective.” — Oncologist

The Guardian

• “But for now, healthy people should know there is no blood test (or “total body scan”) for cancer that saves lives.” — Oncologist

The Guardian