The idea of blood tests for cancer seems almost too good to be true: a simple blood sample that could search for signals from multiple types of cancer at the same time, even before a person notices any symptoms.
Traditionally, early cancer detection has relied primarily on tests designed for specific organs. Mammograms look for suspicious changes in the breasts, colonoscopies help identify lesions in the colon, and CT scans can be used to detect lung cancer.
However, there is an important underlying problem. Many types of cancer still do not have routine screening tests for people without symptoms. As a result, early detection is often difficult, and many cases are diagnosed at advanced stages, when the prognosis may be less favorable.
With this problem in mind, multi-cancer detection tests, known as MCD or MCED (multi-cancer early detection) tests, have recently been developed. Instead of examining a single organ, these tests look for molecular signals in the blood that could indicate the presence of different types of cancer.
However, the growing popularity of these new tests has generated an important discussion among researchers, physicians, and regulatory authorities regarding their sensitivity and specificity. In March 2026, the renowned scientific journal Nature addressed precisely this issue: an increasing number of blood tests claim to detect cancer, some of which are already commercially available, but experts warn that they may miss certain cancers or produce false-positive or false-negative results.
The question, then, is no longer simply whether we can find signs of cancer in the blood. The more important question is: what does finding those signals actually mean?
Cancer can leave clues in the blood
A tumor is not completely isolated from the rest of the body. Tumor cells continuously release different fragments and molecules, such as DNA, proteins, and other biological material, some of which can enter the bloodstream.
This has opened a new window for studying cancer without having to directly access the tumor. Modern technologies can analyze enormous amounts of molecular information from a blood sample and attempt to identify patterns associated with the presence of cancer cells.
One of the tests that has received the most attention is Galleri, developed by GRAIL and evaluated by the FDA.
Galleri analyzes circulating cell-free DNA, known as cfDNA, and looks for specific methylation patterns. Methylation refers to chemical modifications of DNA that participate in the regulation of genes—in simple terms, they can influence whether a gene is “turned on” or “turned off.” Cancer cells can develop methylation patterns that differ from those normally observed in healthy cells, making these changes potentially useful signals of cancer.
In addition, when the test detects a signal associated with cancer, it attempts to predict where in the body that signal may have originated.
However, there is an important point that needs to be clarified. If a blood test reports that it has detected a cancer-related signal, this does not automatically mean that the person has a confirmed cancer diagnosis. Tests of this kind can support the diagnostic process, but they represent only one part of the overall picture. A physician will still need to investigate further and may rely on other diagnostic tools, such as imaging studies or, if a suspicious lesion is identified, a biopsy.
For this reason, it is useful to think of these tests as the beginning of an investigation rather than necessarily its conclusion.
What if the result says no cancer signal was detected?
Just as a positive result does not necessarily mean that a patient has cancer, a negative result should also be interpreted with caution. Although it may be reassuring, it does not mean exactly the same thing as saying: “You do not have cancer”.
None of these tests can detect every cancer, and their performance depends on several factors, including the type of tumor, its biology, its location, and potentially the amount of tumor-derived material that reaches the bloodstream.
There is also an important paradox. Small, early-stage cancers are precisely one of the main targets of these diagnostic technologies, but they may also be among the most difficult to detect through a blood sample because they can release extremely small amounts of tumor DNA.
More than 140,000 people put this idea to the test
One of the most important evaluations of this technology has been the NHS-Galleri trial, conducted in England.
The study randomized 142,250 people between the ages of 50 and 77. In the intervention group, Galleri was added to usual care, and participants underwent three annual rounds of testing.
The published results help illustrate both the possibilities and the limitations of the test.
Specificity was very high, at approximately 99.50% to 99.60%. In other words, the vast majority of people without cancer did not receive a positive cancer signal.
However, sensitivity was considerably more variable. The test identified approximately 27 to 37 out of every 100 cases in people who actually had cancer. This figure improved when researchers focused on a specific group of only 12 cancer types, where approximately 48 to 63 out of every 100 cases were detected.
Therefore, when all patients are considered regardless of cancer type, the test can still miss a substantial proportion of existing cancers. This is why a negative result cannot be interpreted as a guarantee that cancer is absent.
However, when the test correctly detected cancer, its prediction of the likely site of origin of the cancer signal was accurate in more than 90% of cases across all three screening rounds.
The primary objective of the NHS-Galleri trial was to determine whether adding this test could reduce the incidence of cancers diagnosed at advanced stages compared with the control group. However, the study did not achieve this primary objective.
Figure 1. Circulating molecular biomarkers can provide information about cancer-related signals and their possible tissue of origin. The clinical value of these tests depends on their ability to detect cancer while minimizing false-positive and false-negative results.
So… is this technology useful?
In reality, every cancer detection strategy must deal with the possibility of false positives and false negatives. Although current tests can achieve very high specificity, even a small proportion of false-positive results becomes important when a technology is used in hundreds of thousands or potentially millions of people.
Beyond the financial cost and the potential risks associated with additional diagnostic procedures, another factor should not be overlooked: the anxiety a patient may experience after being told that a cancer signal has been detected while waiting for additional testing to confirm or rule out the presence of cancer.
These tests can be useful, but they do not replace medical judgment or other diagnostic tools. It is important to understand their limitations and consider them as an additional layer of cancer detection, while interpreting their results carefully.
The ability to analyze a single blood sample and simultaneously search for signals associated with multiple cancers would have seemed extraordinary only a few decades ago. Today, this technology is being evaluated in studies involving tens of thousands of people and is being examined by regulatory authorities.
But enthusiasm must advance at the same pace as the evidence.
The Galleri trial is neither a story of absolute success nor one of failure. Instead, it represents a technology that is still defining the role it should play in cancer detection.
For patients, perhaps the most important conclusion is also the simplest: a blood test can provide extraordinarily valuable information, but that information must always be interpreted within the broader medical context.
Main reference:
Neal, R. D., Dolly, S., Johnson, P., Jones, H., Kumar, S. H., Lee, L. Y. W., Lee, Y., Liang, W., Round, T., Smittenaar, R., Warwick, J., Sasieni, P., & Swanton, C. (2026). Performance of a multi-cancer early detection test in the randomized controlled NHS-Galleri trial. Nature medicine, 10.1038/s41591-026-04652-8. Advance online publication. https://doi.org/10.1038/s41591-026-04652-8
Other references:
Fleming N. Cancer blood tests are everywhere — here’s why you should be cautious. Nature. 2026;651:22–24. doi:10.1038/d41586-026-00661-2. https://www.nature.com/articles/d41586-026-00661-2
Marinac, C.R., Rebbeck, T.R. & O’Donnell, E.K. Some answers, more questions for multi-cancer early detection tests. Nature Medicine (2026).
https://doi.org/10.1038/s41591-026-04688-w
