How accurate are multi-cancer early detection blood tests like Galleri?

Multi-cancer early detection (MCED) tests, such as the Galleri test, are a developing area of medicine. Their accuracy is generally measured by two key metrics: sensitivity (the ability to correctly identify cancer) and specificity (the ability to correctly identify those without cancer).

Here is the current understanding of their accuracy:

• Specificity is generally high: These tests are designed to be very good at avoiding "false positives." The Galleri test, for example, has demonstrated a specificity of over 99%, meaning it rarely tells a healthy person they have cancer when they do not.
• Sensitivity varies by stage and cancer type: Sensitivity is much higher for late-stage cancers than for early-stage (Stage I or II) cancers. Because the goal of these tests is to catch cancer before it becomes symptomatic, the lower sensitivity for early-stage disease is a significant limitation. Sensitivity also varies widely depending on the specific type of organ the cancer originates from.
• The "False Negative" risk: Because sensitivity for early-stage cancer is not 100%, these tests can produce "false negatives." There is a concern that some people may receive a negative result and subsequently feel a false sense of security, potentially ignoring symptoms or delaying standard-of-care screenings (like mammograms or colonoscopies).

Important context:

• Not a replacement: Medical organizations emphasize that these tests are intended to be used as a supplement to—not a replacement for—standard cancer screening programs.
• Clinical validation: While these tests can detect a "cancer signal," they are still undergoing large-scale clinical trials (such as the NHS-Galleri trial) to determine how they impact long-term patient outcomes, such as mortality rates. Knowing that cancer is present is only the first step; the clinical benefit depends on whether the test leads to earlier, more effective treatment.

If you are considering such a test, it is best to discuss it with a physician who can review your specific risk factors, family history, and which standard screenings you may already be due for.