A patient presents with persistent fatigue and unexplained weight loss. The doctor prescribes a blood test. The results reveal abnormalities in certain markers, and the diagnostic process accelerates.
This scenario illustrates the growing role of blood analyses in the early detection of cancer. A blood test does not function like a metal detector: it guides, it alerts, it refines. Understanding what it can actually identify, and within what limits, prevents confusing a sorting tool with a definitive diagnosis.
See also : How to Become a Locksmith?
Circulating tumor DNA and MCED tests: what standard analyses do not see
Standard blood tests (complete blood count, protein levels, electrolyte panel) detect biological imbalances compatible with cancer, without being able to confirm its presence. We remain in the realm of indication, not proof. Where things change is with multi-cancer early detection tests, known as MCED tests.
These tests rely on the detection of small fragments of circulating tumor DNA in the blood. The principle: when cancer cells develop, they release DNA into the bloodstream. By analyzing these fragments, we can identify signatures specific to several dozen types of cancers, including those for which no organized screening exists, such as pancreatic cancer.
Related reading : How long does a puff last?
The large randomized NHS-Galleri trial, conducted with nearly 143,000 participants, showed that the addition of a multi-cancer blood test resulted in a reduction of about 14% in stage IV diagnoses. Meanwhile, stage I-II diagnoses increased by about 16% compared to the standard screening group.
The notion of cancer detectable by a blood test takes on a concrete meaning here, even though the authors of the study emphasize that no proof of reduced mortality has yet been provided.
Blood tumor markers: useful, but rarely sufficient alone
Tumor markers are substances produced by cancer cells or by the body in response to cancer. They are measured in the blood to guide a diagnosis or monitor the effectiveness of treatment. Among the most well-known are: PSA (prostate), CA 125 (ovary), ACE (colon, lung), and CA 19-9 (pancreas).
The problem is that a high marker does not necessarily mean cancer. Inflammation, infection, or a benign condition can elevate these values. Conversely, a normal marker does not exclude the presence of a tumor. A cancer diagnosis is never made solely based on a tumor marker.
Their value lies mainly in three specific situations:
- Confirming a clinical suspicion already supported by other tests (imaging, biopsy), directing towards a particular type of cancer
- Monitoring treatment response: if the marker decreases after surgery or chemotherapy, it is a favorable signal
- Detecting a recurrence during post-treatment follow-up, sometimes before symptoms reappear
In mass screening, classic tumor markers lack specificity. This is why MCED tests, which combine the analysis of circulating DNA with classification algorithms, represent a different approach: they do not seek a single marker, but an overall profile.
Blood tests for risk stratification: the example of lung cancer
Beyond direct detection, some blood tests are used to assess an individual’s risk of developing cancer. The goal is no longer to find an existing tumor, but to better target individuals who would benefit from imaging screening.
A study coordinated by the IARC developed the INTEGRAL-Risk model, which combines a panel of 13 blood proteins with age and smoking history to predict the occurrence of lung cancer. This type of approach changes the logic: instead of offering a chest scan to all smokers above a certain age, heavy examinations are concentrated on high-risk profiles identified by the blood test.
The benefit is twofold. It reduces the number of unnecessary scans (and the anxiety-inducing false positives that result). It improves detection in patients who need it most. This model is not yet routinely deployed, but it illustrates how a simple blood test can restructure a screening program.
Current limitations and necessary caution regarding commercial tests
Tests like miCheckup, based on signatures of circulating microRNAs, are already offered in France for multi-cancer early detection. They can be ordered and performed. The problem: they are neither integrated into an organized screening program nor recommended by health authorities at this stage.
The distinction is significant. A commercially available test is not a test validated in the general population. Data on sensitivity (the ability to detect an existing cancer) and specificity (the ability not to trigger a false alarm) vary depending on the type and stage of cancer. Feedback also varies on the management of results: what do we do concretely when a test indicates a risk, without any imaging exam confirming the presence of a tumor?
False positives generate additional tests, anxiety, and sometimes unnecessary interventions. False negatives provide a false sense of security. These are real limitations that promotional content often overlooks.
- A positive MCED test always requires confirmation by imaging or biopsy before any therapeutic decision
- The sensitivity of these tests remains lower for stage I cancers than for advanced stages
- No blood test replaces existing organized screenings (mammography, colonoscopy, Pap smear)
Blood tests are becoming more precise each year in the field of cancer screening. MCED tests, protein stratification, and tumor markers form a complementary arsenal, not a substitute for reference examinations. Real progress is measured by the number of diagnoses made at a treatable stage, not by the number of tests marketed.