Can a Blood Test Detect Cancer Early?

One of the most common questions I hear is:

"Can I have a blood test to check for cancer?"

It seems like a simple question. After all, modern medicine can sequence our DNA, perform whole-body MRI scans, and analyse hundreds of biomarkers from a single blood sample. Surely we should be able to detect cancer before it causes symptoms.

The reality is more complicated.

Despite remarkable advances in medicine, we still have very few tests that reliably detect cancer early in healthy people. Understanding why helps explain why evidence-based cancer screening looks very different from what many people expect.

Why cancer is different

Unlike conditions such as diabetes or high cholesterol, cancer is not a single disease. It is hundreds of different diseases, each developing in different organs, behaving differently, and producing different biological signals.

Some cancers grow slowly over many years. Others develop rapidly. Some release detectable molecules into the bloodstream early, while others remain virtually invisible until they are relatively advanced.

This makes the idea of a single "cancer blood test" far more difficult than it first appears.

What about routine blood tests?

Many health assessments include a full blood count (FBC), which measures your red cells, white cells and platelets.

While this test is extremely useful for diagnosing many medical conditions, it is a poor screening test for most cancers.

A full blood count may occasionally detect blood cancers such as leukaemia, but a normal result tells us very little about whether you have—or will develop—the vast majority of cancers.

The same applies to many other blood tests that are often referred to as "tumour markers."

Tests such as CEA, CA 19-9 and CA-125 can be useful in patients who already have a diagnosed cancer or where there is a specific clinical concern. They were never designed to be screening tests for healthy people because they can produce both false positives and false negatives.

PSA is one of the exceptions

One notable exception is the Prostate Specific Antigen (PSA) blood test.

PSA can detect prostate cancer earlier than would otherwise be possible, but it also illustrates why cancer screening is rarely straightforward.

Some prostate cancers grow so slowly that they would never cause harm during a person's lifetime. Others are aggressive and require treatment. PSA cannot always distinguish between the two.

This means the decision to have PSA screening should involve an informed discussion about both the potential benefits and the potential downsides, including overdiagnosis and unnecessary treatment.

Does genetic testing help?

Genetic testing has become increasingly popular, but it is often misunderstood.

Most genetic tests do not tell you whether you have cancer.

Instead, they estimate your risk of developing particular cancers in the future.

For example, inherited variants such as BRCA1, BRCA2 or genes associated with Lynch syndrome can substantially increase the lifetime risk of certain cancers. This information can be extremely valuable because it may justify earlier or more intensive screening.

However, most people do not carry these high-risk variants, and even if they do, genetics still cannot predict whether or when cancer will develop.

Genes influence risk. They are not a diagnosis.

What about a whole-body MRI?

Whole-body MRI has generated a lot of interest as a potential early detection strategy.

It can sometimes detect cancers before they cause symptoms. But there's a lot to understand about what whole-body MRI is—and isn't.

Whole-body MRI is not used to estimate your future risk of cancer. It is used to look for cancer that is already present.

It also often detects incidental findings. These are unexpected abnormalities unrelated to cancer that would never have caused symptoms. While some incidental findings are important, many lead to further scans, referrals to specialists, biopsies and anxiety before ultimately being shown to be harmless.

This doesn't mean whole-body MRI is without a role. It simply means its benefits and limitations need to be understood before assuming that more imaging automatically improves health.

What about the new cancer blood tests?

Scientists are working to develop a new generation of tests known as multi-cancer early detection (MCED) tests, also known as liquid biopsies.

These tests look for very small fragments of DNA that cancers shed into the bloodstream.

The science is very exciting, and there is real potential for these technologies to change cancer screening over the coming decades.

The question remains:

Do they reduce the number of people who die from cancer?

Finding more cancers is not the same as improving health outcomes.

To become routine, screening tests first need to show that they do more good than harm.

What actually works?

Effective cancer screening strategies are often those that don't get the headlines.

For those who are eligible, screening programs such as bowel cancer screening, cervical screening, mammography and low-dose CT scans for selected high-risk smokers have been shown to reduce deaths from cancer.

These programs are recommended because they have been carefully studied over many years.

Importantly, many of the biggest reductions in cancer risk come from lifestyle rather than technology: not smoking, maintaining a healthy weight, exercising regularly, limiting alcohol, and protecting your skin from excessive sun exposure.

The Peak Perspective

When it comes to cancer, we are all susceptible to the allure of believing that more tests are always better.

But medicine is rarely that simple.

Every test has strengths, limitations and potential downsides. The challenge is not just finding more tests—it is knowing which tests are likely to help, which are unlikely to change your care, and which may lead to unnecessary cost, anxiety or further investigations.

Good preventive healthcare is not about ordering every test available.

It is about understanding your individual risk, applying the best available evidence, and selecting investigations that are genuinely likely to improve your long-term health.