You may have read about new blood tests that can detect cancer or risk of cancer at an early stage.

Many practices are now offering these tests as part of a platinum screening option for their patients.

  • For example, the Galleri® blood test is promoted as “Adding a multi-cancer early detection test to your cancer screenings allows you to go beyond what’s currently possible.”

There are other screening test that you have read about – for example, whole body MRI scans, coronary calcium scans for early coronary artery disease detection.

As part of our screening tests, we do not routinely offer these screening tests. Why is that?

Our decision not to routinely offer these screening tests stems from concerns about their effectiveness and potential harms. While they promise early detection, the sensitivity of some tests may be too low to reliably detect early-stage disease, leading to false reassurance. Additionally, false positives can cause undue anxiety and lead to further unnecessary invasive procedures without finding anything actually wrong.

It’s crucial to weigh the benefits against the risks before incorporating new screening tests into routine practice.

Balancing sensitivity and specificity is essential for the effectiveness of any screening program..The important criteria for screening include: 

  • Sensitivity ensures that the test can detect early disease reliably, minimising false negatives and ensuring that potential cases are not missed.
  • Specificity, on the other hand, ensures that the test accurately identifies those without the disease, reducing false positives and unnecessary anxiety or interventions.
Let’s look at early cancer blood tests

For a cancer screening test to work, it must find disease before it has caused symptoms — when it is in an early or premalignant stage. Screening tests that detect a cancer in an advanced stage are not of value.

Many of these blood tests have an average sensitivity of <20% for picking up stage 1 (i.e. early treatable cancer) – meaning that up to 80% will be missed, the patient will be given false reassurance.

But surely a test picking up a cancer must be worthwhile?

 “…while the risks of possible harm should be weighed against the potential benefits for the individual, the best chance of not dying from cancer comes from catching cancer early through screening.”

This is the common sense rational for cancer screening.

 Unfortunately, positive tests from screening can be positively harmful!

  • There is a very small chance you could be harmed directly by the screening test.
  • You might also suffer from anxiety related to the test and the results. This anxiety will be especially aggravating if you get a false positive result.
  • The test could also lead to over-diagnosis, the diagnosis of a cancer that does not need to be treated. (For example, the chance of this is about 6/1000 for 10 years of mammograms)
  • You could be diagnosed with a cancer that needs to be treated but whose early treatment provides no benefit. This means that you would have the same outcome had we waited until you developed symptoms from the cancer. In this case, the only thing that screening has done is make you live longer as a “cancer patient.”
  • Just because you apparently find an early disease – it does not mean that there are any effective treatments
  • False positive tests can mean a patient is subjected to harmful investigations or treatments when they do not have the disease.
  • Most people who are screened for a particular type of cancer will not benefit because. most of the people will never get, never be harmed by, and/or never die from the cancer for which they are being screened.
Let us examine screening for pancreatic cancer with one of the commercially available tests.

Pancreatic cancer is often referred to as the silent killer, we are not sure why people get it, and it is one of the few cancers where a healthy lifestyle does not seem to help.

Let us imagine screening for pancreatic cancer in a 64-year-old man.
  • The sensitivity of the blood test for stage 1 pancreatic cancer is 62%.
  • In other words, it will pick up nearly 2/3 of early treatable disease but miss 1/3.
  • The test is very specific, quoted at 100%, meaning that the rate of false positive should be really low, i.e if it tells you have it, then you do.
  • Surely this is worthwhile?
But unfortunately, this is where the science gets complicated!

Sensitivity (false negatives) and specificity (false positives) are hugely influenced by how likely the person is to have the disease you are testing for.

  • In the case above, the chance of having stage 1 pancreatic cancer in this patient is 0.03%, i.e only 3 patients in 10,000 will have the disease.
  • A negative test merely confirms what was always likely to be the case.
  •  BUT very importantly a positive test in someone that is very unlikely to have what you are testing for is likely to be wrong – however accurate the test appears to be on paper.
So, for our 64-year-old man, if the test tells him he has early stage 1 pancreatic cancer it is >95% likely to be wrong!
  • We know that this sounds counterintuitive, but it is true.
  • The patient may then go on to have further scanning and invasive tests to prove that the test is wrong.
  • If this further evaluation is positive, and we have managed to diagnose asymptomatic, pancreatic cancer, the likelihood of survival is probably, at best, 50%.

To summarise for every 100 patients who we tell may have early pancreatic cancer, 95 of them will not, it is a false positive. These patients will be caused unnecessary worry, subjected to further tests, and importantly so far, we do not have any evidence that this screening test will prevent any deaths.

Would you still want to have this test?