July 24th, 2018
What if you could take a blood test to find out if you had cancer, far before symptoms appeared? That’s the vision for a “liquid biopsy” that scientists are currently developing.
As CNN reported recently, researchers in Australia have created a blood test to screen for melanoma and find the cancer before it becomes aggressive. The article describes the test as having a “high degree of accuracy” and proclaims that the test “could speed up the diagnosis process, saving thousands of lives.” But doctors are warning of serious downsides to this test, when and if it becomes available.
The researchers still have a long way to go before the test is usable; currently it can only identify people with melanoma with 79% accuracy and people without a melanoma with 84% accuracy. That means that it does not find melanoma in 1 in 5 people who have it, and gives a false positive to 1 in 6 healthy people. Screening an entire population with these rates of uncertainty would expose thousands to harmful overuse and underuse.
“Liquid biopsies are a recipe for more health anxiety, more procedures, more complications, and more overdiagnoses. Not to mention, more out-of-pocket costs for our patients,” wrote Dr. H. Gilbert Welch, professor of medicine at the Dartmouth Institute, in an op-ed earlier this year.
Even if the test was 90% accurate, as the researchers are hoping to accomplish soon, there are still serious issues for how the test would work in practice. A brilliant Twitter thread from dermatologist Dr. Ade Adamson points out some of the flaws with the liquid biopsy using a thought experiment:
“As you can see this could open up a cascade of tests, procedures, worry, and overdiagnosis. A veritable Pandora’s box of harm,” Adamson concludes.
Another way to look at this, says dermatologist David Elpern, is an extension of the misguided fervor to catch melanoma early. Over the past few decades, dermatologists have made a push to look for more melanomas and increase public awareness of skin cancer signs. As a result, we are diagnosing about three times as many melanomas today as we did 30 – 40 years ago. However, the actual number of deaths from melanoma has barely increased.
“The truth is that newer diagnostic approaches have increased the discovery of early lesions that would never have killed anyone,” said Elpern. So why do we keep looking? Because finding early lesions keeps melanoma centers busy, even though it creates more health anxiety and overtreatment.
“There are many ways to look at this new test, but the bottom line is that early diagnosis should not be our goal,” said Elpern.