By Eva Briggs, MD
A question that patients sometimes pose is, “Why isn’t there one blood test that can test for cancer?”
Or the similar request, “Just test me for everything.”
Part of the answer is that cancer is not just one disease. There are many different types of cancer, arising in different body tissues or organs. And each has its own unique biological and chemical composition.
For a test to be useful as a screening test, it must fulfill certain criteria:
• The disease tested for is a significant public health problem. Screening tests look for relatively common conditions that cause significant morbidity (symptoms and suffering) or mortality (death). Sometimes a condition is detected that does not progress to cause symptoms or even resolves spontaneously. That’s called pseudo-disease. Treatment won’t help, might be expensive and have side effects, and the person with pseudo-disease experiences unnecessary stress and worry.
• The disease has an available treatment that increases the chance of cure when the condition is detected early. There is no sense in finding an illness if there is no way to treat it. And if the treatment does not work better when the disease is found at an earlier stage, the test is not beneficial.
• The test can detect the disease before symptoms appear. Once symptoms begin, testing is diagnostic and no longer a screening test. The phase where a person has the disease, but no symptoms is called the preclinical phase. Finding disease during the preclinical phase is the aim of a screening test.
• The test must be safe. A test that harms patients is not useful.
• The cost must be reasonable.
• The test must demonstrate that it leads to better outcomes. There is no sense to find a disease early if treating it before symptoms appear does not increase the chance of cure or decrease symptoms and suffering.
• The test must be widely available and the treatments for the condition it finds must also be widely available. If the treatment is unaffordable or almost impossible to obtain, there is no real benefit.
All these factors contribute to the difficulty of developing a screening test for a single disease.
Yet there are some tests available that claim to screen for multiple conditions from one sample. These are called multi-cancer early detection (MCED) tests. None are FDA approved, although at least one has a CLIA waiver because it is performed in a central laboratory. Therefore, doctors can order it.
This MCED test is called GRAIL Galleri test. Unfortunately, it has not yet been scientifically proven to meet all the above criteria. There is a risk that this test may be of little or no benefit. It could cause harm such as false negative tests that incorrectly reassure people that they don’t have disease. False positives could lead to further testing, worry, and side effects. And it could cost money better used elsewhere. If you decide to pursue this or other similar tests, be aware of these potential drawbacks. This test is not a substitute for current known effective screening tests such as mammograms, PAP tests, HPV tests, colonoscopy, PSA blood test, and low dose CT scans for lung cancer.
In the future MCEDs may be discovered that are safe, effective, and accurate. Scientists continue to work on such tests.
Eva Briggs is a retired medical doctor who practiced in Central New York for several decades. She lives in Marcellus.