Today’s Managing Health Care Costs Number is $1196
Pathway Genomics, which a few years ago suggested that its genetic testing might lower employers’ medical claims costs by “scaring people skinny” has announced new product – the “liquid biopsy.” The company will seek genetic hints of cancer by checking a blood test. The company recommends the test quarterly – meaning that the annual cost is about $1200. The test was featured in the Washington Post the same week that 23 and me has returned to the market with FDA approved medical interpretations of its genetic tests, almost two years after the FDA has asked the company to discontinue a more comprehensive set of reporting that was not fully backed by evidence.
I’ve reviewed the Pathways website – and here are a few terms I can’t find:
· Sensitivity(how many true positives are picked up by the test)
· Specificity(how few true positives are missed by the test)
· Predictive value of a positive (How likely a positive test is to represent real disease)
· Predictive value of a negative (How likely a negative test is correct)
· Number needed to treat (How many people must be tested for one meaningful diagnosis)
· Number needed to harm (How many people must be tested for one person to be harmed- for instance by a false positive or by overdiagnosis)
There’s really no way to evaluate the appropriateness of this test without such information. The value of a test is also highly dependent on prevalence of disease in the population. If a disease is rare, false positives will outweigh true positives even if the test is exceptionally accurate (high sensitivity and high specificity). Pathways will be aiming this test on well-off customers who are probably not especially old, so the reservoir of undetected cancer will be small. The reservoir of undetected cancer where detection will positively change a patient’s lifewill be smaller still.
It's been a bad last few years for screening, even screening that is far more focused than these "liquid biopsies."
We’ve seen hundreds of thousands of men in the US have radical prostatectomieswhich probably only decreased mortality by a tad (or maybe not at all). We’ve seen massive overdiagnosis of thyroid cancer in South Korea with advent of widespread intensive screening. Many women and men have had thyroidectomies; they will be on thyroid replacement forever, and hopefully only a few will have trouble with calcium metabolism if their parathyroid glands were inadvertently removed as well. We have belatedly recognized that mammography leads to substantial overdiagnosis of ductal carcinoma in situ. We are finding more and more early breast cancer, but death from breast cancer has only budged a bit.
We love to prevent disease, and we hate the uncertainty that there might be a cancer lurking somewhere in our bodies. However, the potential to do medical harm by overscreening is high. Someday, the specificity of such testing (lack of false positives) will be good enough that perhaps we will all get regular blood tests to be sure we are cancer free. Better yet, we will be able to recommend these tests for the portion of the population at highest risk. That time isn’t now, though. Americans should not get “liquid biopsies” to detect hidden cancer.
