These are tests of circulating cell-free DNA (cfDNA), not circulating cells. The fraction of circulating _cancer cells_ is too low (outside of blood cancers or metastatic cancers) to be technically feasible to detect; ditto in pregnancy. By contrast, while there isn't much cfDNA (DNA fragments floating around in blood plasma outside of cells) -- on the order of 1-10 ng/mL -- there's enough to extract reliably, and critically, the fraction of that that is tumor-derived (or fetally-derived, for prenatal testing) can be high. It's already high single to low double digit percent at 10-12 weeks of gestation, and can go much higher in some cancers.But the biggest challenge for these tests is that this "tumor fraction" can be very very low in early stage cancers, which is why stage I sensitivity tends to be quite poor.
tsoukase|1 year ago