So far the accuracy of the blood-based tests is in dispute. Foundation Medicine, a company in Cambridge, Massachusetts, that offers a DNA test for biopsied tissue samples, says they may yield false results and steer patients to the wrong drugs. “We get many calls from doctors with results from [a tissue biopsy] and then totally different results from a liquid biopsy,” says the company’s chief scientific officer, Phil Stephens. “The results are completely discordant and all over the map. That is bad for physicians and bad for patients.” Guardant CEO Helmy Eltoukhy, however, says his company has data from over 3,000 patients indicating that the two test types agree more than 90 percent of the time. “We don’t look at it as experimental,” he says.Though it might seem that Foundation Medicine sometimes produces results that aren't useful because they're 'discordant', the results between genetic tests from solid tumors and plasma DNA don't have to match at all. A service provider sequencing a tumour may be able to get over 90% purity of tumour DNA in their samples, which makes it dead easy to call mutations. Plasma DNA, however, contains a ton of normal unmutated DNA, which dilutes the signal from mutated DNA significantly, making it harder to find anything of potential importance. In effect, you might be trying to use a liquid biopsy to find rare mutated DNA that's present at only 1% or less. (Guardant has a vaguely written patent about how they deal with this issue.)
One final note:
Whether DNA tests really help patients when they’re used to guide treatment is also a matter of dispute. Foundation Medicine issued a press release in May describing work by MD Anderson Cancer Center that compared 175 people who got its test, called FoundationOne. It said those who received a therapy matched to one of the mutations it uncovered lived three months longer on average than those who didn’t.I'd say that's evidence liquid biopsies are helping patients.