With most of these (cell free DNA) assays, its not a single site that you are measuring but a statistical aggregate about profiles that have been defined for a

With most of these (cell free DNA) assays, its not a single site that you are measuring but a statistical aggregate about profiles that have been defined for a certain cell type. If you know the overall profile of prostate cells and the profile of normal blood, you can subtract the expected profile from a patient sample and the analyse the left-over fraction for whether it is just noise or is matching to one or multiple known cell types. The surprise from this publication is only that they detect such signatures 4 years earlier than other non-invasive testing. (although nobody in the cfDNA field would be surprised about that).