Finding a needle in a haystack: Measures of circulating tumor DNA (ctDNA) to guide treatment decisions in oncology

Liquid biopsy is a non-invasive molecular test that uses biofluid to detect cancer. This is a rapidly growing field that encompasses a diverse class of assays that rely on circulating nucleic acids, proteins, and cells to identify cancer. Recently, there is particular interest in using circulating tumor DNA (ctDNA) to screen for cancer, to monitor treatment response, and to help guide therapeutic decisions. Here, we will review the procedural aspects of ctDNA assays, as well as the various applications of ctDNA assays in cancer care. The utility of various biofluids to conduct ctDNA assays, the technical considerations for performing the assays, and several possible available assay formats that can be used for ctDNA analysis are reviewed. Several protocols that can be used for ctDNA isolation, detection, and quantification are presented, including targeted ctDNA qPCR.