Abstract
Liquid biopsies offer the potential to monitor cancer response and resistance to therapeutics in near real-time. However, the plasma cell free DNA (cfDNA) level can be low and the fraction of circulating tumour DNA (ctDNA) bearing a mutation - lower still. Detection of tumour-derived mutations in ctDNA is thus challenging and requires highly sensitive and specific assays. Droplet digital PCR (ddPCR) is a technique that enables exquisitely sensitive detection and quantification of DNA/RNA markers from very limiting clinical samples, including plasma. The Bio-Rad QX200 ddPCR system provides absolute quantitation of target DNA molecules using fluorescent dual-labelled probes. Critical to accurate sample analysis are validated assays that are highly specific, reproducible, and with known performance characteristics, especially with respect to false positives. We present a systematic approach to the development and optimisation of singleplex and multiplex ddPCR assays for the detection of point mutations with a focus on ensuring extremely low false positives whilst retaining high sensitivity. We also present a refined method to determine cfDNA extraction efficiency allowing for more accurate extrapolation of mutational levels in source samples. We have applied these approaches to successfully analyse many ctDNA samples from multiple clinical studies and generated exploratory data of high quality.