Results of the first external quality assessment scheme (EQA) for isolation and analysis of circulating tumour DNA (ctDNA)

Background: Circulating tumour DNA (ctDNA) is considered to have a high potential for future management of malignancies. This pilot external quality assessment (EQA) scheme aimed to address issues of analytical quality in this new area of laboratory diagnostics. Methods: The EQA scheme consisted of three 2-mL EDTA-plasma samples spiked with fragmented genomic DNA with a mutant allele frequency ranging from 0% to 10% dedicated to the analysis of nine known sequence variations in KRAS codon 12/13 and of BRAF V600E. Laboratories reported: (1) time elapsed for processing, (2) storage temperatures, (3) methods for extraction and quantification, (4) genotyping methodologies and (5) results. Results: Specimens were sent to 42 laboratories from 10 European countries; 72.3% reported to isolate cell-free DNA (cfDNA) manually, 62.5% used the entire plasma volume for cfDNA isolation and 38.5% used >10% of cfDNA extracted for downstream genotyping. Of the methods used for quantification, PicoGreen demonstrated the lowest coefficient of variation (33.7%). For genotyping, 11 different methods were reported with the highest error rate observed for Sanger sequencing and the lowest for highly sensitive approaches like digital PCR. In total, 197 genotypes were determined with an overall error rate of 6.09%. Conclusions: This pilot EQA scheme illustrates the current variability in multiple phases of cfDNA processing and analysis of ctDNA resulting in an overall error rate of 6.09%. The areas with the greatest variance and clinical impact included specimen volume, cfDNA quantification method, and preference of genotyping platform. Regarding quality assurance, there is an urgent need for harmonisation of procedures and workflows.