In studies comparing ctDNA with tissue sequencing results, one potential caveat is that tumors can evolve new mutations between the time of tissue biopsy and the blood draws, Dr Torga acknowledged. “But that caveat should not apply to our study to explain discrepancies on ctDNA mutations harbored in the same blood samples,” he added.

The study cannot, furthermore, determine whether one liquid biopsy panel is more accurate overall.

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“The main findings,” Dr Torga said, “are that even when the companies were analyzing DNA from the same blood drawn, their results rarely matched each other.”

In 15% (6/40) of patients, both tests’ results matched for at least 1 of the reported mutations, he added. In 40%, both companies’ tests agreed and identified no mutations in genes potentially covered by both panels.

“The certification process for these technologies should be re-evaluated if we want to move forward in precision medicine,” Dr Torga said.

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According to Andrew J. Armstrong, MD, associate professor of medicine, surgery, pharmacology, and cancer biology at the Duke Cancer Institute in Durham, North Carolina, this study highlights the need for cross-validation studies comparing different liquid biopsies with clinical outcomes.

“Their data suggest an unacceptable level of variability between assays, which may be assay-specific and/or related to the low level of tumor burden in many of these patients. Larger scale studies are needed to determine the reproducibility of liquid biopsy results for actionable genetic targets in prostate cancer.”

Dr Torga provided study data to both companies. Personal Genome Diagnostics has reanalyzed those data and submitted a letter to the editor of JAMA Oncology.


  1. Torga G, Pienta KJ. Patient-paired sample congruence between 2 commercial liquid biopsy tests. JAMA Oncol. 2017 Dec 14. doi: 10.1001/jamaoncol.2017.4027 [Epub ahead of print]
  2. Lanman RB, Mortimer SA, Zill OA, et al. Analytical and clinical validation of a digital sequencing panel for quantitative, highly accurate evaluation of cell-free circulating tumor DNA. PLoS One. 2015;10:e0140712.