When Tissue Is the Issue: Expanding Cell-Free DNA "Liquid Biopsies" to Supernatants and Nonplasma Biofluids

While tissue biopsy remains the gold standard for tumor biomarker testing, assays using plasma-derived cfDNA, aka circulating-tumor DNA (ctDNA), have recently demonstrated validity in the setting of limited tissue or recurrent disease. Tumor-derived cfDNA is also present in nonplasma biofluids and supernatants procured through interventional procedures. Evaluation of cfDNA extracted from these fluids may have benefits at nearly every stage of cancer patient management, from diagnosis and prognosis to monitoring disease progression and predicting therapeutic response. This review will focus on preanalytical, analytical, and postanalytical variables that must be considered when analyzing "liquid biopsies" outside the plasma compartment.