What Is a Liquid Biopsy?

## Traditional Biopsy vs. Liquid Biopsy A **traditional (tissue) biopsy** involves physically removing a piece of tumor tissue — through a needle, endoscope, or surgery — so it can be examined under a microscope and tested molecularly. A **liquid biopsy** is a blood test. It looks for traces of cancer in your bloodstream — primarily **ctDNA** (circulating tumor DNA) but also circulating tumor cells (CTCs) and exosomes. ## How It Works When cancer cells die or divide, they release DNA fragments into the bloodstream. A liquid biopsy captures these fragments and uses highly sensitive sequencing technology to: 1. Detect the presence of cancer DNA 2. Identify mutations in that DNA 3. Measure how much cancer DNA is present (an indirect measure of tumor burden) ## Key Uses in Oncology **Biomarker testing when tissue isn't available or sufficient** If a biopsy was too small for full molecular profiling, or re-biopsy is too risky, a liquid biopsy can provide mutation information needed to select targeted therapy. **Resistance testing** When cancer stops responding to a targeted drug, liquid biopsy can identify the new resistance mutation — often faster and less invasively than a repeat tissue biopsy. **Treatment monitoring** Serial blood draws over time can show whether ctDNA levels are rising or falling, giving insight into treatment response before imaging changes are visible. **Post-treatment surveillance (MRD)** After surgery, a positive ctDNA test predicts higher recurrence risk — even when scans are clear. This is called **Minimal Residual Disease (MRD) testing**. ## Advantages Over Tissue Biopsy - Non-invasive (just a blood draw) - Can be repeated easily over time - Can capture mutations from multiple tumor sites simultaneously (addressing tumor heterogeneity) - Results often faster than tissue processing ## Limitations - Less sensitive for very small tumors or early-stage disease - Can miss mutations if ctDNA levels are too low - Cannot diagnose cancer on its own — a positive ctDNA result requires tissue confirmation - Not appropriate to replace tissue biopsy for initial diagnosis ## When to Ask About Liquid Biopsy - "Has my tumor been molecularly profiled? If tissue was insufficient, should I have a liquid biopsy?" - "Should we do a liquid biopsy to look for resistance mutations now that my cancer is progressing?" - "Is there a ctDNA monitoring test appropriate for my cancer type?"