What Is ctDNA Testing (Liquid Biopsy)?

## What Is ctDNA? **ctDNA (circulating tumor DNA)** is small fragments of DNA shed by cancer cells into the bloodstream. A **liquid biopsy** is a blood test that detects and analyzes this ctDNA. Unlike a traditional tissue biopsy (which requires a needle, scope, or surgery to obtain tumor tissue), a liquid biopsy requires only a standard blood draw. ## What Can a Liquid Biopsy Find? A liquid biopsy can detect: - **Specific gene mutations** driving cancer growth (EGFR, KRAS, BRAF, etc.) - **Resistance mutations** that develop when cancer stops responding to treatment - **Copy number changes** — gene amplifications or deletions - **MSI status** (in some tests) - **Fusion genes** (e.g., ALK, ROS1) ## When Liquid Biopsies Are Used **1. When tissue biopsy is difficult or impossible** Some tumors are in hard-to-reach locations. A blood test can provide molecular profiling without a risky procedure. **2. Monitoring treatment response** ctDNA levels fall when treatment is working and rise when cancer is growing. Serial ctDNA measurements provide real-time information between scans. **3. Detecting resistance mutations** When cancer stops responding to a targeted therapy, a liquid biopsy can identify the new mutation driving resistance — helping your oncologist select the next treatment. **4. Minimal Residual Disease (MRD) detection** After surgery or treatment, ctDNA can detect microscopic disease that doesn't show up on imaging. A positive ctDNA test after surgery predicts higher recurrence risk. **5. Early detection (emerging)** Research programs are developing ctDNA tests for early cancer detection in people without symptoms. Multi-cancer early detection (MCED) tests like Galleri are being studied. ## Limitations of Liquid Biopsy - **False negatives:** If ctDNA levels are very low (early-stage cancer, low tumor burden), the test may miss it - **Not a replacement for tissue biopsy** in most cases — tissue remains gold standard for initial diagnosis - **Tumor heterogeneity** — may not capture all mutations present in different parts of the tumor - **Interpretation requires expertise** — variants of uncertain significance can create confusion ## Major Liquid Biopsy Tests Available - **Guardant360** — comprehensive liquid biopsy for advanced solid tumors - **FoundationOne Liquid CDx** — FDA-approved for multiple indications - **Tempus xF** — broad molecular profiling - **Signatera** — personalized MRD monitoring after surgery (custom test built from your tumor's specific mutations)