The Erythrocyte Sedimentation Rate (ESR) is a blood test that measures how quickly red blood cells, or erythrocytes, settle at the bottom of a test tube. A higher rate can indicate inflammation or the presence of certain diseases, as it reflects changes in blood proteins that occur during inflammatory processes. ESR is often used alongside other tests to help diagnose conditions such as infections, autoimmune diseases, and some cancers. While it is a useful screening tool, it is not specific to any one disease, so further testing is usually required for a definitive diagnosis.