The Erythrocyte Sedimentation Rate (ESR) is a blood test that measures the rate at which red blood cells (erythrocytes) settle at the bottom of a test tube over a specific period, usually one hour. A faster than normal rate can indicate inflammation in the body, as certain proteins released during inflammation cause red blood cells to clump together and settle more quickly. ESR is commonly used to monitor conditions such as infections, autoimmune diseases (like rheumatoid arthritis), and certain cancers. It is a non-specific test, meaning it can indicate the presence of inflammation but does not identify its cause.