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 to help diagnose conditions such as rheumatoid arthritis, lupus, and infections. While it is a useful screening tool, it is not specific to any one disease, so further testing is usually required to determine the exact cause of the inflammation.