ESR, or Erythrocyte Sedimentation Rate, is a blood test that measures how quickly red blood cells settle at the bottom of a test tube. A faster-than-normal rate may indicate inflammation in the body, which can be caused by various conditions, including infections, autoimmune diseases, and certain cancers. The test is simple and involves drawing a small amount of blood, which is then placed in a vertical tube. The rate at which the red blood cells fall is measured in millimeters per hour. While ESR is a useful screening tool, it is not specific to any one disease and is often used alongside other tests for diagnosis.