Erythrocyte Sedimentation Rate (ESR), also known as the Westergren method, is a non-specific blood test that measures how quickly red blood cells settle to the bottom of a test tube over one hour. This test is a marker of inflammation in the body, as inflammatory conditions cause changes in blood proteins that make red blood cells clump together and fall faster. Elevated ESR levels can indicate various inflammatory and infectious conditions including rheumatoid arthritis, lupus, temporal arteritis, infections, and certain cancers. The ESR is particularly useful for monitoring disease activity in conditions like temporal arteritis and polymyalgia rheumatica, where it serves as a sensitive indicator of inflammation. While ESR is not diagnostic of any specific condition, it provides valuable information about the presence and severity of inflammation, helping guide further diagnostic testing and monitor treatment response.