Sedimentation rate: A blood test that detects and monitors inflammation in the body. Abbreviated sed rate. The sed rate measures the rate at which red blood cells in a test tube separate from blood serum over time, becoming sediment at the bottom of the test tube. The sed rate increases as inflammation increases. The sed rate can also become elevated in diseases that feature the production of abnormal proteins, such as multiple myeloma. Also known as erythrocyte sedimentation rate, or ESR.