The sedimentation rate (sed rate) blood test measures how quickly red blood cells (erythrocytes) settle in a test tube in one hour. When inflammation is present in the body, certain proteins cause red blood cells to stick together and fall more quickly than normal to the bottom of the tube. The more red cells that fall to the bottom of a special test tube in one hour, the higher the sed rate. These proteins are produced by the liver and the immune system under many abnormal conditions, such as an infection, an auto-immune disease, or cancer. There are many possible causes of an elevated sedimentation rate. For this reason, a sed rate is done with other tests to confirm a diagnosis. Once a diagnosis has been made, a sed rate can be done to help monitor the course of the disease or the effectiveness of treatment. Why It Is Done A sedimentation rate (sed rate) test is done to: Determine whether a condition causing inflammation is present. Monitor the response to treatment of some conditions, such as an infection or some auto-immune diseases.