The erythrocyte sedimentation rate (ESR) blood test also called a sedimentation rate (sed rate) is the rate that red blood cells settle in a period of 1 hour. The more red cells that settle in one hour (measured in mm/h), the higher the rate. The ESR blood test has been used for years to detect diseases associated with inflammation, such as an infection, an autoimmune disease, heart disease or cancer. ESR is also increased in pregnancy or with rheumatoid arthritis. Because the sed rate can be affected by other conditions besides inflammation, the ESR is typically prescribed with other tests like the C-reactive protein test to determine the cause of the inflammation.