Erythrocyte Sedimentation Rate Does this test have other names? ESR, sed rate What is this test? Erythrocyte sedimentation rate (ESR) is a blood test. It measures how quickly erythrocytes, or red blood cells, separate from a blood sample that has been treated so the blood will not clot. During this test, a small amount of your blood will be put in an upright tube. A lab specialist will measure the rate that your red blood cells settle toward the bottom of the tube after 1 hour. If you have a condition that causes inflammation or cell damage, your red blood cells tend to clump together. This makes them heavier, so they settle faster. The faster your red blood cells settle and fall, the higher your ESR. A high ESR tells your healthcare provider that you may have an active disease process in your body.  Why do I need this test? You may need this test if you have symptoms of one of the diseases that may cause ESR to go up. You may also need this test if you have already been diagnosed wi