Sed rate, or erythrocyte sedimentation rate (ESR), is a blood test that can reveal inflammatory activity in your body. A sed rate test isn't a stand-alone diagnostic tool, but it may help your doctor diagnose or monitor the progress of an inflammatory disease.When your blood is placed in a tall, thin tube, red blood cells (erythrocytes) gradually settle to the bottom. Inflammation can cause the cells to clump together. Because these clumps of cells are denser than individual cells, they settle to the bottom more quickly.The sed rate test measures the distance red blood cells fall in a test tube in one hour. The farther the red blood cells have descended, the greater the inflammatory response of your immune system.