The sed rate by modified Westergren is a blood test that measures the rate at which red blood cells settle to the bottom of a tube over one hour due to the influence of gravity. The tube used for the test is called the Westergren tube, which is a standardized, upright, elongated tube with an internal diameter of 2.5 mm and lengths of 190 to 300 mm. The test is useful in differentiating inflammatory and neoplastic diseases and as an index of disease severity. The sed rate test measures how fast red blood cells fall to the bottom of a tube, and inflammation creates proteins that make red blood cells fall more quickly. A high sed rate is a sign that a person has a disease that causes inflammation in the body. The sed rate test is often used with other tests to help diagnose or check the progress of an inflammatory disease. The sed rate test can only tell a doctor that a person has inflammation somewhere in the body, but it cannot show where the inflammation is or what caused it.