A sed rate test, also known as an erythrocyte sedimentation rate or ESR test, works by measuring the distance red blood cells fall in a test tube within an hour, according to Mayo Clinic. The greater the distance red blood cells fall, the greater the inflammatory response of the immune system.
A sed rate test is conducted by placing blood in a test tube, explains Mayo Clinic. The red blood cells settle to the bottom of the tube over the course of an hour. Inflammation in the body can cause clumping together of the red blood cells. When the red blood cells clump together, they are larger than other cells, which causes them to fall to the bottom of the tube faster. A sed test be itself does not diagnose a medical condition.
Sed rate tests are not as accurate as other tests that measure inflammatory activity and do not pinpoint the cause of inflammation, states Mayo Clinic. Sed rate tests are commonly used to monitor treatment and are often accompanied by other tests, such as a C-reactive protein test. A doctor may use a sed rate test when a patient is suspected of having certain conditions, such as giant cell arteritism, polymyalgia rheumatica and rheumatoid arthritis.