ESR or Erythrocyte Sedimentation Rate tests have been available for longer than most other blood tests.
Although the test is now performed more rapidly, in the past a tube of whole blood would have been placed on one end. The blood cells slowly settle to the bottom leaving clear serum above. The ESR is the number of millimetres the blood cells settle in one hour - inflammatory proteins in the plasma make cells settle more quickly.
A high ESR level is usually associated with inflammation in the body but it gives no information on its own about the cause of the inflammation so is usually interpreted with other tests such as a CBC.
Although a highly sensitive test for inflammation, it has largely been replaced by the cheaper and faster CRP test, which generally provides the same information.