Erythrocyte Sedimentation Rate (ESR) is a nonspecific laboratory test of the speed at which erythrocytes settle out of unclotted blood.
The speed at which the cells settle depends on how many red blood cells clump together. Clumping is increased by the presence of acute-phase proteins, released during inflammation. Therefore, the sedimentation rate indicates the presence of inflammation and is used to track the onset and progress of inflammatory disorders. An elevated ESR helps to differentiate an acute myocardial infarction from angina pectoris because no inflammation is present in the latter state.