This document provides information about the erythrocyte sedimentation rate (ESR) test. It describes how the test works by allowing anticoagulated blood to stand vertically undisturbed for 1 hour, then measuring the distance the erythrocytes have settled. There are three stages of sedimentation over this time period. Two common methods for performing the ESR test are described - the Westergren and Wintrobe methods. Reference ranges for ESR are provided. Potential sources of error and limitations of the test are also outlined.