Final answer:
The ESR (erythrocyte sedimentation rate) test measures how quickly red blood cells settle out of the plasma. The test is performed using an ESR tube, also known as a Westergren tube.
Step-by-step explanation:
The ESR (erythrocyte sedimentation rate) test measures how quickly red blood cells settle out of the plasma over a set period of time. To perform this test, blood is collected in an ESR tube, also known as a Westergren tube. The ESR tube is a long, narrow tube that is open at the top and has a graduated scale on the side to measure the sedimentation rate.