ESR tubes are tall, thin vacuum tubes for collecting blood to measure the Erythrocyte Sedimentation Rate (ESR) test. They are typically plastic or glass, contain a buffer of 3.2% sodium citrate, and have a black cap. The tube's vacuum and the citrate anticoagulant are designed for a precise 1:4 blood-to-citrate ratio, allowing for automated or manual measurement of how fast red blood cells settle, which can indicate inflammation or other diseases.