Vacuum blood collection tubes with sodium citrate 3.8% and a double cap are specifically designed for coagulation tests, where the sodium citrate acts as an anticoagulant by binding calcium ions, preventing blood clotting. These tubes are typically used for determining the Erythrocyte Sedimentation Rate (ESR) and other coagulation studies like PT and APTT. The double cap design, often a "sandwich" tube, helps to maintain the vacuum and prevent evaporation, ensuring accurate blood-to-additive ratios for reliable test results.