Coagulation testing requires plasma that stays anticoagulated and free from substances that could prematurely affect clotting times. Citrate achieves this by binding calcium, a necessary factor in the coagulation cascade, effectively preventing clot formation. Collecting the first tube with citrate preserves the proper blood-to-anticoagulant ratio (blue-top tube) and minimizes contamination from other additives in later tubes, which is crucial for accurate PT and aPTT results. If a different additive were drawn first or if subsequent tubes contaminated the sample, calcium availability and coagulation factor activity could be altered, leading to inaccurate measurements. Other common additives serve different purposes—EDTA is used for CBCs, heparin for certain plasma tests, and fluoride for glucose measurement—so they’re not suited to be the first tube for coagulation studies.