SEQUENCE OF FILLING OF TUBES Following order of filling of tubes should be followed after withdrawal of blood from the patient if multiple investigations are ordered: First tube: Blood culture. Second tube: Plain tube (serum). Third tube: Tube containing anticoagulant (EDTA, citrate, or heparin). Fourth tube: Tube containing additional stabilizing agent like fluoride. Further Reading: USES OF ANTICOAGULANTS FOR HEMOTOLOGICAL INVESTIGATIONS PLAIN TUBES (WITHOUT ANY ANTICOAGULANT) AND FLUORIDE TUBES FOR COLLECTION OF BLOOD USE OF PLASMA VS. SERUM