Are discard tubes necessary in coagulation studies?

We evaluated the necessity of drawing a discard tube on prothrombin time (PT) and activated partial thromboplastin time (APTT) results in outpatients taking oral anticoagulant therapy. We drew two blue-stoppered tubes on 380 patients rather than the traditional practice of filling a red-top tube before filling the coagulation tube. We ran PTs and APTTs on both samples and compared results. We evaluated two sodium citrate concentrations and two prothrombin reagents, one sensitive and the other less sensitive. When PT results are reported as international normalized ratio values, no statistically significant difference exists in values between the first and second tubes filled regardless of reagent sensitivity or citrate concentration. We measured APTTs using two citrate concentrations and a factor-sensitive APTT reagent. Similarly, APTT results showed no significant difference between tubes.