Effect of Drawing a Discard Tube on PT and APTT Results in Healthy Adults

Abstract

OBJECTIVE: The purpose of this study was to compare results obtained for the prothrombin time (PT) and the activated partial thromboplastin time (APTT) using specimens drawn with and without a discard tube in healthy adults.

DESIGN: A specimen of blood in a 3.2% sodium citrate, 5.0 mL tube was drawn from one arm with a discard tube and from the other arm without a discard tube on 35 healthy adults. A PT and APTT were performed on each specimen using a fibrometer.

SETTING: The PT and APTT tests were all performed in the student laboratory of the Clinical Laboratory Science Program at East Carolina University.

PATIENTS OR OTHER PARTICIPANTS: Study subjects included technician, technologist, and phlebotomist employees of the clinical laboratory at Pitt County Memorial Hospital, and faculty and junior and senior students in the Clinical Laboratory Science Program at East Carolina University, Greenville, North Carolina. All participants signed an informed consent prior to venipuncture.

INTERVENTIONS: Two coagulation specimens were drawn from each subject. Specimens in a discard (no anticoagulant) tube and then an anticoagulated tube were drawn from one arm, and a specimen in a single anticoagulant tube was drawn from the other arm. The PT and APTT were performed using standardized procedures with Pacific Hemostasis reagents and controls and a BBL FibroSystem fibrometer.

MAIN OUTCOME MEASURES: The degree of difference between PT results and between APTT results for specimens drawn with and without a discard tube.

RESULTS: Based on paired t-test analyses, no difference in mean PT results and no difference in mean APTT results were found between specimens drawn with a discard tube and those drawn without a discard tube at an alpha of 0.05. Paired-samples correlation coefficients were significant for both the PT and the APTT at an alpha of 0.05, showing precision between results with and without a discard tube for both coagulation tests.

CONCLUSIONS: Relative to sampling from a population of healthy adults, drawing a discard tube before a sodium citrate tube for coagulation testing appears to make an insignificant difference. Replication of these results with patients receiving anticoagulant therapy and/or patients with abnormal coagulation results, would offer cost savings by justifying elimination of discard tubes for blood draws for coagulation testing only. Such a change in protocol would also reduce the likelihood of nosocomial blood loss in vulnerable patient populations.

ABBREVIATIONS USED: APTT = activated partial thromboplastin time; NCCLS = National Committee for Clinical Laboratory Standards; PT = prothrombin time.