Measurement of prothrombin time and activated partial thromboplastin time in citrated whole blood samples from clinically ill dogs following storage

Objectives To assess the reliability of prothrombin time and activated partial thromboplastin time results generated from citrated whole blood samples following short-term storage at room temperature. Methods Clotting times were measured in blood samples from 40 dogs that showed a variety of clinical signs. Before measurement of prothrombin time and activated partial thromboplastin time in citrated plasma, whole blood samples were split in three aliquots; one was processed within 30 minutes of collection (fresh) while the remaining two were stored unseparated at room temperature for 24 (24RT) or 48 (48RT) hours. Results The median prothrombin time for the 24RT (7 seconds) and 48RT (7.2 seconds) samples were not significantly different to those obtained from the fresh (7.1 seconds) samples but the median activated partial thromboplastin time for the 24RT (12.6 seconds) and 48RT (12 seconds) samples were significantly shorter than those obtained from the fresh samples (14.2 seconds). Clinical Significance Storage of citrated whole blood at room temperature for 24 or 48 hours did not significantly alter the measurement of prothrombin time but resulted in significantly shorter activated partial thromboplastin time results. Extrapolating from these findings, it is proposed that unseparated clinical samples that are submitted to an external diagnostic laboratory for the performance of clotting times, may generate reliable prothrombin time but unreliable activated partial thromboplastin time results.