you are running at prothrombin time (pt) and activated partial thromboplastin time (aptt) for pre-op testing that came from an outpatient clinic. you get an alarm from your analyzer indicating that there is no endpoint clotting time. the patient has no history of bleeding problems and is on no medication that would cause excessive long coagulation times. the specimen was in a 3.2% sodium citrate tube that was received into the lab 20 minutes after it was collected and spun immediately upon arrival. what may be the cause for the failure to clot?