Pre-instrumental variables in coagulation testing.

A number of variables thought to affect measurement of prothrombin time (PT) and partial thromboplastin time (PTT) were examined in an effort to determine more precisely their effects on these measurements. On the basis of these studies, it is proposed that blood specimens be anticoagulated with one part 3.8% (w./v.) sodium citrate solution to 19 parts whole blood to avoid excessive anticoagulation of blood samples drawn from patients with polycythemia. Because of the smaller amount of plasma in such samples, relatively larger amounts of anticoagulant are used, and spuriously prolonged PT and PTT measurements commonly result. No deleterious effect on anemic specimens is evident when the smaller amount of citrate is used. Studies of the stability of these specimens indicate that unopened, vacuum-drawn specimens do not noticeably deteriorate for as long as 6 hours, even when kept at room temperature. Prothrombin time measurements remain constant for as long as 24 hours. However, a 10--15% lengthening of the partial thromboplastin time is evident after 24 hours of storage.