ARTIFACTUAL PROLONGATION OF THE ACTIVATED PARTIAL THROMBOPLASTIN TIMEASSOCIATED WITH HEMOCONCENTRATION IN DOGS

An inappropriate blood-to-anticoagulant ratio can cause an artifactual prolongation of the activated partial thromboplastin time (APTT) and prothrombin time (PT). In a drug safety study in dogs, we observed a 4 - to 5-second increase in the APTT from baseline coincident with incre ased hematocrit values (56% to 65%) secondary to drug-induced vomiting and diarrhea, The PT and platelet counts were unchanged, and there wa s no clinical evidence of bleeding associated with venipuncture. Altho ugh we were unable to sample the same dogs to investigate the possible effect of hemoconcentration on the prolonged APTT, the question was a ddressed by an in vitro study. The hematocrit value for citrated blood samples collected from healthy beagle dogs was increased by the addit ion of aliquots of red blood cell/plasma mixtures in vitro while maint aining a 9:1 blood-to-anticoagulant ratio. There was a 2- to 4-second prolongation of the APTT associated with hematocrit values of 55% to 6 1%, but the PT was not prolonged, Adjustment of the blood-to-anticoagu lant ratio corrected the prolongation, This study emphasizes the impor tant relationship of the blood-to-anticoagulant ratio when measuring c oagulation tests in hemoconcentrated samples. Copyright (C) 1995 by th e American College of Veterinary Internal Medicine.