Reference Values for Coagulation Time of Dogs in Porto Alegre, RS, Brazil

Background: Prothrombin time (PT) and activated partial thromboplastin time (PTT) are screening tests for coagulopathies in dogs. Both tests measure the clotting ability by the activation of different parts of the coagulation cascade. These tests vary widely in terms of reference parameters, mainly due the considerable diversity of reagents and analyzers available. In addition, there are many variations inherent to different populations, and little has been published about coagulation reference parameters for the local dog population. The main objective of the present study was to determine a clotting time reference range of a dog population in Porto Alegre, Rio Grande do Sul, Brazil. Materials, Methods & Results: Hemostatic reference range was determined from citrated plasma of 268 clinically heathy dogs of both genders. The animals did not present bleeding diathesis or thrombocytopenia history. All dogs were previously submitted to clinical examination (cardiopulmonary auscultation, abdominal palpation, and rectal temperature) and laboratory screening (complete blood count, creatinine, albumin, and alanine aminotransferase). PT and PTT of 71 and 258 samples were measured, respectively. Blood samples were collected into 2.7 mL 3.2 % sodium citrate tubes (9 parts blood : 1 part citrate) by vacuum. Blood samples were centrifuged; the plasma was harvested and stored at -30 degrees C upon analyses. All analyses were performed using the viscosity detection method in semi-automatic coagulometer according to manufacturer's guidelines. The reference ranges were determined in accordance with the American Society for Veterinary Clinical Pathology. The PT and PTT reference ranges were between 6.06 to 9.32 s and between 15.25 to 24.57 s, respectively. Discussion: The increased activity of the extrinsic pathway clotting factor generates narrow values in the PT assay, in comparison to PTT results. PT results were similar to previous reports, which may be attributed to minor variations that exist between the PT reagent kits. All PT kits are comprised of tissue thromboplastin and Ca2+. Basically, there are three sources of thromboplastin for the PT reagents: recombinant tissue factor, tissue thromboplastin, and combined thromboplastin. This narrow range of components make TP results vary little, irrespective of the brand reagent. On the other hand, PTT results were broader and differed from previous reports. PTT reagents are more diverse and less standardized than PT reagents. It is the combination of the clotting activator, phospholipid, calcium chloride molarity, ionic strength, buffers, and stabilizer system that determines the properties of the reagent. The clotting activators most commonly used are ellagic acid, kaolin, micronized silica, and celite. These compounds have different properties, and are all used in different PTT reagents as clotting activators. Phospholipids are platelet substitutes and accelerate the reactions, and may also come from different sources. Furthermore, no reference range for PTT using reagents based on micronized silica as the clotting activator is available for dogs, which may explain the different reference range obtained for this parameter. Moreover, the higher PTT values may be a characteristic of this specific dog population. The PT and PTT values obtained in this study can be used as reference values in local laboratories based on the same methodology.