Clotting Times (aPTT and PT)

• Published in: Interpretation of Equine Laboratory Diagnostics pp. 139 - 140
• Main Authors: Ness, SallyAnne L, Brooks, Marjory B
• Format: Book Chapter
• Language: English
• Published: Hoboken, NJ, USA John Wiley & Sons, Inc 07.12.2017
• Subjects: activated clotting time; activated partial thromboplastin time; clot formation; coagulation screening tests; hemostasis; horse
• ISBN: 9781118739792; 1118739795
• DOI: 10.1002/9781118922798.ch20

Coagulation screening tests in horse are traditionally categorized into tests of primary hemostasis (platelet plug formation), secondary hemostasis (fibrin clot formation), or fibrinolysis (clot breakdown). Activated partial thromboplastin time (aPTT) evaluates the intrinsic and common pathways of secondary hemostasis. It is diagnostically similar to activated clotting time (ACT) but unlike the ACT, is less influenced by platelet abnormalities or hematocrit. Blood for aPTT and PT should be collected via minimally traumatic venipuncture into tubes containing sodium citrate (3.2 or 3.8%) anticoagulant (blue top tube). Both aPPT and PT are used as screening tests to identify abnormalities in the intrinsic, extrinsic, and/or common coagulation pathways. Both tests involve the addition of specific coagulation‐activating reagents to citrated plasma and subsequent measurement of the time to clot formation. APTT and PT values that exceed 120% of the upper end of normal reference range are considered abnormal, however, different reagents and assay configurations influence sensitivity to detect mild factor deficiencies.