Which tests comprise complete coagulation testing referenced in veterinary medicine?

• A. Platelet count and fibrinogen
• B. D-dimer and fibrinogen degradation products
• C. Prothrombin time (PT) and activated partial thromboplastin time (aPTT)
• D. International normalized ratio (INR) and bleeding time

Answer: (C)

Coagulation testing is fundamentally about assessing the two main pathways that generate a clot: the extrinsic pathway and the intrinsic pathway, along with the common pathway that they share. The best way to screen both pathways together is to measure prothrombin time (PT), which evaluates the extrinsic pathway and the common pathway, and activated partial thromboplastin time (aPTT), which evaluates the intrinsic pathway and the common pathway. Using PT and aPTT together provides a broad view of the clotting cascade and helps detect most common coagulation disorders seen in veterinary patients, such as liver disease affecting clotting factor production, vitamin K–dependent factor deficiencies, or disseminated intravascular coagulation.

Other options don’t provide the same comprehensive view. D-dimer and fibrin degradation products reflect clot breakdown rather than formation and are useful for diagnosing ongoing thrombosis or DIC but don’t measure the clotting factors themselves. Platelet count and fibrinogen give information about platelets and a general inflammatory/clotting state, not the functional status of the coagulation cascade. INR is a standardized version of PT but, on its own, doesn’t add new information beyond PT, and bleeding time is an outdated, unreliable measure of platelet function.