American Board of Family Medicine (ABFM) Practice Exam

What is the main distinction between Partial Thromboplastin Time (PTT) and Prothrombin Time (PT)?

1. PTT measures intrinsic pathway clotting, while PT measures extrinsic pathway

2. PTT is used primarily for liver function, while PT is for heart health

3. PTT measures vitamin K levels, while PT measures platelet count

4. PTT is performed only in emergency situations, while PT is routine

The correct answer is: PTT measures intrinsic pathway clotting, while PT measures extrinsic pathway

The main distinction between Partial Thromboplastin Time (PTT) and Prothrombin Time (PT) lies in the specific clotting pathways they assess. PTT evaluates the intrinsic and common clotting pathways, which involve factors related to blood clot formation that are activated by internal events affecting the vascular system. This includes factors such as VIII, IX, XI, and XII, among others. On the other hand, PT measures the extrinsic pathway of coagulation, which is triggered by external mechanisms, primarily involving tissue factor and factor VII. This test helps assess the function of factors that are dependent on vitamin K, including II, VII, IX, and X. Understanding this distinction is crucial in clinical practice; it allows healthcare providers to diagnose coagulation disorders or monitor anticoagulant therapy, as the PTT is often used to manage patients on heparin, while PT is commonly used to monitor warfarin therapy. The other options do not accurately reflect the primary distinction between PTT and PT, as they either incorrectly attribute functions to the tests or conflate their purposes without addressing the core difference in the pathways they evaluate.