A test for prothrombin time is used to identify deficiencies in the extrinsic clotting pathway; prothrombin time is prolonged if any of the factors are deficient. A test for activated partial thromboplastin time is used in a similar fashion to detect deficiencies in the intrinsic clotting pathway. Which factor would be deficient if a person had a prolonged prothrombin time but a normal partial thromboplastin time?

Verified step by step guidance

1. Understand the difference between the extrinsic and intrinsic clotting pathways: the prothrombin time (PT) test measures the extrinsic pathway, while the activated partial thromboplastin time (aPTT) test measures the intrinsic pathway.
2. Recall which clotting factors are involved in each pathway. The extrinsic pathway primarily involves factor VII, while the intrinsic pathway involves factors VIII, IX, XI, and XII.
3. Note that a prolonged prothrombin time (PT) indicates a deficiency in the extrinsic pathway factors, whereas a normal activated partial thromboplastin time (aPTT) suggests that the intrinsic pathway factors are functioning properly.
4. Identify the factor that is unique to the extrinsic pathway and would cause a prolonged PT but normal aPTT if deficient. This factor is factor VII.
5. Conclude that a deficiency in factor VII would result in a prolonged prothrombin time with a normal activated partial thromboplastin time.

Key Concepts

Extrinsic Clotting Pathway

The extrinsic pathway is one of the two initial routes that trigger blood coagulation, primarily involving factor VII. It is activated by external trauma causing blood to escape the vascular system. Prothrombin time (PT) tests assess this pathway, and deficiencies in factors here prolong PT.

Intrinsic Clotting Pathway

The intrinsic pathway involves factors XII, XI, IX, and VIII and is activated by damage inside the blood vessel. Activated partial thromboplastin time (aPTT) tests evaluate this pathway. A normal aPTT indicates that the intrinsic pathway factors are functioning properly.

Interpretation of Prolonged PT with Normal aPTT

A prolonged PT with a normal aPTT suggests a deficiency in a factor exclusive to the extrinsic pathway, most commonly factor VII. Since the intrinsic pathway is unaffected, factors measured by aPTT remain normal, helping pinpoint the specific clotting factor involved.