Deficiencies of each of the following coagulation factors would cause an abnormal Prothrombin Time (PT) except
A Factor V
B Factor X
C Factor VII
D Factor IX
The correct answer and explanation is:
The correct answer is: D Factor IX
Explanation:
Prothrombin Time (PT) is a blood test that measures how long it takes blood to clot via the extrinsic and common coagulation pathways. It primarily evaluates the function of clotting factors involved in these pathways.
- Extrinsic pathway factors: Mainly Factor VII
- Common pathway factors: Factors X, V, II (prothrombin), and fibrinogen (Factor I)
PT is prolonged (abnormal) if there is a deficiency or dysfunction in any of these factors.
Why Factors Affect PT or Not:
- Factor V: Part of the common pathway. Deficiency prolongs PT.
- Factor X: Part of the common pathway. Deficiency prolongs PT.
- Factor VII: Part of the extrinsic pathway. Deficiency prolongs PT.
- Factor IX: Part of the intrinsic pathway, which is evaluated by the Activated Partial Thromboplastin Time (aPTT), not PT. Therefore, deficiency in Factor IX does not affect PT but instead prolongs aPTT.
Summary:
- PT tests the extrinsic and common pathways: Factors VII, X, V, II, and fibrinogen.
- Deficiency in these factors prolongs PT.
- Factor IX belongs to the intrinsic pathway, so its deficiency does not prolong PT but prolongs aPTT instead.
Additional Notes:
- PT test clinical use: Often used to monitor warfarin therapy and assess liver function or vitamin K deficiency.
- aPTT test: Used to assess intrinsic and common pathways, including Factors VIII, IX, XI, XII.
- Understanding which factors prolong PT or aPTT is key in diagnosing different bleeding disorders.