Prothrombin Time Screening Test
The correct answer is E (A and C): The Prothrombin Time screens for Factor X, Factor V, Factor II (prothrombin), Fibrinogen (option A) AND Factor VII (option C). 1, 2, 3
Factors Assessed by PT
The PT evaluates the extrinsic and common pathways of the coagulation cascade, specifically measuring the activity of:
- Factor VII (extrinsic pathway - half-life 6 hours) 1
- Factor X (common pathway) 1, 3
- Factor V (common pathway) 1, 3
- Factor II/Prothrombin (common pathway - half-life 60-72 hours) 1, 3
- Factor I/Fibrinogen (common pathway) 1, 3
Why Option E is Correct
The PT responds to reduction of 3 of the 4 vitamin K-dependent procoagulant clotting factors (II, VII, and X) that are reduced by warfarin at rates proportionate to their respective half-lives. 1 During the first few days of warfarin therapy, the PT reflects mainly reduction of Factor VII due to its short 6-hour half-life, with subsequent contributions from Factors X and II. 1
Factor V and Fibrinogen, while not vitamin K-dependent, are also assessed by the PT as they participate in the common pathway. 3, 4
Why Other Options are Incorrect
Option B factors (FXII, FXI, FIX, FVIII) are NOT assessed by PT - these belong to the intrinsic pathway and are evaluated by the activated partial thromboplastin time (aPTT), not PT. 1, 3 The aPTT evaluates the intrinsic and common pathways, measuring factors XII, XI, IX, VIII, X, V, II, and I. 3
Clinical Mechanism
The PT assay is performed by adding calcium and thromboplastin (tissue factor) to citrated plasma. 1 The thromboplastin contains both tissue factor and phospholipid necessary to promote activation of Factor X by Factor VII, initiating the extrinsic pathway. 1 This bypasses the intrinsic pathway factors entirely, which is why FXII, FXI, FIX, and FVIII do not affect PT results. 3
Diagnostic Algorithm
When PT is prolonged:
- Measure Factors II, V, VII, and X to identify the specific deficiency 2
- Perform 50:50 mixing study to differentiate factor deficiency from inhibitor 2
- If mixing study corrects, proceed with specific factor assays 2
- Isolated Factor VII deficiency causes isolated PT prolongation without aPTT abnormality 5