Questions
1–2 questions in university exams
Difficulty
Medium
Importance
High yield for BMLT and MBBS pathology papers
Overview
Coagulation studies are essential diagnostic tests used to evaluate the body's hemostatic mechanism and blood clotting integrity. Mastering these tests, specifically PT and aPTT, is crucial for interpreting patient clinical states and is a staple in medical and laboratory science examinations.
The Coagulation Cascade
The coagulation cascade is a sequential series of enzyme activations that lead to the formation of a stable fibrin clot. It is divided into extrinsic, intrinsic, and common pathways, each requiring specific clotting factors and cofactors.
- Intrinsic pathway involves factors XII, XI, IX, and VIII
- Extrinsic pathway is triggered by Tissue Factor (Factor III)
- Common pathway starts with activation of Factor X
- Thrombin converts fibrinogen into insoluble fibrin strands
- Calcium (Factor IV) is essential for multiple steps in the cascade
Prothrombin Time (PT)
Prothrombin Time measures the integrity of the extrinsic and common pathways of the coagulation cascade. It is the primary screening tool for monitoring oral anticoagulant therapy, particularly Vitamin K antagonists like Warfarin.
- Tests factors VII, X, V, II, and fibrinogen
- Measured in seconds compared to a control
- Reported as INR (International Normalized Ratio) to standardize results
- Normal range is typically 11–13.5 seconds
- Prolonged by Vitamin K deficiency and liver disease
Activated Partial Thromboplastin Time (aPTT)
The aPTT evaluates the intrinsic and common pathways of the coagulation cascade and is the gold standard for monitoring unfractionated heparin therapy. It detects deficiencies in most coagulation factors except Factor VII.
- Tests factors XII, XI, IX, VIII, X, V, II, and fibrinogen
- Requires contact activator and phospholipid reagent
- Normal range is typically 25–35 seconds
- Prolonged by Heparin, Hemophilia A (Factor VIII), and Hemophilia B (Factor IX)
- Used to screen for Von Willebrand disease
Formula Sheet
INR = (Patient PT / Control PT) ^ ISI
Exam Tip
Always remember the mnemonic 'Table Tennis'—PT is for the extrinsic pathway (TEN, like 10-7=3) and aPTT is for the intrinsic pathway (TABLE, as in 12, 11, 9, 8).
Common Mistakes
- Confusing which pathway is assessed by PT versus aPTT in the cascade
- Forgetting that Factor VII is the only factor not included in the aPTT test
- Failing to mention the INR as the standardized reporting method for PT results
More Revision Notes
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