Prothrombin Time (PT) and Activated Partial Thromboplastin Time (aPTT)
Key Points
- Basic screening tests for coagulation disorders
- PT evaluates extrinsic and common pathways
- aPTT assesses intrinsic and common pathways
- Essential for monitoring anticoagulation therapy
- Age-dependent reference ranges critical in pediatrics
Prothrombin Time (PT)
- Mechanism
- Measures time to clot formation after adding tissue factor
- Evaluates factors VII, X, V, II, and fibrinogen
- Results expressed in seconds and INR
- Technical Aspects
- Citrated plasma required
- Temperature-controlled conditions
- Standardized reagents necessary
- ISI calibration for INR calculation
Activated Partial Thromboplastin Time (aPTT)
- Mechanism
- Measures intrinsic pathway activation time
- Evaluates factors XII, XI, IX, VIII, X, V, II, and fibrinogen
- Phospholipid and contact activator dependent
- Technical Considerations
- Pre-analytical variables critical
- Sample stability limitations
- Reagent sensitivity variations
- Quality control requirements
Primary Applications
- Bleeding Risk Assessment
- Preoperative screening
- Trauma evaluation
- Bleeding disorder workup
- Anticoagulation Monitoring
- Warfarin therapy (PT/INR)
- Heparin therapy (aPTT)
- DOACs monitoring considerations
- Disease Monitoring
- Liver dysfunction
- DIC assessment
- Vitamin K deficiency
Specific Clinical Scenarios
- Emergency Situations
- Acute bleeding assessment
- Urgent surgery preparation
- Trauma management
- Chronic Conditions
- Hemophilia monitoring
- Liver disease follow-up
- Thrombotic disorders
Age-Specific Reference Ranges
- Neonatal Period (0-30 days)
- PT: 13.0-20.0 seconds
- aPTT: 31.3-54.3 seconds
- Infants (1-12 months)
- PT: 11.5-15.3 seconds
- aPTT: 28.1-42.9 seconds
- Children (1-16 years)
- PT: 11.0-14.5 seconds
- aPTT: 26.0-36.0 seconds
Developmental Hemostasis
- Physiological Changes
- Maturation of coagulation system
- Age-related factor levels
- Hormonal influences
- Clinical Impact
- Interpretation adjustments
- Treatment modifications
- Monitoring frequency
Pattern Recognition
- Isolated PT Prolongation
- Factor VII deficiency
- Early vitamin K deficiency
- Warfarin effect
- Isolated aPTT Prolongation
- Factor VIII, IX, XI, XII deficiencies
- von Willebrand Disease
- Lupus anticoagulant
- Heparin therapy
- Combined Prolongation
- Multiple factor deficiencies
- Vitamin K deficiency
- Liver disease
- DIC
Critical Actions
- Result Verification
- Sample quality check
- Repeat testing protocols
- Mixing study indications
- Further Investigation
- Factor assays
- Specialized coagulation tests
- Genetic testing considerations
Common Pitfalls
- Pre-analytical Errors
- Incorrect blood-to-citrate ratio
- Sample activation
- Storage conditions
- Interpretation Errors
- Age-specific range oversight
- Missing medication effects
- Incomplete clinical correlation
Further Reading
