INR and aPTT Goals for Pulmonary Embolism Anticoagulation
For patients with pulmonary embolism, the therapeutic INR target is 2.0-3.0 (target 2.5), and the aPTT target is 1.5-2.5 times control (corresponding to 45-75 seconds), which correlates with an anti-Xa level of 0.3-0.7 IU/mL. 1, 2, 3
Initial Heparin Therapy and aPTT Targets
Weight-based unfractionated heparin (UFH) should be initiated with an 80 IU/kg IV bolus followed by 18 IU/kg/hour continuous infusion, targeting an aPTT of 1.5-2.5 times control. 3, 1 This aPTT range corresponds to 45-75 seconds depending on laboratory calibration 1, 3 and correlates with an anti-Xa activity of 0.3-0.6 IU/mL 1.
- The first aPTT should be measured 4-6 hours after initiating the heparin infusion 1
- After any dose adjustment, recheck aPTT in 6-10 hours 1
- Once therapeutic, monitor aPTT daily 1
Alternative Monitoring: Anti-Xa Assay
Anti-Xa monitoring (target 0.3-0.7 IU/mL) is increasingly preferred over aPTT as it provides more accurate and reproducible assessment of heparin's anticoagulant effect. 4 This is particularly important because:
- Recent data shows that 93% of patients achieve sustained therapeutic anticoagulation at 48 hours using anti-Xa guided protocols 4
- In contrast, aPTT-guided protocols result in only 28% of patients achieving therapeutic range at 48 hours, with no patient maintaining all therapeutic aPTT values 5
- Anti-Xa monitoring should be considered in patients with aPTT resistance or when aPTT results are inconsistent 1
Transition to Warfarin and INR Targets
Warfarin should be started simultaneously with heparin on day 1 at a dose of 5-10 mg daily for the first 2 days, with subsequent dose adjustments to maintain INR 2.0-3.0. 1, 2, 6, 3
- Heparin must be continued for at least 5 days AND until INR is ≥2.0 for at least 24-48 hours on two consecutive measurements. 2, 6, 3 This overlap is critical to prevent a gap in anticoagulation 3
- Initial INR monitoring should occur every 1-2 days until stable in therapeutic range 1, 2
- The target INR of 2.0-3.0 (target 2.5) should be maintained throughout the entire treatment period 2, 7
Critical Pitfalls to Avoid
- Never use fixed-dose heparin without weight adjustment - this leads to delayed therapeutic anticoagulation and increased recurrence rates 3
- Never delay heparin while awaiting diagnostic confirmation in patients with intermediate or high clinical probability of PE 1, 3
- Never discontinue heparin before INR is therapeutic for 24-48 hours - this creates a dangerous anticoagulation gap 2, 3
- Never assume aPTT accurately reflects anticoagulation status - consider anti-Xa monitoring for more reliable assessment, especially given that most patients fail to achieve therapeutic aPTT within 24 hours using standard protocols 4, 5
- Each laboratory should determine its specific aPTT range that corresponds to the target anti-Xa activity, as reagent variability affects results 1