What is the cut off for Activated Partial Thromboplastin Time (APTT) in the context of heparin therapy?

APTT Therapeutic Range for Heparin Therapy

The therapeutic APTT range for unfractionated heparin should be 60-85 seconds (or 1.5-2.5 times the control value), corresponding to a plasma heparin level of 0.2-0.4 U/mL by protamine titration or 0.35-0.7 U/mL by anti-factor Xa assay. 1, 2

Critical Monitoring Parameters

Initial monitoring:

• Measure aPTT 6 hours after the initial heparin bolus dose 1, 2
• Continue monitoring aPTT approximately every 4 hours during initiation, then at appropriate intervals once therapeutic 2
• For intermittent IV injection, perform coagulation tests before each injection during initiation 2

Pediatric targets:

• Target aPTT of 60-85 seconds in children, reflecting an anti-factor Xa level of 0.35-0.70 U/mL 1, 2

Dose Adjustment Protocol

Follow this weight-based nomogram 1:

• Initial dose: 80 units/kg bolus, then 18 units/kg/h continuous infusion 1

Adjustments based on aPTT results:

• aPTT <35 seconds: Give 80 units/kg bolus, then increase infusion by 4 units/kg/h 1
• aPTT 35-45 seconds: Give 40 units/kg bolus, then increase infusion by 2 units/kg/h 1
• aPTT 46-70 seconds: No change needed 1
• aPTT 71-90 seconds: Decrease infusion rate by 2 units/kg/h 1
• aPTT >90 seconds: Hold infusion for 1 hour, then decrease infusion rate by 3 units/kg/h 1

Critical Pitfalls to Avoid

Reagent variability is the most important consideration:

• Different aPTT reagents have dramatically different sensitivities to heparin, with therapeutic ranges varying from 1.6-1.9 (least responsive reagent) to 2.2-2.9 (most responsive reagent) for the same heparin concentration 1, 3
• Your institution must establish its own therapeutic range based on the specific aPTT reagent and coagulometer used 1
• A fixed ratio of 1.5-2.5 times control may result in significant underanticoagulation with some reagents 4, 5

Clinical consequences of subtherapeutic anticoagulation:

• Patients with aPTT <50 seconds have a 15-fold increased risk of recurrent venous thromboembolism 1, 6
• Subtherapeutic aPTT values (50-59 seconds) still carry increased thrombotic risk 1

Excessive anticoagulation risks:

• aPTT >90 seconds increases bleeding risk without additional antithrombotic benefit 1, 6
• Monitor platelet counts, hematocrit, and occult blood in stool throughout therapy regardless of route 2

Special Circumstances

Heparin resistance:

• When patients require unusually high doses (≥35,000 units/day) to achieve therapeutic aPTT, consider switching to anti-factor Xa monitoring with target range 0.35-0.7 units/mL 1
• Heparin resistance may result from AT deficiency, increased heparin clearance, or elevated factor VIII/fibrinogen levels 1

Lower doses for acute coronary syndromes:

• For unstable angina/NSTEMI: 60-70 units/kg bolus (max 5,000 units), then 12-15 units/kg/h (max 1,000 units/h) 1
• For STEMI with fibrinolytics: 60 units/kg bolus (max 4,000 units), then 12 units/kg/h (max 1,000 units/h) 1

Combination therapy adjustments:

• Dosing regimens must be modified when heparin is combined with thrombolytic therapy or platelet GP IIb/IIIa antagonists 1, 6