Heparin Drip Management When PTT Results Are Unobtainable
Continue the heparin infusion without interruption and pursue immediate troubleshooting of the laboratory issue, as stopping heparin for 2 hours to obtain a PTT result would create an unacceptable period of subtherapeutic anticoagulation that increases thrombotic risk 15-fold. 1, 2
Rationale for Continuing Heparin
Subtherapeutic anticoagulation carries severe consequences: Patients with aPTT values <50 seconds have a 15-fold increased risk of recurrent venous thromboembolism compared to those in therapeutic range. 1, 2
Heparin's pharmacokinetics do not support a 2-hour hold: Therapeutic doses of heparin are cleared through both rapid saturable mechanisms and slower renal clearance, meaning a 2-hour interruption would result in subtherapeutic levels without providing meaningful diagnostic clarity. 1
The first aPTT should be drawn 6 hours after the initial bolus: This timing allows for steady-state anticoagulation to be achieved, and subsequent measurements guide dose adjustments. 1, 2 Drawing a PTT after only a 2-hour hold would not reflect steady-state conditions anyway.
Immediate Troubleshooting Steps
Address the laboratory issue directly rather than modifying therapy:
Contact the laboratory immediately to identify why results cannot be obtained. Common causes include: 1
- Specimen clotting in the collection tube
- Insufficient sample volume
- Contamination with heparin from IV line draws
- Reagent-specific issues with the aPTT assay
Ensure proper specimen collection technique: Blood must be drawn from a site remote from the heparin infusion, with adequate discard volume if drawn from a central line. 1
Consider alternative monitoring if aPTT remains unavailable: Anti-factor Xa levels (target 0.35-0.7 U/mL) can be used when aPTT testing is problematic, particularly in critically ill patients where aPTT may not correlate well with heparin effect. 2, 3
Clinical Decision Algorithm
If PTT results remain unobtainable after troubleshooting:
Continue current heparin infusion rate if the patient was recently started on a weight-based protocol (80 U/kg bolus, then 18 U/kg/h). 2, 4
Switch to anti-factor Xa monitoring with target range 0.35-0.7 U/mL, which does not have the same specimen collection issues as aPTT. 2, 4, 3
Monitor for clinical signs of over-anticoagulation: Check for bleeding, falling hemoglobin/hematocrit, or hemodynamic changes that would suggest excessive anticoagulation. 1
Assess platelet count to rule out heparin-induced thrombocytopenia, which could be causing laboratory interference. 1
Critical Pitfalls to Avoid
Never hold therapeutic anticoagulation to obtain a laboratory value when the underlying thrombotic condition remains active. The risk of recurrent thromboembolism far exceeds the risk of continuing empiric dosing for a brief period. 1, 2
Do not assume the heparin is "working" without objective measurement: While continuing the infusion is correct, you must obtain some form of anticoagulation monitoring (aPTT or anti-Xa) within 6 hours of the bolus dose. 1, 2
Recognize that aPTT reagent variability exists: The therapeutic range must be adapted to your institution's specific reagent responsiveness, which is why establishing why results cannot be obtained is critical. 1, 4
Avoid drawing specimens from heparinized lines: This is a common cause of unobtainable results due to specimen contamination, requiring peripheral venipuncture or proper central line draw technique with adequate discard volume. 1
Alternative Monitoring Strategy
If aPTT continues to be problematic:
Transition to anti-factor Xa monitoring as the primary method, particularly in critically ill patients where elevated factor VIII and fibrinogen can cause heparin resistance and aPTT unreliability. 3
Consider LMWH as an alternative if the clinical situation allows, as it requires no routine monitoring and has more predictable pharmacokinetics. 2