Alternative Monitoring for Antithrombotic Agents at UWMC

Heparin Activity (Anti factor Xa level)

Used to monitor low molecular weight heparins; or instead of aPTT in patients with antiphospholipid antibodies that might interfere with aPTT, or in patients with presumed heparin resistance as measured by aPTT.

  • Therapeutic range for low molecular weight heparins (LMWH):
    • Peak for twice daily SQ dosing: 0.5-1 units/mL (3-4 hours after dose)
    • Peak for once daily SQ dosing: 1-1.5 units/mL (3-4 hours after dose)
    • Trough: < 0.5 units/mL
  • Therapeutic range for IV unfractionated heparin: 0.3-0.7 units/mL
  • Test order code: HEPACT
  • Specimen collection: 5mL blue top

HEPARIN DOSING AND MONITORING USING HEPARIN ACTIVITY (ANTI-XA) LEVEL

The UWMC nurse-managed protocol for UFH is based on aPTT monitoring. The UWMC protocol uses weight-based initiation, followed by dosing adjustments in units/hr.

In patient cases in which aPTT is suspected of being invalid and in which LMWH use as an alternative to UFH is contraindicated, heparin can be monitored using anti-factor Xa levels (referred to by UWMedicine lab services at Heparin Activity Level)

Suggested Dosing Nomogram (* note that dose adjustments are in units/kg/hr)

NOTE: When this nomogram is used to monitor heparin, all infusion adjustments must be written:

  1. As a PHYSICIAN order ( this is NOT a nurse-managed protocol)
  2. In units/hr, with the PHYSICIAN to perform all dose calculations to convert from units/kg/hr to units/hr
Heparin Activity (AntiXa) Level Heparin Bolus Infusion Adjustment Next Heparin Activity Level
< 0.20 units/ml 26 units/kg* Increase by 4 units/kg/hr* 6 hours after dose adjustment
0.20 – 0.29 units/ml none Increase by 2 units/kg/hr* 6 hours after dose adjustment
0.30 – 0.70 units/ml none No change qAM
0.71 – 0.80 units/ml none Decrease by 1 unit/kg/hr* 6 hours after dose adjustment
0.81 – 0.99 units/ml none Decrease by 2 units/kg/hr* 6 hours after dose adjustment
>= 1.0 units/ml none Stop infusion for 1 hour, then decrease by 3 units/kg/hr* 6 hours after dose adjustment

Smith ML, Wheeler KE. Weight-based heparin protocol using antifactor Xa monitoring. Am J Health-Syst Pharm 2010; 167:371-4.

Order Writing

If a physician wishes to use this monitoring method, he/she should:

  1. DC all orders for the UWMC nurse-managed heparin protocol
  2. Check patient weight in ORCA and perform all dose calculations (from units/kg/hr to units/hr)
  3. Write orders for each infusion rate change in units/hr (converted from the nomogram’s units/kg/hr adjustments) in the patient’s order book

Example heparin order

~ increase heparin to 1500 units/hr
~ check heparin activity (antiXa) level 6 hours after infusion rate change
~ page MD when results are available

 

Direct Thrombin Inhibitor Assay

Used instead of aPTT to assess DTI therapy. Preferred over aPTT due to better sensitivity, and is not affected by antiphospholipid antibodies. Cost, turn-around time and 24/7 availablity at UWMC are similar to aPTT.

  • Therapeutic range
    • For argatroban: 60-100 seconds
    • For bivalirudin: 60-90 seconds
    • For lepirudin: 90-160 seconds
    • For dabigatran: 50-90 seconds*
      *NOT INTENDED FOR ADJUSTMENT of the fixed dosing (150mg bid for Clcr>30ml/min; 75mg bid for Clcr 15-30ml/min) recommended for stroke prevention in AF, but can be used:
      1. To assess compliance
      2. to evaluate possible underanticoagulation in treatment failures
      3. to evaluate possible overanticoagulation in cases of hemorrhage
      4. to assure appropriate dabigatran clearance prior to invasive procedures
      5. to assure appropriate dabigatran clearance prior to thrombolytic therapy for acute ischemic stroke
  • Test order code: DTI
  • Specimen collection: 3mL or 5mL blue top

 

Love JE, Ferrill C and Chandler WL: Monitoring direct thrombin inhibitors with a plasma diluted thrombin time. Thromb Haemost. 2007 Jul;98(1):234-42.

Chromogenic Factor X

Used instead of INR in patients with antiphospholipid antibodies that might interfere with INR, or in patients on concurrent direct thrombin inhibitors that might interfere with INR.

  • Therapeutic range for warfarin: 45-25% = INR 2-4
  • Test order code: CHRF10
  • Specimen collection: 3mL or 5mL light top tube (citrate)