How should activated partial thromboplastin time (aPTT) and prothrombin time (PT) be used in the Rashke heparin dosing protocol?

Use of aPTT and PT in the Raschke Heparin Protocol

The Raschke protocol uses aPTT exclusively for monitoring unfractionated heparin (UFH) therapy, targeting an aPTT of 1.5-2 times control (typically 46-70 seconds), while PT/INR is not used for heparin monitoring and should only be checked when transitioning to warfarin therapy. 1, 2

aPTT Monitoring in the Raschke Protocol

The Raschke protocol is a weight-based heparin dosing nomogram that relies entirely on aPTT for dose adjustments:

• Initial dosing: 80 units/kg IV bolus, followed by 18 units/kg/hour continuous infusion 1
• Target aPTT range: 46-70 seconds (corresponding to anti-Xa activity of 0.3-0.7 units/mL) 1
• Monitoring frequency:
  • Check aPTT approximately every 4 hours when initiating therapy 2
  • Continue at appropriate intervals once therapeutic range is achieved 2
  • For intermittent IV dosing, check before each injection during initiation 2

Dose Adjustment Algorithm Based on aPTT

The protocol provides specific dose adjustments based on aPTT results 1:

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

PT/INR Has No Role in Heparin Monitoring

PT should not be used to monitor heparin therapy. The evidence is clear on this separation:

• Heparin prolongs PT minimally and unpredictably 2, 3
• PT is insensitive to therapeutic heparin levels 3
• Within therapeutic aPTT ranges, PT changes are not statistically significant 3

When PT Becomes Relevant

PT/INR only becomes important during warfarin bridging when transitioning from heparin to oral anticoagulation 4, 2:

• Wait at least 5 hours after last IV heparin dose before drawing PT for warfarin monitoring 2
• Wait 24 hours after last subcutaneous heparin dose 2
• This prevents heparin interference with obtaining a valid PT/INR for warfarin dose adjustment

Critical Pitfalls to Avoid

Do not use PT to guide heparin dosing - this is a common error that can lead to inadequate anticoagulation. The Raschke protocol achieved therapeutic aPTT in 91% of low-bleeding-risk patients using aPTT-guided dosing alone 5.

Local aPTT calibration is essential - the therapeutic range of 46-70 seconds must be validated at your institution to correspond to anti-Xa levels of 0.3-0.7 units/mL 1, 6. Different aPTT reagents have varying sensitivities to heparin 7, 6.

Consider anti-Xa monitoring in heparin resistance - if patients require >35,000 units/day to achieve therapeutic aPTT, switch to anti-Xa level monitoring (target 0.35-0.7 units/mL) 1.

Additional Monitoring Beyond aPTT

While aPTT drives dosing decisions, comprehensive monitoring includes 2:

• Platelet counts (periodically throughout therapy to detect HIT)
• Hematocrit
• Occult blood in stool
• These parameters assess safety but do not guide dose adjustments