Annotations Initiation, Maintenance and Duration of Therapy

This guideline is not intended for patients on ECMO, cardiac bypass pump, hemodialysis or continuous renal replacement therapy. Refer to specific guideline for these situations.


  • Obtain blood for CBC, PT, PTT.
  • Loading heparin dose: 75 units/kg IV over 10 minutes.
  • Calculate initial maintenance heparin dose based on age and weight.

Initial maintenance dose:
 ≤ 1 year 28 units/kg/hour IV
> 1 year 20 units/kg/hour IV

  • Obtain blood for PTT and/or heparin level 4 hours after administration of the loading dose (no earlier).
  • Adjust heparin to maintain the PTT at 70-100 sec (equivalent to a heparin level of 0.35 to 0.7) using Table 1.
  • Obtain blood for PTT and/or heparin level 4 hours after every change in infusion rate.
  • Use heparin level to calibrate PTT levels. If PTT is highly variable, heparin level may provide a more reliable measure.

Table 1 - Heparin Adjustment




Hold infusion

Rate change

Repeat PTT

< 60




4 hours




↑ 10%

4 hours




No Change

24 hours





4 hours





4 hours

> 120




4 hours

Maintenance and monitoring

  • Once a therapeutic PTT or heparin level is achieved obtain blood for CBC, PT, PTT and/or heparin level daily.
  • Once PTT is in therapeutic range draw a heparin  level in combination with the PTT. Adjust heparin dose to achieve a heparin level of 0.35-0.7 units/ml. Use this heparin level to calibrate PTT levels for the patient.
  • Measure platelet counts daily. If platelet count decreases below 150,000/microL or drops by ≥ 50% determine if the decrease in platelet count is related to the underlying disorder or is potentially due to heparin therapy. If likely due to heparin, discontinue heparin; initiate an alternative therapy and consult hematology. The risk for heparin-induced thrombocytopenia (HIT) is greater after 5 days of heparin.


Duration of therapy

  • The duration of heparin therapy is dependent upon the primary problem.
    • General recommendations:



 Duration of heparin or LMW heparin prior to warfarin


3-7 day minimum

Extensive DVT or large PE

7-14 days

  • Start Warfarin using above duration guideline when clinical situation allows.
  • May convert to LMW heparin/enoxaparin when clinical situation allows.
  • Consult Hematology on day 1 or 2 of heparin therapy to assist with this decision for infants < 30 days and all patients not in critical care units or on cardiology / cardiothoracic surgery service.



These guidelines do not establish a standard of care to be followed in every case. It is recognized that each case is different and those individuals involved in providing health care are expected to use their judgment in determining what is in the best interests of the patient based on the circumstances existing at the time. It is impossible to anticipate all possible situations that may exist and to prepare guidelines for each. Accordingly these guidelines should guide care with the understanding that departures from them may be required at times.

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