Heparin sliding scale

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Unfractionated heparin can be administered intravenously as a sliding scale. A bolus dose is often given and the subsequent amounts of heparin is determined by the APTT. The time taken for a change in heparin dose can be a few hours, so APTT levels are often checked at least 6 hours after a change. The theory and knowledge base is complex, for example heparin has non linear kinetics[1] but in practice safe heparin infusion is possible with frequent enough monitoring and correct size adjustments.




  • Situations where:
    • Quick reversal required (eg just turning off the infusion pump or possibility of using protamine for severe bleed)
    • LMWH or other alternatives have safety/unavailability issues that mitigate against the complexity of heparin infusion monitoring


  • Usually made up to 1000 units/ml
  • e.g. 24000 units made up into 24ml of 0.9% saline

Prescribing & Monitoring

Textbooks often state 5000 units stat, followed by 1000 units/hour with titrations according to table below in simple APPT adjustment. This actually may lead to underdosing in average adult who is likely to require about 30000 units/24 hours, but was used historically until the 1990s and is certainly better than nothing if the indication exists and is easy to remember.

Simple APPT adjustment
APTT ratio Adjustment
1.5 - 2.5 unchanged
2.5 - 3.0 reduce by 50 units/hour
3.0 - 4.0 reduce by 100 units/hour
4.0 - 5.0 reduce by 300 units/hour
5.0 - 7.0 reduce by 500 units/hour

More complex regimes were then developed and are probably best used eg:

  • IV Bolus 5000 units then
  • IV Infusion 30,000 units/24 hours
    • Patients below 50kg and above 70kg in weight – start at 500 units/kg/24hrs
    • Usual minimum dose 20,000 units/24hrs
    • Usual maximum dose 50,000 units/24hrs

Check APTT daily and adjust heparin to achieve a ratio of between 1.5 – 2.5.

Complex APPT adjustment without usually having to weigh
APTT ratio Adjustment
>5.0 stop for 1 hour THEN decrease by 12000 units/24hrs- recheck APTT in 2 – 6 hours
4.1 – 5.0 decrease by 8000 units/24hrs - recheck APTT in 2 – 6 hours
3.1 – 4.0 decrease by 4000 units/24hrs - recheck APTT next day
2.6 – 3.0 decrease by 2000 units/24hrs - recheck APTT next day
1.5 – 2.5 no change - recheck APTT next day
1.2 – 1.4 increase by 6000 units/24hrs - recheck APTT next day
<1.2 increase by 10000 units/24hrs - recheck APTT in 2 – 6 hours

It is possibly even more preferable to do it by weight: bolus of 80 units/kg followed by 18 units/kg/hour maintenance [2] with adjustments as per first table). Even better is it to do it by volume of distribution. See for example http://www.rxkinetics.com/heparin.html

Whatever in practice many only got it right just as oral anticoagulation was achieved as monitoring blood specimens were not done frequently enough. Hence the popularity of LMWH in most indications. This also does nor require a weekly monitoring of platelet count.

Introduction of warfarin

Heparin should overlap with warfarin for at least 3 days once the INR is in the therapeutic range. Some, knowing that the APPT will increase on warfarin reduce the dose of UFH before they get the next daily APPT.