Heparin monitoring

aPTT, obtain before initiation of heparin, every six hours thereafter until stable, then daily.

•	A patient is considered stabilized when two aPTT's drawn 24 hours apart are within the therapeutic range.

•	When the aPTT is checked at 6 hours or longer, steady-state kinetics can be assumed.

•	Heparin can cause a severe and potentially fatal form of thrombocytopenia commonly referred to as HIT. This complication is of an immune origin and usually occurs after 7 to 14 days of heparin therapy.

•	Avoid the use of aspirin and aspirin containing drugs during heparin therapy. Aspirin inhibits platelet function. Current evidence indicates that aspirin can increase the risk of bleeding in patients anticoagulated with heparin.

The recommended therapeutic range for the treatment of venous thrombosis is based on studies which demonstrated that thrombus extension is prevented by a heparin dose that prolonged the aPTT ratio to 1.5 to 2.5, which corresponds to a heparin level of 0.2 to 0.4 units/ml.

The "control" value used to determine the ratio is poorly defined, it may be the mean value of the normal range for the aPTT, or the patient's baseline aPTT (the aPTT measured before any heparin). Use of the patient's baseline as the control has gained popularity based on the fact that many patients undergoing thrombotic episodes have shortened aPTTs.

Unfortunately, the different commercial aPTT reagents vary in their responsiveness to heparin. An approximation of the therapeutic range of 0.2 to 0.4 units/ml can be made by testing the aPTT reagent in a plasma system that has been calibrated by addition of a range of clinically relevant concentrations of heparin. This heparin response curve can then be entered into the Kinetics program and used as the basis for dosage adjustments (highly recommended!).

The effect of warfarin on the aPTT is not well documented, although it appears that the aPTT is increased following initiation of warfarin and this effect may be cumulative with successive doses. Some patients may be at risk of over-coagulation when heparin and warfarin are combined.

Many clinicians reduce heparin dosage when warfarin is initiated in anticipation of an increase in the aPTT.