Monitoring heparin with the aPTT

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Hemostasis index

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Heparin was discovered in 1916 by a second year medical student working in a research lab at Johns Hopkins. Subsequent study found high concentrations of the substance in liver-- thus it was called heparin. Heparin is produced in mast cells.

Administered as an anticoagulant, heparin parallels the function of the proteoglycan heparan sulfate, which is found on vessel endothelium. It activates Antithrombin III (AT III), which becomes a potent inhibitor of clotting factors.

Two approaches and two target ranges.
1. Heparin is typically monitored using the activated partial thromboplastin time (aPTT)-- measuring the anticoagulant effect of prolongation. Text references refer to a therapeutic aPTT target of 1.5- 2.0 times the mean of the reference range. Applying that guideline in our aPTT system, using the current mean of our reference range, we yield a target aPTT range of 42-56 sec.

2. An alternative approach is to use a heparin response table. Studies in our lab, adding the hospitalís heparin to normal plasma and performing the aPTT, yield the data given in the table.

Using this data you can determine the aPTT target for your therapeutic goal. For instance, if your therapeutic goal is to maintain heparin between 0.2 and 0.4 IU/ml., your aPTT target would be 50- 86 seconds. This target range might change in your practice at other hospitals. Heparin response data varies by institution because different products are used.

The bottom line?
There is a significant difference in target ranges between the two methods discussed. You should consult with your staff for target ranges on individual patients. We offer this data as a guideline for the design of your therapeutic strategy. Heparin monitoring is complex and depends both on the activity of the heparin and the sensitivity of the aPTT system (reagent and instrument). MCL Pathology offers consultation on difficult cases.


© 2004 Medical Center of Louisiana
Page updated: 9/24/04