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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 hospital’s 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.
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