Laboratory monitoring is widely recommended to measure the anticoagulant effect of unfractionated heparin and to adjust the dose to maintain levels in the target therapeutic range.The most widely used laboratory assay for monitoring unfractionated heparin therapy is the activated partial thromboplastin time (aPTT).nfractionated heparin dose appears to be more important than the aPTT in predicting clinical efficacy. Despite serious limitations, the reliance on the aPTT is likely to continue because of its ready availability and familiarity of clinicians with the test.