Activated Partial Thromboplastin time (aPTT)

The activated partial thromboplastin time (aPTT) is a common screening test done to evaluate function of the intrinsic clotting system.

Reference values for aPTT vary among laboratories, but generally range between 25 and 38 seconds. The aPTT of a newborn will usually be prolonged and may be up to 55 seconds at birth. It decreases to the adult range by 6 months of age. (Note: each lab has its own reference values based upon the equipment and reagents used.)

The aPTT is the most commonly used test to monitor heparin therapy. The therapeutic goal for a patient being anticoagulated with heparin, is an aPTT approximately 1.5 to 2.5 times the mean normal value. Heparin is most often administered as an initial intravenous bolus followed by a continuous intravenous infusion. The aPTT is evaluated every 6 hours during the first day of heparin therapy and 6 hours after any dosage change. If the aPTT is therapeutic, it can be checked once daily while patients are on heparin. (Note: If low molecular weight heparin is given for anticoagulation, a prolonged aPTT does not occur, so another test(s) may be indicated to monitor therapy.)

If the aPTT is increased beyond the therapeutic range, the physician may order the heparin IV flow slowed or briefly discontinued. As the half-life of heparin is quite short, these measures will often allow the aPTT to rapidly return to a therapeutic range. Protamine sulfate may also be given to block the action of heparin. In some serious situations, such as active bleeding, the physician will order a transfusion of whole blood or plasma to increase clotting factors. Surgery may be postponed in a patient who has an increased aPTT, unless it is an emergency procedure.

A prolonged aPTT in non-heparinized patients can occur due to:

A decreased aPTT can occur due to:

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The aPTT is a measurement of the intrinsic coagulation system.

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