Representative normal ranges are around 12 to 15 seconds for the PT and 25 to 35 seconds for the aPTT but are defined by each laboratory using its own equipment, reagents, and normal specimens. Testing is routinely performed at 37°C, but hypothermia in a patient impairs the enzymatic reactions of clot formation.
Most assays measure the functional conversion of fibrinogen to fibrin, although the fibrinogen protein level also can be measured for comparison to assess fibrinogen dysfunction. Normal fibrinogen levels are around 150 to 400 mg/dL.
Mixing Studies. To investigate unexpectedly elevated PT or aPTT values, the test should be repeated after mixing the patient's plasma with equal volumes of normal plasma.
In severe factor deficiencies, PT or aPTT shows substantial correction toward normal in a mixing study.
If the patient's plasma contains an inhibitor or an anticoagulant, the normal plasma will also be affected, and PT or aPTT will not correct.
Individual factor-level activities are determined by the degree of correction that patient plasma gives when mixed with factor-deficient plasma.
Coagulation inhibitors are substances, usually antibodies, that block one or more clotting factors.
Disseminated intravascular coagulation (DIC) is unchecked coagulation initiated by pathologic systemic activation of the intrinsic clotting pathway.
Other tests that are commonly performed during surgery with whole-blood specimens are the activated clotting time (ACT), ecarin clotting time (ECT), and viscoelastic whole-blood clotting with thromboelastography.
The ACT is a point of care test that assesses the intrinsic clotting pathway and is used mainly to monitor heparin anticoagulation and its protamine reversal during cardiopulmonary bypass or vascular surgery.
Whole-blood clotting and fibrinolysis can be assessed by viscoelastic testing in thromboelastography.