The PTT assesses the coagulation activity of the intrinsic and common pathways of coagulation. It is the best screening test for the diagnosis of disorders of coagulation that do not involve factor VII (extrinsic pathway) or platelet function. The conventional prefix "activated " � is obsolete; there is no nonactivated PTT in use. The "activation " � reflects a technical aspect of the assay because the reagents contain a negatively charged surface that accelerates the rate of the reaction.
Normal range: 22.3 " �34.0 seconds (varies slightly from lot to lot of reagent, type of the commercial reagent used, and equipment).
Use
Screening for hemophilia A and B and other possible coagulopathies (except factors VII and XIII). PTT is not affected by single clotting factor defects above 40% of normal.
Detection of clotting inhibitors: This is best performed by mixing studies once an otherwise unexplained prolonged PTT is found. Mixing equal parts of patient and normal plasma (1:1) for 1 " �2 hours at 37 � �C normalizes the prolonged PTT if it is caused by a coagulation factor deficiency but not if it is caused by an inhibitor. The inhibitor is commonly a factor VIII inhibitor; LA can also prolong the PTT if LA-sensitive reagents are used.
Monitoring of therapy with unfractionated heparin. It is not useful in monitoring low molecular weight heparins or fondaparinux; these anticoagulants can be monitored with anti-Xa assay.
Not recommended for preoperative screening in patients without a personal or immediate family history of unprovoked bleeding.
Interpretation
Increased (>36 seconds) In
Single clotting factor deficiencies, the most common being factor VIII deficiency
Inhibitors
Therapy with unfractionated heparin
Therapy with warfarin (variable response)
Therapy with antithrombin agents such as hirudin and its derivatives, argatroban, and newer antithrombin (e.g., dabigatran), as well as anti-Xa agents (e.g., rivaroxaban)
High-titer LA
Moderate to severe von Willebrand disease
Decreased (< 22 seconds) In
Excessive generation of thrombin. No clinical correlation with a predisposition to thromboembolism has been definitely demonstrated.
Normal In
Thrombocytopenias and thrombocytopathies without associated clotting defects
Majority of cases of mild von Willebrand disease
Isolated defects of factor VII or XIII
Limitations
Preanalytic Pitfalls
Partial clotting of sample due to insufficient mixture with anticoagulant
Overfilling or underfilling the test tube, thereby changing the 9 (blood)-to-1 (anticoagulant) ratio
Use of wrong anticoagulant rather than the recommended 3.2% sodium citrate (currently used in blue top tubes)
Analytic Pitfalls
Hemolyzed, severely icteric, or hyperlipemic blood may affect results (modern equipment may override icteric or hyperlipidemic blood)
Other Limitations: Drugs
Short values may be seen with estrogen therapy or oral contraceptives.
Prolonged values may result from diphenylhydantoin, naloxone, and radiographic contrast agents.