Monitoring and Interpretation of Activated Partial Thromboplastin Time

Introduction to Activated Partial Thromboplastin Time

The American College of Cardiology recommends that activated partial thromboplastin time (TTPa) be measured 6 hours after any change in unfractionated heparin (HNF) dose to allow for pharmacokinetic steady state 2, 3

The American Heart Association suggests that the therapeutic target for TTPa is 1.5 to 2.5 times the control value (for example, for a control of 30 seconds, the target would be 45-75 seconds) 2, 3
Once two consecutive TTPa values are therapeutic, measurements can be made every 24 hours 2, 3
Any significant clinical change (recurrent ischemia, bleeding, hypotension) should trigger an immediate TTPa measurement 2, 3

The American College of Cardiology recommends that TTPa be used as a screening test for lupus anticoagulant, particularly when it is part of a panel of tests for lupus anticoagulant 1

The American Heart Association recommends that during extracorporeal membrane oxygenation (ECMO), TTPa should be measured at least daily with anti-FXa levels, prothrombin time, fibrinogen, platelet count, and antithrombin III levels to confirm adequate anticoagulation 5
The target during ECMO is a prolonged TTPa of 1.5 to 2.5 times the control value and an anti-FXa level of 0.3 to 0.7 U/mL 5

The American College of Cardiology notes that TTPa reagents vary considerably in their sensitivity to heparin: the therapeutic range of 1.5-2.5 may not be appropriate for all reagents 2, 3
Each institution should establish specific nomograms designed to achieve target TTPa values based on their local reagents 2, 3
Delays in laboratory response time can be a source of variability in care, leading to over- or under-anticoagulation for prolonged periods 2, 3
The American College of Cardiology recommends that TTPa is not reliable for evaluating direct oral anticoagulants (DOACs): specialized tests such as diluted thrombin time or calibrated anti-Xa levels are necessary 6

For low molecular weight heparins (LMWH), TTPa is generally not affected and anti-Xa levels are preferred for monitoring 2
In patients with liver disease, TTPa may be prolonged but does not reliably predict bleeding risk 7