Activated partial thromboplastin time [PTT] - Clinical assay interference and limitations- Department of Laboratory Medicine

Department of Laboratory Medicine
University of Washington

Clinical assay interference and limitations

Activated partial thromboplastin time [PTT]

Unfractionated Heparin (Standard Heparin)

Full anticoagulation, patients on continuous IV heparin. Suggested therapeutic aPTT range represents heparin concentrations in vivo of approximately 0.2 to 0.4 U/mL protamine titration units in most patients without pre-existing coagulation defects.

Therapeutic aPTT 60 to 100 sec

CRITICAL aPTT >120 sec

Low Molecular Weight Heparin (Enoxaparin, measured 4 hours after dose)

Treatment of DVT

Therapeutic Heparin Activity (anti-Xa units)

Twice-daily subcutaneous dosing 0.5 to 1.0 U/mL

Once daily subcutaneous dosing 1.0 to 2.0 U/mL

Prophylaxis of Thrombosis

Prophylactic Heparin Activity (anti-Xa units) 0.4 to 0.6 U/mL

Prophylaxis in Pregnancy (anti-Xa units) 0.2 to 0.4 U/mL

Prophylaxis in Newborns (anti-Xa units) 0.5 to 1.0 U/mL

Suggested populations to monitor LMW Heparin: renal insufficiency, pregnancy, newborns, markedly obese or underweight patients (pharmacokinetic differences), patients on prolonged therapeutic doses (malignancy, APA, refractory or failed warfarin) to prevent excessive of insufficient anticoagulation.

Hirudin Therapy (Arch Pathol Lab Med 1998; 122:799)

Hirudin is a specific thrombin inhibitor developed from the leach. The current drug is a recombinant form of leach hirudin. Drug is cleared through the kidneys, check creatinine before starting. For use in patients with thrombosis associated with heparin-induced thrombocytopenia, unstable angina or myocardial infarction.

Deep Venous Thrombosis Prophylaxis and Treatment of Venous Thromboembolism, Unstable Angina, or Myocardial Infarction

For subcutaneous injection draw sample 3 hours after last dose.

Monitoring using aPTT: 60 to 80 sec

Monitoring using Direct Thrombin

Inhibitor assay (dilute TT) 90 to 160 sec

Hirudin can be used to anticoagulate patients during cardiopulmonary bypass. The levels used are much higher and an ecarin clotting time must be used for monitoring.

Cardiopulmonary Bypass >5 ug/mL

Argatroban Therapy (Arch Pathol Lab Med 1998; 122:799)

Argatroban is a synthetic reversible specific thrombin inhibitor. Argatroban is cleared through the liver; check bilirubin and baseline PT prior to starting. For use in patients with thrombosis associated with heparin-induced thrombocytopenia and for post-angioplasty anticoagulation.

Monitoring using aPTT: 60 to 80 sec

Monitoring using Direct Thrombin

Inhibitor assay (dilute TT): 60 to 100 sec

To switch from argatroban to warfarin, start warfarin and adjust until INR = 3.5 with patient still on argatroban. Once INR = 3.5, stop argatroban and stabilize warfarin INR as argatroban clears.

Bivalirudin Therapy (Angiomax - Best Pract Res Clin Haematol 2004;17:105)

Bivalirudin is a synthetic peptide thrombin inhibitor related to hirudin (hirulog). The drug is cleared through cleavage by thrombin and by the kidneys (t 1/2 = 30 minutes), check creatinine before starting. For use in patients with thrombosis associated with heparin-induced thrombocytopenia.

Monitoring using aPTT: 60 to 80 sec

Monitoring using Direct Thrombin

Inhibitor Assay (Dilute TT): 60 to 90 sec

Cardiopulmonary Bypass: Use ACT