Which tests are used to monitor heparin therapy?

• A. PT and INR
• B. PTT and ACT
• C. PT only
• D. INR only

Answer: (B)

Monitoring heparin therapy relies on tests that reflect how much the clotting cascade is slowed by the drug. Unfractionated heparin enhances antithrombin III, which in turn inhibits thrombin and factor Xa, causing a measurable delay in clot formation. The standard laboratory test to gauge this effect over a typical dosing range is the activated partial thromboplastin time (aPTT or PTT). Prolongation of the aPTT indicates effective heparin activity, and dosing is adjusted to stay within a therapeutic range.

During procedures that require higher, rapid-acting anticoagulation, the activated clotting time (ACT) is used. This whole-blood, point-of-care test gives a quick readout of heparin’s effect and helps guide dosing in real time, such as during PCI or cardiopulmonary bypass.

Tests like PT and INR assess the extrinsic pathway and are primarily used to monitor warfarin therapy, not heparin. They don’t reliably reflect heparin’s effect, so they’re not appropriate for monitoring unfractionated heparin.