CPT® Code 85300

The CPT® Code 85300 refers to a laboratory test that measures the activity of antithrombin III, a critical protein in the regulation of blood coagulation. Antithrombin III functions as a natural anticoagulant, inhibiting the activity of several enzymes in the coagulation cascade, particularly thrombin and factor Xa. This test is essential for evaluating the anticoagulant properties of a patient's blood, especially in cases where there is a suspicion of clotting disorders or when monitoring patients on anticoagulant therapy. The procedure involves taking a diluted blood sample and mixing it with antithrombin-deficient plasma, which contains the necessary intrinsic coagulation enzymes. An AT augmenting compound, such as heparin, is also added, along with phospholipids and an activator that initiates the contact phase of the intrinsic coagulation pathway. After the addition of calcium ions, the coagulation time is measured and compared to a reference standard, allowing for the determination of the antithrombin III activity level in the sample. This test is crucial for diagnosing conditions related to abnormal clotting and for managing patients who may be at risk for thromboembolic events.