Type of test :  Blood

Normal findings

 70-120 sec Therapeutic range for anticoagulation: 150-600 sec (Normal ranges and anticoagulation ranges vary according to type of laboratory procedure and particular therapy.)

Possible critical values:  Depend on use for the test and clinical situation

 Test explanation and related physiology

The ACT is primarily used to measure the anticoagulant effect of heparin or other direct thrombin inhibitors during cardiac angioplasty, hemodialysis, and cardiopulmonary bypass (CPB) surgery. This test measures the time for whole blood to clot after the addition of particulate activators. It is similar to the activated partial thromboplastin time (APTT) in that it measures the ability of the intrinsic pathway to begin clot formation by activating factor XII (see Figure 1). By checking the blood clotting status with ACT, the response to unfractionated heparin therapy can be monitored.

fig1. Secondary hemostasis (fibrin clot formation) and fibrinolysis (fibrin clot dissolution). Primary hemostasis involves platelet plugging of the injured blood vessel. Secondary hemostasis, as described here, takes place most rapidly on the platelet surface after attachment to the fractured endothelium. Four different reactions result in the formation of fibrin. As seen beneath the dark line in the figure, the fibrin clot supports the platelet clump so that the clot does not get swept away by the tremendous shear forces of the fast moving blood cells. Fibrinolysis follows formation of the fibrin clot in order to prevent complete occlusion of the injured blood vessel.

Both the APTT and the ACT can be used to monitor heparin therapy for patients during CPB. However, the ACT has several advantages over the APTT. First, the ACT is more accurate than the APTT when high doses of heparin are used for anticoagulation. This makes it especially useful during clinical situations requiring high-dose heparin, such as during CPB, when high dose anticoagulation is necessary at levels 10 times those used for venous thrombosis. The APTT is not measurable at these high doses. The accepted goal for the ACT is 400 to 480 seconds during CPB.

Second, the ACT is both less expensive and more easily per formed, even at the bedside. This allows for immediate accessibility and decreased turnaround time. The capability to perform the ACT at the point of care makes the ACT particularly useful for patients requiring angioplasty, hemodialysis, and CPB.

A nomogram is often used as a guide to reach the desired level of anticoagulation. This nomogram is used in determining the dose of protamine to neutralize the heparin upon completion of these procedures. The ACT is used in determining when it is safe to remove the vascular access upon completion of these procedures. The benefits of the modified ACT test are that it requires a smaller-volume blood specimen; it can be automated; it can use standardized blood/reagent mixing; and it provides faster clotting time results than the conventional ACT. The mod ified ACT is now used more frequently.

Interfering factors

 • The ACT is affected by biologic variables, including hypothermia, hemodilution, and platelet number and function.

 • Factors affecting the pharmacokinetics of heparin (e.g., kidney or liver disease) and heparin resistance can affect ACT measurements.

 • A clotted specimen can increase ACT measurements.

 Procedure and patient care

• See inside front cover for Routine Blood Testing.

 • Fasting: no

 • Blood tube commonly used: verify with laboratory

• Less than 1 mL of blood is collected and placed in a machine at the bedside. When a clot forms, the ACT value is displayed.

 • If the patient is receiving a continuous heparin drip, the blood sample is obtained from the arm without the intravenous catheter.

• The bleeding time will be prolonged because of anticoagulation therapy.

 • Assess the patient to detect possible bleeding. Check for blood in the urine and all other excretions, and assess the patient for bruises, petechiae, and low back pain.

Abnormal findings

 Increased levels

- Heparin administration

 - Clotting factor deficiencies

 - Cirrhosis of the liver

- Lupus inhibitor

 - Warfarin administration

Decreased levels

 Thrombosis