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Activated Partial Thromboplastin Time

The activated partial thromboplastin time (APTT) is used to evaluate all the clotting factors of the intrinsic pathway - except platelets - by measuring the time required for formation of a fibrin clot after the addition of calcium and phospholipid emulsion to a plasma sample. An activator, such as kaolin, is used to shorten clotting time.

Purpose

• To aid preoperative screening for bleeding tendencies.
• To screen for congenital coagulation deficiencies of the clotting factors
• To monitor heparin therapy

Patient preparation

• Explain to the patient that this test is used to determine if blood clots normally.
• Tell him that a blood sample will be taken. Explain who will perform the venipuncture and when.
• Reassure him that drawing a blood will luke less than 3 minutes.
• Explain that he may feel slight discomfort from the tourniquet pressure and the needle puncture.
• When appropriate, tell the patient receiving therapy that this test may be repeated at regular intervals to assess the response to treatment.
• inform the patient that food or fluids need not be restricted before the test.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml blue-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.

Precautions

• Completely fill the collection tube, invert it gently several times, and send it to the laboratory on ice.
• To prevent hemolysis, avoid excessive probing at the venipuncture site and handle the sample gently.
• For a patient on anticoagulant therapy, additional pressure may be needed at the venipuncture site to control bleeding.

Reference values

Normally, a fibrin clot forms 21 to 35 seconds after addition of reagents. For a patient on anticoagulant therapy, ask the attending doctor to specify the reference values for the therapy being.

Abnormal findings

Prolonged APTT may indicate a deficiency of certain plasma clotting factors, the presence of heparin, or the presence of fibrin split products, fibrinolysins, or circulating anticoagulants that are antibodies to specific clotting factors

Interfering factors

• Failure to fill the collection tube completely, to use the proper anticoagulant, or to adequately mix the sample and the anticoagulant.
• Hemolysis due to rough handling of the sample or to excessive probing at the venipuncture site.
• Failure to send the sample to the laboratory immediately or to place it on ice.

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