Tag Archives: APTT

The APTT is a screening test of the intrinsic clotting system. It will detect the inhibition or deficiency of one or more of the following factors: prothrombin, V, VIII (antihaemophilic factor), IX, X, XI, XII and fibrinogen. The APTT is also used to monitor patients being treated with heparin.

Principle of test

Kaolin (surface activator) and platelet substitute (phospholipid) are incubated with citrated plasma at 37°C for the time
specified in the test method. Calcium chloride is added and the time taken for the mixture to clot is measured.

Reagents

Kaolin/platelet substitute mixture
Purchase from a reliable manufacturer in lyophilized form and reconstitute as instructed.

Calcium chloride, 0.025 mol/l (25 mM)
This is best obtained ready made unless the laboratory has facilities to make the reagent accurately and standardize it.

See also:

APTT TEST PROCEDURE

Test the control plasma and patient’s plasma in duplicate.

1. Pipette 0.2 ml of well-mixed kaolin/platelet substitute in a small glass tube.

2. Add 0.1 ml of plasma, mix, and incubate at 37° C for exactly 2 minutes (tilting the tube at intervals).

3. Add 0.1 ml 0.025 mol/l calcium chloride, mix and start the stop-watch. Hold the tube in the water bath and tilt the mixture back and forth, looking for clot formation. When a clot forms, stop the stop-watch and record the time.

4. Report the patient’s APTT (average of the duplicate tests) providing the APTT of the normal control plasma is satisfactory.

Reference APTT range

36–50 seconds

What causes prolonged APTT?

• DIC (involving several clotting factors)
• Deficiency of clotting factors: prothrombin, V, VIII, IX, X, XI or XII due to vitamin K deficiency, liver disease, heparin or warfarin anticoagulation, or less commonly an inherited coagulation disorder.

See also:

The Principle of APTT test