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:
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