APTT

4.5 mL blood added to 0.5 mL citrate.

The sample must not be collected into heparinised syringes or tubes, or from heparinised lines.

See Table 1.

Recalcification of platelet-poor plasma at 37°C in the presence of an activator and platelet substitute.

Measure time to clot.

Refer to laboratory. Commonly between 25-35 seconds (guide only), but varies with reagents and method.

The therapeutic interval for continuous infusion heparin varies with the reagents used and should be confirmed with the laboratory.

Used as an initial test with the PT/INR when a coagulopathy is suspected.

A baseline APTT prior to heparin therapy may detect a lupus inhibitor or other coagulopathies.

The APTT is used to monitor full dose continuous infusion IV heparin therapy but is usually not required to monitor 'prophylactic' subcutaneous heparin.

If monitoring is required for low molecular weight heparin therapy an Anti factor Xa assay is required.

See also Coagulation profile for further information.

A normal APTT does not exclude mild, but clinically significant, coagulation factor deficiency (eg, as in mild haemophilia, von Willebrand disease) as many reagents give a prolonged APTT only at coagulation factor levels of 30% or below, presumably.

An isolated prolongation of the APTT (PT normal) suggests deficiency of factor VIII, IX, XI or XII.

Prolongation of both the APTT and PT suggests factor X, V, II or I (fibrinogen) deficiency, all of which are rare. The APTT is normal in factor VII deficiency (PT prolonged) and factor XIII deficiency. See Figure 1.

A prolonged APTT which is not corrected by the in vitro addition of normal plasma suggests the presence of a coagulation factor (eg, VIII or IX) inhibitor or a lupus inhibitor.

Artefactual prolongation of the APTT may be due to:

• the presence of heparin in the sample;
• difficult or slow collection;
• addition of an incorrect volume of blood to the tube;
• delay in mixing blood with the citrate anticoagulant; or
• suboptimal specimen storage or a prolonged interval between collection and testing.

See also Bleeding disorders and Anticoagulant monitoring.