Activated partial thromboplastin time (APTT) (UFH monitoring)

The activated partial thromboplastin time (APTT) is a screening test which isolates the 'intrinsic' and 'common' pathways of the in vitro coagulation cascade model. Coagulation factors and cofactors within each pathway operate in concert to generate a fibrin clot end-point, the time taken to form the clot being the APTT.

Patient plasma is first incubated with a contact activator to activate FXII and commence the intrinsic pathway. Although not a significant reaction in vivo with respect to bleeding, the activated FXII (FXIIa) activates FXI independently of thrombin, which proceeds to activate FIX independently of FVIIa/TF. FIXa generates FXa to begin the 'common' pathway which generates thrombin via the prothrombinase complex to form a fibrin clot. The APTT reagent also contains the 'partial' thromboplastin, comprising phospholipid but not tissue factor, which facilitates reactions involving the vitamin K dependent factors of both pathways but excludes FVII. The process of timing to clot formation begins upon the addition of calcium ions to replace those removed by the tri-sodium citrate anticoagulant and thereby facilitate functioning tenase and prothrombinase complexes.

Our routine APTT reagent, used on automated analysers employing photo-optical clot detection, is lupus anticoagulant insensitive. For rare occasions where interfering factors compromise APTT analysis on the automated analysers, we have an alternative reagent used on a semi-manual coagulometer employing a mechanical clot-detection technique.