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.
Clinical details: 
Unfractionated heparin (UFH) is a parenteral anticoagulant used for treatment and prophylaxis of venous thrombosis disorders. Its main mechanism of action is through binding to antithrombin, which leads to a conformational change that potentiates antithrombin activty up to 1000-fold. Bioavailability of UFH varies with dose, and also between individuals because it binds to plasma proteins, platelet factor-4, macrophages, fibrinogen, lipoproteins, and endothelial cells.

APTT is the most commonly used test for UFH monitoring. Between-reagent variability exists for the sensitivity of APTT reagents to the UFH anticoagulant effect
Reference range: 

Therapeutic range: 1.5 - 2.5 (ratio)

Sample type and Volume required: 
External requests: Citrated platelet poor plasma
800µL x 1 aliquot
Internal requests: please refer to EPR label
Turnaround time: 
4 hours
Special sample instructions: 

The sample should be analysed within 4 hours of venepuncture. Please ensure sample tubes are filled exactly to the fill-line as underfilling creates a dilution error and leads to inaccurate results.

Diagnostic Haemostasis and Thrombosis Department
020 7188 2797
St Thomas' Hospital
North Wing - 4th and 5th Floors
Westminster Bridge Road
London SE1 7EH

Laboratory opening times
For clinical advice or interpretation of results, please contact the laboratory in the first instance.

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Last updated: 08/03/2017