Activated partial thromboplastin time (APTT)

Description: 
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: 
An elevated APTT can be due to one or more of the following:

● hereditary or acquired deficiencies of factors II, V, VIII, IX, X, XI, XII, prekallikrein, high molecular weight kininogen, fibrinogen
● some sub-types of von Willebrand disease (due to the associated FVIII deficiency)
● autoantibodies/inhibitors against the above coagulation factors
● vitamin K deficiency
● liver disease
● disseminated intravascular coagulation
● lupus anticoagulant
● anticoagulant therapy with vitamin K antagonists, UFH, LMWH, fondaparinux, direct thrombin inhibitors, direct-FXa inhibitors

An APTT below the reference range can be due to:
● elevated FVIII, FIX or fibrinogen
● activated sample

Unexpectedly elevated APTTs additionally receive a mixing test, which is an APTT performed on a mixture of equal volumes of patient and normal plasma. Most factor deficiencies will return into the reference range as the normal plasma supplies a sufficient level of the missing or reduced factor(s) to restore a normal clotting time. Conversely, most inhibitors will exert their effect on the normal plasma as well as the patient plasma and the APTT remains elevated.
Reference range: 

"Ratio: 0.8 -1.2 Seconds: 22.3 - 30.5"

Synonyms or keywords: 
APTT, APTTr, PTT
Units: 
Standard reporting format is ratios Reporting of clotting time is also available
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.

Contacts:
Diagnostic Haemostasis and Thrombosis Department
St Thomas': 020 7188 2797; Guy's: 020 7188 7188 ext. 53860
St Thomas' Hospital
North Wing - 4th and 5th Floors
Westminster Bridge Road
London SE1 7EH

Laboratory opening times
24/7

Guy's Hospital
Southwark Wing - 4th Floor
Great Maze Pond
London SE1 9RT

Outside core hours, contact Duty Haemostasis Biomedical Scientist
For clinical advice or interpretation of results, please contact the laboratory in the first instance.

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Last updated: 10/11/2021