Prothrombin time/INR

Description: 
The prothrombin time (PT) is a screening test which isolates the 'extrinsic' 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 PT.

Patient plasma is incubated with thromboplastin reagent, which contains recombinant tissue factor, synthetic phospholipids & calcium ions. FVIIa forms a complex with tissue factor in a phospholipid and calcium dependent manner to autoactivate FVII and then FX to FXa to begin the 'common' pathway which generates thrombin via the prothrombinase complex to form a fibrin clot. The process of timing to clot formation begins upon the addition of calcium ion-containing thromboplastin to replace those removed by the tri-sodium citrate anticoagulant and thereby facilitate functioning tenase and prothrombinase complexes.

For rare occasions where interfering factors compromise PT analysis on the automated analysers, we have an alternative rabbit brain-derived reagent used on a semi-manual coagulometer employing a mechanical clot-detection technique. It can also aid detection of lupus anticoagulants and dysreactive FVIIs.
Clinical details: 
An elevated prothrombin time (PT) can be due to one or more of the following:

● hereditary or acquired deficiencies of factors II, V, VII & X
● autoantibodies/inhibitors against the above coagulation factors
● vitamin K deficiency
● liver disease
● disseminated intravascular coagulation
● lupus anticoagulant (rare)
● anticoagulant therapy with vitamin K antagonists, direct thrombin inhibitors, direct-FXa inhibitors

Unexpectedly elevated PTs additionally receive a mixing test, which is an PT 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 PT remains elevated.
Reference range: 

"INR 0.8 - 1.2 PT (s) 9.9 - 11.6"

Units: 
International normalised ratio (INR) Seconds
Sample type and Volume required: 
External requests: Citrated platelet poor plasma
500µL 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
020 7188 2797
St Thomas' Hospital
North Wing - 4th and 5th Floors
Westminster Bridge Road
London SE1 7EH

Laboratory opening times
24/7
For clinical advice or interpretation of results, please contact the laboratory in the first instance.

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Last updated: 09/03/2017