Prothrombin time/INR (VKA monitoring)

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.

Conversion of prothrombin time results to international normalised ratio (INR) exists to standardise VKA monitoring between alternative reagents and analytical platforms as far as is possible. Our laboratories undertake local international sensitivity index (ISI) calibration of thromboplastin reagents to maximise INR accuracy.
Clinical details: 
Pharmaceutical impairment of vitamin K metabolism inhibits production of functional vitamin K dependent coagulation factors (FII, VII, IX & X) and inhibitor molecules (proteins C,S & Z).
Reference range: 

Therapeutic range: 2.0 - 4.0

International normalised ratio (INR)
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.

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: 09/03/2017