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 - 11.2"

Units: 
i
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
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.

Print as a PDF

Last updated: 03/10/2022