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: â— hereditary or acquired deficiencies of factors II, V, VII & X â— autoantibodies/inhibitors against the above coagulation factors â— vitamin K deficiency â— liver disease â— disseminated intravascular coagulation â— lupus anticoagulant (rare) â— 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.
- Related condition
- Reference range
"INR 0.8 - 1.2 PT (s) 9.9 - 11.2"
- Units
- i
- Testing site
- Synnovis : Blood Sciences : Guy's Hospital|St Thomas' Hospital
- Laboratory
- Diagnostic Haemostasis and Thrombosis
- Sample type and volume required
- External requests: Citrated platelet poor plasma 500µL x 1 aliquot Internal requests: please refer to EPR label
- 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.
- Turnaround time
- 4 hours
- Contacts
Diagnostic Haemostasis and Thrombosis Department
St Thomas’ Hospital
Phone: 020 7188 2797
St Thomas’ Hospital
North Wing – 4th and 5th Floors
Westminster Bridge Road
London SE1 7EHGuy’s Hospital
Phone: 020 7188 7188 ext. 53860
Guy’s Hospital
Southwark Wing – 4th Floor
Great Maze Pond
London SE1 9RT
Outside core hours, contact Duty Haemostasis Biomedical Scientist