Prothrombin Time (PT)/INR - Black Country Pathology Services

Investigation Name:Prothrombin Time (PT)/INR

Alias or Abbreviation:PT/INR

Accreditation:UKAS ISO15189:2022 Accredited test: On scope

Intro:

The Prothrombin Time (PT) and International Normalised Ratio (INR) test is used to assess blood clotting and is most commonly used to monitor patients on warfarin therapy. It helps ensure that anticoagulation remains within a safe and effective range. The test evaluates the extrinsic and common pathways of the coagulation cascade, making it useful not only in monitoring anticoagulant treatment but also in the investigation of bleeding disorders and the assessment of liver function. The INR provides a standardised result, allowing for consistent interpretation across different laboratories and testing systems.

In the laboratory, automated coagulation analysers are used to perform the test. Patient plasma is incubated with a thromboplastin reagent, which contains tissue factor, and calcium ions. This initiates the extrinsic pathway, resulting in the conversion of fibrinogen to fibrin and the formation of a solid clot within the sample cuvette. The time taken for this clot to form is measured and reported as the PT, and standardised as the INR for consistency across testing platforms.

Pathology Laboratory:Haematology Laboratory

Requestable Seperately?No

Part of Profile:Coagulation Screen

Tests in Profile:Prothrombin Time, INR

E-Requesting Tab:BASIC-Blds

Lower Reference Range Limit:9.5

Upper Reference Range Limit:12.5

Units:seconds

Minimum Sample Volume:Filled to line on bottle

Expected Turnaround Time:<4 hours

Test Code:PTI

Sample Type:

Sodium Citrate Tube (Blue top)

Maximum Add on Time:24 hours

Complex Reference Range:If patient is on warfarin, the INR should be used for monitoring (The INR range varies depending on the reason for the anticoagulation).

Age specific paediatric/neonatal reference range :-
Premature (24-29 weeks - Day 1) PT 11.7 - 21.6 seconds
Premature (30-36 weeks - Day 1) PT 10.6 - 16.2 seconds
Premature (30-36 weeks - Day 30) PT 10.0 - 13.6 seconds
Full term (37-40 weeks - Day 1) PT 10.1 - 15.9 seconds
Full term (37-40 weeks - Day 5) PT 10.0 - 15.3 seconds

Collection Conditions / Other Information:Full sample should be tested ASAP however the INR is stable for 24 hours.

Paediatric sample tubes are available to order for children and difficult to bleed patients. These tubes require 1ml of blood to be taken.

Underfilled or overfilled samples will not be processed as the ratio of anticoagulant to blood in the tube will be incorrect.
Clotted and grossly haemolysed samples cannot be processed.
Haemolysis, Icterus and lipaemia may interfere with testing and so manual methods may be required to gain results.

Referred to Another Laboratory?No

Storage Requirements:Samples should be stored at room temperature and sent to the laboratory without delay.

Methodology:Light transmission

Site Sample Tested:New Cross Hospital - Haematology Laboratory
Walsall Manor Hospital - Haematology Laboratory
Midland Metropolitan University Hospital - Haematology Laboratory
Russells Hall Hospital - Haematology Laboratory