Source-checked
Coagulation
Prothrombin time (PT)
Source-checked · Content last updated · 1 min read ·
This is the date the draft was written, not a medical review date.
The prothrombin time assesses the extrinsic and common coagulation pathways (factors VII, X, V, II and fibrinogen). It is sensitive to vitamin K deficiency, warfarin and liver synthetic dysfunction.
Reference interval
| Conventional (seconds) | SI (seconds) | |
|---|---|---|
| Adults | 11–13.5 seconds | 11–13.5 seconds |
Specimen: Citrated plasma (3.2 % sodium citrate, 9:1 blood to citrate) · Conventional × 1 = SI
Causes of high values
Warfarin therapy, vitamin K deficiency (malabsorption, biliary obstruction, antibiotics), liver disease, DIC, direct oral anticoagulants (reagent-dependent), factor VII deficiency and hypofibrinogenaemia.
Causes of low values
A shortened PT is usually not clinically meaningful (hypercoagulable state, sampling or handling artefact).
Pre-analytical and method notes
Reference range depends on the thromboplastin reagent and analyser (typically 11-13.5 s, up to about 15 s in some laboratories), so each laboratory must define its own interval; the INR standardises vitamin K antagonist monitoring. Underfilled tubes, high haematocrit above about 55 % and heparin contamination affect results. In France and some countries the result is also reported as percentage activity (taux de prothrombine, normally about 70-100 %).
References
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. Elsevier — Tier 3 (textbook)
- Loscalzo J, et al.. Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022) — Tier 3 (textbook)
References are listed only where the article cites them. See the sources page for the full policy. How sources are chosen and credited