Prothrombin Time / INR
PT/INR· also: INR, Protime, PT/INR
Clinical Overview
Prothrombin time (PT) measures how long it takes blood to clot via the extrinsic coagulation pathway, which includes factors I, II, V, VII, and X. The International Normalized Ratio (INR) standardizes PT results across different laboratories. PT/INR is used to monitor warfarin therapy and assess liver synthetic function.
Why This Test Matters
PT/INR is the primary test for monitoring warfarin (Coumadin) anticoagulation therapy. It also screens for deficiencies in extrinsic pathway clotting factors, severe liver disease (where factor synthesis is reduced), and vitamin K deficiency. A normal PT with prolonged aPTT suggests hemophilia A or B (intrinsic pathway defects).
Reference RangesWHO/IFCC standards
| Reference Range | Unit | Notes |
|---|---|---|
| 11 – 13.5 | seconds | Prothrombin Time (PT) |
Also reported in: INR.
Critical (Panic) Values
Critical High: > 5 seconds. Values outside these limits require immediate clinical attention.
What Causes Abnormal Results?
High PT/INR Causes
- Warfarin therapy (most common cause of prolonged INR)
- Severe liver disease (reduced factor synthesis)
- Vitamin K deficiency (malnutrition, malabsorption, antibiotics)
- Disseminated intravascular coagulation (DIC)
- Factor II, V, VII, or X deficiency (rare inherited)
Low PT/INR Causes
- Not clinically significant (low INR or short PT is not a recognized problem)
Signs & Symptoms to Watch For
How to Prepare for This Test
For routine coagulation testing: no special preparation. For warfarin monitoring: test at the same time of day consistently. Inform the lab if you are on heparin, as it can affect the assay.
Factors That Can Affect Results
- Warfarin and other vitamin K antagonists (main reason for elevated INR)
- Antibiotics (suppress gut bacteria that produce vitamin K)
- Dietary vitamin K changes (green leafy vegetables lower INR)
- Many medications interact with warfarin (amiodarone, fluconazole, metronidazole raise INR)
- Liver disease
- Underfilled sample tube (wrong citrate-to-blood ratio falsely prolongs PT)
Related Topics
Frequently Asked Questions
What is the difference between PT and INR?
PT (prothrombin time) is the raw clotting time in seconds, which varies between laboratories depending on the reagent used. INR (International Normalized Ratio) is a mathematical correction that standardizes the PT result so that a value of 2.0 means the same thing regardless of which lab runs the test. For clinical decisions, INR is always used.
What INR is dangerous?
An INR above 4–5 is associated with significantly increased bleeding risk even without active bleeding. An INR above 8–10 is a medical emergency if the patient is actively bleeding. Reversal agents include vitamin K (oral or IV), fresh frozen plasma (FFP), and 4-factor prothrombin complex concentrate (4F-PCC). 4F-PCC works within minutes while FFP takes hours.
What does INR tell you about liver disease?
The liver produces most clotting factors. In severe liver disease, factor synthesis decreases, prolonging PT/INR. An INR above 1.5 is incorporated into the MELD score (Model for End-Stage Liver Disease) used to prioritize liver transplant listing. However, PT/INR does not fully capture the complex coagulopathy of liver disease, which involves both pro- and anticoagulant factor deficiencies.