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Prothrombin Time (PT)
(Key Coagulation Test for Extrinsic & Common Pathway - Critical for Liver Disease, Vitamin K Status & Warfarin Monitoring)
Synonyms
- Prothrombin Time
- PT
- ProTime
- Quick time
- Extrinsic pathway clotting time
- Coagulation factor II / VII / X test
Units of Measurement
- seconds (s)
- sec
No other units are used for the raw PT value.
Description
Prothrombin Time (PT) is a clot-based test that measures the time it takes for plasma to clot after the addition of:
- Tissue factor (thromboplastin)
- Calcium
PT evaluates the extrinsic and common coagulation pathways:
Extrinsic Pathway
- Factor VII
Common Pathway
- Factors II (prothrombin), V, X, and fibrinogen
PT is highly sensitive to deficiencies in:
- Vitamin K–dependent factors (II, VII, X)
- Liver dysfunction
- Warfarin (coumadin) therapy
- Disseminated intravascular coagulation (DIC)
Because thromboplastin reagents differ between labs, PT is standardized using INR (International Normalized Ratio) for warfarin management.
Physiological Role
PT reflects the functional ability of the coagulation cascade to form a fibrin clot via tissue-factor–mediated pathway activation.
Clinical Significance
HIGH PT (Prolonged PT)
Primary abnormality; indicates delayed clot formation.
Major Causes
1. Vitamin K Deficiency
- Malnutrition
- Prolonged antibiotics
- Fat malabsorption
- Obstructive jaundice
2. Warfarin Therapy (Most Common Cause)
Directly prolongs PT/INR.
3. Liver Disease
Decreased synthesis of clotting factors:
- II, VII, IX, X
- V, fibrinogen
4. Disseminated Intravascular Coagulation (DIC)
5. Factor Deficiencies
- II, V, VII, X
6. Massive Transfusion
Dilution of clotting factors.
7. Rare Inhibitors
- Factor VII inhibitor
- Lupus anticoagulant (rarely affects PT)
LOW PT
Not clinically relevant — normal PT simply indicates adequate extrinsic/common pathway function.
Reference Intervals
(Tietz 8E + Mayo + ARUP + ISTH)
Prothrombin Time (PT)
- 11 – 15 seconds
(but varies with reagent sensitivity)
Each lab must provide its own reference range.
INR (International Normalized Ratio)
Standardized measure for warfarin therapy.
| Clinical Context | Target INR |
| Normal (not on anticoagulants) | 0.8 – 1.2 |
| Warfarin — most indications | 2.0 – 3.0 |
| Mechanical mitral valve | 2.5 – 3.5 |
Diagnostic Uses
1. Warfarin Monitoring
Primary use of PT/INR.
2. Liver Disease Assessment
Chronic or acute liver failure:
- PT prolongation is an early clue
- Reflects decreased hepatic synthesis of clotting factors
- Most reliable marker of liver synthetic function
3. Suspected Vitamin K Deficiency
Malabsorption, antibiotics, newborn bleeding.
4. Bleeding Disorders
Evaluate extrinsic/common pathway deficits.
5. DIC Workup
PT prolonged alongside:
- Low fibrinogen
- Elevated D-dimer
- Low platelets
6. Preoperative Coagulation Screening
Not routine. Used if:
- Bleeding history
- Liver disease
- Anticoagulant use
- Malnutrition
7. Newborn Hemorrhagic Disease
Vitamin K deficiency → prolonged PT.
Analytical Notes
- Blue-top citrate tube (9:1 ratio required)
- Underfilling → falsely prolonged PT
- Hemolysis interferes minimally
- Reagent sensitivity differs between labs
- INR should not be used for DOAC therapy (rivaroxaban, apixaban)
Clinical Pearls
- Factor VII has the shortest half-life → PT prolongs early in vitamin K deficiency/warfarin therapy.
- PT/INR is a crucial prognostic marker in acute liver failure (King’s College criteria).
- INR is not reliable in pregnancy or in non-warfarin anticoagulation.
- If PT prolonged but aPTT normal → think factor VII deficiency or early vitamin K deficiency.
- Repeat PT after vitamin K injection helps differentiate deficiency from liver failure.
Interesting Fact
Vitamin K deficiency first affects factor VII, giving PT high diagnostic sensitivity - this is why PT rises long before aPTT in early deficiency.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Coagulation Testing
- ISTH Guidelines - Coagulation Standards
- ACCP Guidelines - Anticoagulation Therapy
- Mayo Clinic Laboratories - Prothrombin Time
- ARUP Consult - PT/INR Interpretation
- MedlinePlus / NIH - PT Test