PT/INR

Overview of the PT/INR Test

Prothrombin Time (PT):
- Definition: A coagulation test that measures the time (in seconds) it takes for plasma to clot after the addition of thromboplastin and calcium
- Purpose:
  - Evaluate the extrinsic and common pathways of coagulation
  - Monitor oral anticoagulant therapy with warfarin (Coumadin)
  - Assess liver function
  - Evaluate for coagulation factor deficiencies
- Principle:
  - Thromboplastin (Tissue Factor): A reagent that contains tissue factor, phospholipids, and calcium
  - Addition of Thromboplastin: Activates the extrinsic pathway of coagulation, leading to thrombin formation and fibrin clot formation
  - Clot Detection: The time to clot formation is measured, either manually or using an automated coagulation analyzer
International Normalized Ratio (INR):
- Definition: A standardized ratio calculated from the patient’s PT result that is used to monitor warfarin therapy
- Purpose:
  - To reduce inter-laboratory variability in PT results due to differences in thromboplastin reagents
  - To provide a consistent and reliable measure of the degree of anticoagulation
- Calculation:
  - INR = (Patient PT / Mean Normal PT)^ISI
  - Patient PT: The patient’s prothrombin time in seconds
  - Mean Normal PT (MNPT): The average PT value from a group of healthy individuals tested in the same laboratory using the same thromboplastin reagent
  - International Sensitivity Index (ISI): A value assigned to each thromboplastin reagent by the manufacturer, reflecting its sensitivity to vitamin K-dependent coagulation factors

Components of the PT/INR Test

Specimen Collection:
- Collection Tube: Sodium citrate (light blue top) tube with a 3.2% or 3.8% sodium citrate concentration
- Blood-to-Anticoagulant Ratio: The correct ratio is critical for accurate results:
  - 9:1 ratio of blood to anticoagulant
  - Underfilling the tube can lead to falsely prolonged PT/INR results
  - Overfilling the tube can lead to falsely shortened PT/INR results
Reagents:
- Thromboplastin: Contains tissue factor, phospholipids, and calcium ions, which initiate the extrinsic pathway of coagulation
- Calcium Chloride: Provides calcium ions, which are essential for the activity of coagulation factors
- Standard: Used to create a clot based on a known sample.
Instrumentation:
- Automated Coagulation Analyzer: Detects clot formation using optical or mechanical methods:
  - Optical Method: Measures changes in light transmission as the clot forms
  - Mechanical Method: Detects the movement of a probe or wire as the clot forms

Procedure for Performing the PT/INR Test

Sample Preparation:
- Centrifuge the sodium citrate tube to obtain platelet-poor plasma (PPP)
- PPP should have a platelet count < 10 x 10^9/L
Automated Method:
- Follow the instrument manufacturer’s instructions
- Add thromboplastin reagent to the plasma
- Add calcium chloride to initiate coagulation
- Measure the time to clot formation (PT in seconds)
INR Calculation:
- The instrument automatically calculates the INR using the formula: INR = (Patient PT / Mean Normal PT)^ISI
Quality Control:
- Run control materials at regular intervals to assess the accuracy and precision of the test
- Review control results and take corrective action if necessary

Interpreting Results

Prothrombin Time (PT):
- Reference Range: Varies depending on the thromboplastin reagent and instrument used (e.g. 11-13 seconds)
- Prolonged PT: Indicates a deficiency or dysfunction of one or more factors in the extrinsic or common pathways (Factors II, V, VII, X, or fibrinogen)
  - Liver Disease: Impaired synthesis of coagulation factors
  - Vitamin K Deficiency: Impaired carboxylation of vitamin K-dependent factors
  - Warfarin Therapy: Inhibition of vitamin K-dependent factors
  - Disseminated Intravascular Coagulation (DIC): Consumption of coagulation factors
  - Factor VII Deficiency: Specific deficiency of Factor VII
International Normalized Ratio (INR):
- Target INR Range for Warfarin Therapy:
  - Typically 2.0-3.0 for most indications (e.g., atrial fibrillation, venous thromboembolism)
  - Higher INR ranges (e.g., 2.5-3.5) may be used for patients with mechanical heart valves
- INR < Target Range: Indicates subtherapeutic anticoagulation and increased risk of thrombosis
- INR > Target Range: Indicates supratherapeutic anticoagulation and increased risk of bleeding

Factors Affecting PT/INR Results

Pre-Analytical Variables:
- Improper Collection Technique: Tissue thromboplastin contamination or hemolysis
- Incorrect Blood-to-Anticoagulant Ratio: Underfilling or overfilling the collection tube
- Clotted Sample: Invalidates the results
- Delayed Testing: Coagulation factors can degrade over time
- Improper Storage: Incorrect storage temperatures can affect results
Analytical Variables:
- Instrument Malfunction: Ensure proper calibration and maintenance of the coagulation analyzer
- Reagent Problems: Use fresh, properly stored reagents and follow the manufacturer’s instructions
- Lipemia or Icterus: Can interfere with optical clot detection
- High Factor V Level: Can artificially shorten the PT
Patient-Related Variables:
- Medications: Warfarin, heparin, direct oral anticoagulants, and other medications can affect coagulation test results
- Diet: Vitamin K intake can affect warfarin therapy
- Liver Disease: Impairs the synthesis of coagulation factors
- Genetic Factors: Rare genetic variations can affect coagulation factor levels

Troubleshooting Erroneous Results

If the PT/INR result is inconsistent with the patient’s clinical presentation or previous results:
- Check the sample for clots or hemolysis
- Repeat the test on a fresh sample
- Ensure that the correct blood-to-anticoagulant ratio was used
- Verify the instrument and reagent quality control results
- Investigate potential interfering substances (e.g., lipemia)
- Review the patient’s medication list and medical history
- Perform a mixing study to rule out factor inhibitors

Key Terms

Prothrombin Time (PT): Test of the extrinsic and common pathways
Thromboplastin: Reagent used to initiate the PT test
International Normalized Ratio (INR): A standardized ratio of the PT used to monitor warfarin therapy
Warfarin (Coumadin): An oral anticoagulant that inhibits vitamin K-dependent coagulation factors
Extrinsic Pathway: A pathway in the coagulation cascade initiated by tissue factor
Common Pathway: The final pathway in the coagulation cascade, leading to fibrin formation
Coagulation Factors: Proteins involved in the coagulation cascade
Control Materials: Samples with known values that are used to assess the accuracy and precision of an instrument
Reagents: Substances used to carry out a chemical reaction

Laboratory

APTT