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SOP

Prothrombin Time (PT) and INR

SOP for evaluating the extrinsic and common coagulation pathways and monitoring oral anticoagulant therapy.

Last verified 1 month ago 2 min read Hematology Coagulation Laboratory Techniques #PT #INR #Coagulation #prothrombin time #warfarin #extrinsic pathway #thromboplastin
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Document Control
Version
1.0
Effective
Aug 29, 2026
Reviewed
Aug 29, 2026
Department
Coagulation
Principle

The PT test evaluates the extrinsic (Factor VII) and common (Factors X, V, II, and Fibrinogen) coagulation pathways. Tissue factor (thromboplastin) is added to citrated plasma, initiating the cascade. Calcium is added to reverse the citrate anticoagulation. The time taken for a fibrin clot to form is measured. The INR standardizes this result across different laboratories by accounting for the sensitivity (ISI) of the specific thromboplastin reagent used.

Standard Operating Procedure: Prothrombin Time (PT) and INR

1. Purpose and Scope

To measure the time required for plasma to clot after the addition of tissue factor (thromboplastin) and calcium, and to calculate the International Normalized Ratio (INR) for monitoring warfarin therapy.

2. Specimen Requirements

  • Type: Platelet-poor plasma.
  • Anticoagulant: 3.2% (0.109 M) Sodium Citrate (9:1 blood-to-anticoagulant ratio).
  • Processing: Centrifuge at 1500-2000 × g for 15 mins within 2 hours of collection.
  • Stability: 4 hours at room temperature.
  • Rejection: Clotted, under-filled, or hemolyzed samples.

3. Safety and Precautions

  • Standard PPE. Handle plasma as potentially infectious.

4. Equipment and Reagents

  • Automated coagulation analyzer.
  • PT reagent (thromboplastin containing tissue factor and calcium, or requiring added calcium).
  • Citrate plasma controls (normal and abnormal).

5. Step-by-Step Procedure

  1. Bring reagents and controls to room temperature (or 37°C as per manufacturer).
  2. Run two levels of QC. Verify results are within range.
  3. Load patient plasma into the analyzer.
  4. The analyzer automatically dispenses plasma and pre-warmed PT reagent.
  5. The instrument detects clot formation (via optical or mechanical methods) and records the time in seconds.
  6. The analyzer automatically calculates the INR using the formula: INR = (Patient PT / Mean Normal PT)^ISI.

6. Quality Control

  • Run QC with every new reagent lot, daily, or per lab policy.
  • Verify the ISI (International Sensitivity Index) of the thromboplastin is correctly entered into the analyzer.

7. Reference Ranges

  • The canonical adult reference intervals for PT and INR are shown in the Reference Ranges panel below.

Decision Limits

  • INR (Therapeutic for most indications): 2.0-3.0

8. Reporting Results

  • Report PT in seconds and INR.
  • Flag critical values (e.g., INR > 4.5) for immediate clinical notification.

Reference Ranges

Browse all ranges
INR - Adult

Adult · Any

0.8–1.1 ratio

11–13.5 seconds

Frequently Asked Questions

An incorrect ratio alters the amount of calcium available to reverse the anticoagulation, leading to falsely prolonged (under-filled tube) or shortened clotting times.
The International Sensitivity Index (ISI) indicates how a specific batch of thromboplastin compares to the WHO international reference standard. An ISI of 1.0 means it is equally sensitive.