Prothrombin time (PT) and INR

What it measures

Measures how long plasma takes to clot after tissue factor and calcium are added, testing the extrinsic and common pathways (factors VII, X, V, II, fibrinogen). The INR standardizes results for monitoring warfarin.

Specimen and preparation

• Specimen: Citrated plasma (3.2% sodium citrate, 9:1 blood to anticoagulant)
• Tube: Light_blue top
• Preparation: Fill tube to the line; draw after a discard tube if using a butterfly. Test within 24 h; do not refrigerate whole blood.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Warfarin therapy
• Vitamin K deficiency (malabsorption, antibiotics, newborn)
• Liver disease
• DIC
• Factor VII, X, V, II or fibrinogen deficiency
• Direct oral anticoagulants (variable)

Causes of low values

• Usually not significant
• Overfilled tube or clotted sample (artifact)

Method

Clot-based assay with thromboplastin on optical or mechanical coagulometer; INR = (patient PT/mean normal PT)^ISI

Good to know

An INR above 5 is usually treated as critical. Hematocrit above 55% requires reduced citrate volume to avoid false prolongation.

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