Prothrombin time (PT) and INR

Principle

• Tissue factor with phospholipid (thromboplastin) and calcium are added to citrated plasma, and the time to clot formation is measured.
• Tests the extrinsic and common pathways: factors VII, X, V, II (prothrombin) and fibrinogen.

Specimen

• Venous blood in 3.2% (0.109 M) sodium citrate, ratio 9 parts blood to 1 part citrate; fill tube to at least 90% of nominal volume.
• If Hct is above 0.55, reduce the citrate volume: citrate (mL) = 0.00185 × (100 − Hct%) × blood volume (mL).
• Centrifuge to platelet-poor plasma (under 10 × 10^9/L platelets), e.g. 1500 g for 15 min.
• PT can be tested up to 24 h after collection if kept capped at room temperature.

Procedure

• Pre-warm plasma and reagent at 37 °C.
• Add pre-warmed thromboplastin–calcium reagent to plasma (classically 0.2 mL to 0.1 mL) and time until a clot forms (optical or mechanical detection).

Results

• Typical PT about 11–14 s, depending on reagent and instrument; local ranges apply.
• INR = (patient PT ÷ mean normal PT) raised to the power ISI. The ISI describes reagent sensitivity relative to the WHO reference thromboplastin (ISI 1.0).
• Mean normal PT is the geometric mean from at least 20 healthy adults.
• Usual warfarin target INR 2.0–3.0; 2.5–3.5 for some mechanical heart valves.

Interpretation

• Prolonged: warfarin, vitamin K deficiency, liver disease, DIC, factor VII deficiency (isolated long PT), deficiency of X, V, II or fibrinogen (with long APTT), direct Xa inhibitors (variable).

Quality control and pitfalls

• Under-filled tube (excess citrate) and high Hct falsely prolong results.
• Clotted, haemolysed or heparin-contaminated samples; draw after blood cultures and before serum or EDTA tubes; use a discard tube with winged sets.
• Run normal and abnormal controls each shift and with new reagent lots.
• INR is validated for patients stable on vitamin K antagonists, not for liver disease.

Clinical use

• Monitoring warfarin, liver synthetic function, vitamin K status, pre-operative screening and DIC.