87 Prothrombin index

Prothrombin time, prothrombin index and INR: normal values, warfarin targets and causes of prolonged clotting.

The prothrombin time (PT) measures how quickly plasma clots after tissue factor and calcium are added. It tests the extrinsic and common pathways of coagulation: factors VII, X, V, II (prothrombin) and fibrinogen. Most of these are made in the liver and need vitamin K. PT is reported in seconds, as the prothrombin index or Quick value (in %), and as the INR (international normalised ratio). INR is standardised between laboratories and is used to monitor warfarin and other vitamin K antagonists.

CoagulationLiverAnticoagulants

PT≈ 10–14 s (lab-dependent)

Prothrombin index≈ 70–100% (conventional)

INR, no anticoagulant≈ 0.8–1.2

INR on warfarin (most)2.0–3.0

How the results are expressed

Prothrombin index (%) = PT of normal plasma / PT of patient × 100

INR = (PT of patient / mean normal PT)^ISI

The ISI (international sensitivity index) corrects for the sensitivity of the reagent, so the same INR means the same degree of anticoagulation in any laboratory. The prothrombin index used in Eastern European practice is not standardised in the same way and its reference range is conventional. A lower index or a higher INR means slower clotting.

Figure 1. INR zones. For mechanical mitral valves and some other indications, a higher target range (for example 2.5–3.5) is used.

Prolonged PT (low index, high INR)

Vitamin K antagonists (warfarin, acenocoumarol, phenprocoumon); some direct anticoagulants also prolong PT, but INR is not suitable for monitoring them
Liver disease: cirrhosis, acute hepatitis, liver failure; PT is part of the MELD and Child–Pugh scores
Vitamin K deficiency: poor diet, malabsorption, obstructive jaundice, antibiotics, newborns
Disseminated intravascular coagulation (DIC) and massive transfusion
Rare inherited deficiency of factors VII, X, V or II, or inhibitors

A short PT usually has no clinical significance.

Seek emergency care

for bleeding that does not stop, blood in urine, black stools, vomiting blood, severe headache or a fall with head injury while taking anticoagulants, or if INR is far above the target range.

References

Ageno W, Gallus AS, Wittkowsky A, Crowther M, Hylek EM, Palareti G. Oral anticoagulant therapy: antithrombotic therapy and prevention of thrombosis, 9th ed: American College of Chest Physicians evidence-based clinical practice guidelines. Chest. 2012;141(2 Suppl):e44S–e88S. doi:10.1378/chest.11-2292
Hoffman M, Monroe DM. A cell-based model of hemostasis. Thromb Haemost. 2001;85(6):958–965. doi:10.1055/s-0037-1615947

This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.

Published on 26 April 2024