Prothrombin time and international normalised ratio
Pathway · detail · Extrinsic limb Warfarin
Citrated plasma is recalcified with thromboplastin — tissue factor plus phospholipid — so clotting starts at the extrinsic limb. The INR normalises the result to the reagent's international sensitivity index so that warfarin control is comparable between laboratories.
Traced from the start
- Tissue factor and the extrinsic trigger
- Prothrombin time and international normalised ratio
Detail
- Normal range
- About 11-14 seconds; INR is the patient-to-mean-normal PT ratio raised to the power of the reagent ISI
- Measures
- Factor VII plus the common pathway: factor X, factor V, prothrombin and fibrinogen
- Prolonged by
- Warfarin, vitamin K deficiency, liver disease, DIC, factor VII deficiency and, variably, rivaroxaban
- Targets
- INR 2.0-3.0 for atrial fibrillation and venous thromboembolism; 2.5-3.5 for a mechanical mitral valve
- Prognostic use
- The INR is a component of both the Child-Pugh and MELD scores in chronic liver disease
When it goes wrong
INR above 8 on warfarin without bleeding
Withhold warfarin and give oral vitamin K; with major bleeding give prothrombin complex concentrate plus intravenous vitamin K rather than fresh frozen plasma
Prolonged PT that does not correct after parenteral vitamin K
Points to hepatocellular synthetic failure rather than vitamin K deficiency or biliary obstruction
Practise this structure
No question in the bank is tagged to Prothrombin time and international normalised ratio yet. Questions appear here automatically once one is — the question bank is free to browse in the meantime.