PT/INR Test: Prothrombin Time and International Normalized Ratio

PT/INR Test: Prothrombin Time and International Normalized Ratio — scientific infographic poster

The prothrombin time (PT) measures how long plasma takes to clot through the extrinsic (tissue factor) and common pathways of coagulation. The international normalized ratio (INR) is a calculation from the PT that corrects for differences between laboratory reagents, so that results for people taking warfarin can be compared from one laboratory to another (Ageno 2012). The PT and INR are used to monitor warfarin and other vitamin K antagonists, as part of the assessment of liver synthetic function, and in the work-up of bleeding and clotting problems. MedlinePlus lists bleeding disorders, disseminated intravascular coagulation, liver disease and low vitamin K among the causes of a high INR in people who are not taking a blood thinner.

Table of Contents

  1. Overview — Extrinsic Coagulation Pathway
  2. How PT and INR Are Measured
  3. Normal and Therapeutic Ranges
  4. Pre-Procedure INR Thresholds
  5. Warfarin Therapy and INR Monitoring
  6. PT as a Liver Function Test
  7. Vitamin K and PT/INR
  8. Factor V Leiden and Coagulation Genetics
  9. Point-of-Care INR Testing
  10. Causes of Prolonged PT/INR
  11. aPTT: The Intrinsic Pathway Partner Test
  12. Urgent Reversal Strategies
  13. Key Research Papers
  14. Connections
  15. Featured Videos

Overview — Extrinsic Coagulation Pathway

The extrinsic pathway starts when tissue factor (Factor III), which sits on cells outside the bloodstream, is exposed to blood after a vessel is injured. Tissue factor binds Factor VII, and the tissue factor–Factor VIIa complex activates Factor X and Factor IX. Activated Factor X, together with its cofactor Factor Va on a phospholipid surface (the prothrombinase complex), converts prothrombin (Factor II) to thrombin far faster than Factor Xa can alone. Thrombin cleaves fibrinogen to fibrin, and Factor XIII, also activated by thrombin, cross-links the fibrin into a stable clot. The PT does not measure Factor XIII.

Factors II, VII, IX and X need vitamin K for a final chemical step, the gamma-carboxylation of glutamate residues, without which they cannot bind calcium and phospholipid membranes and do not work as clotting factors. Warfarin and other vitamin K antagonists act at this step (Ageno 2012). Most clotting factors are made in the liver, so the PT is sensitive both to vitamin K status and to liver disease.

Factors Measured by the PT

Despite its name, the prothrombin time does not measure prothrombin alone. It reflects the combined activity of Factor VII and the common-pathway factors X, V, II and fibrinogen (Factor I). A marked deficiency anywhere along that chain prolongs the result:

Factor VII is the only factor the PT measures that the activated partial thromboplastin time (aPTT) does not, so a deficiency or depletion of Factor VII alone prolongs the PT while the aPTT stays normal. When the shared common-pathway factors (X, V, II or fibrinogen) fall far enough, as in severe liver failure or disseminated intravascular coagulation (DIC), both tests are prolonged.

This extrinsic–intrinsic–common cascade is a model for reading laboratory tests and simplifies what happens in the body. The cell-based model of coagulation describes clotting as starting on tissue factor-bearing cells, being amplified by platelets and propagating on the platelet surface. The cascade model remains the usual framework for interpreting PT and aPTT results.

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How PT and INR Are Measured

The prothrombin time was described by the American physician Armand Quick in 1935 (Quick 1935): citrated plasma is recalcified in the presence of a tissue extract and the time to clot formation is measured. The reagents and the clot-detection instruments have changed since then; the principle has not.

Blood is drawn into a light-blue-top tube containing sodium citrate, which binds calcium and stops clotting in the tube. The ratio of blood to citrate matters. An under-filled tube leaves extra citrate for the amount of plasma, and a very high hematocrit does the same because the sample holds less plasma. In a study of 28 patients with hematocrit values of 55% to 72%, PT and aPTT results in tubes with unadjusted citrate differed significantly from adjusted tubes and rose exponentially with the hematocrit; the authors concluded that citrate has to be adjusted in these patients for accurate results (Marlar 2006).

In the laboratory, platelet-poor plasma is separated by centrifugation and warmed to 37°C. A thromboplastin reagent — tissue factor (recombinant or from tissue extracts) with phospholipid — and calcium are added, and an optical or mechanical detector records the time to clot formation. That time, in seconds, is the PT.

Thromboplastin reagents differ in how strongly they respond to the clotting-factor changes caused by vitamin K antagonists, so the same patient sample can give different PT results in different laboratories. To standardize the test, the World Health Organization adopted a scheme in 1977 for calibrating thromboplastins against an International Reference Preparation, and in 1982 a revised calibration model (Kirkwood 1983). That calibration gives each reagent an International Sensitivity Index (ISI), which is used in the INR formula (Ageno 2012):

INR = (Patient PT ÷ Mean Normal PT)ISI

The mean normal PT is established by each laboratory from healthy people tested with the same reagent and instrument. A reagent with an ISI of 1.0 gives an INR equal to the plain PT ratio; the 2012 American College of Chest Physicians (ACCP/CHEST) review notes that highly sensitive thromboplastins with an ISI of about 1.0 are now available (Ageno 2012).

The INR was calibrated with plasma from people taking vitamin K antagonists, and its standardization rests on that use. Liver disease lowers clotting factors in a different pattern, and in a study of 57 patients with cirrhosis tested with 7 thromboplastins, mean INR values differed significantly between reagents (Tripodi 2007). A systematic review found insufficient evidence that a prolonged PT or raised INR predicts bleeding during invasive procedures (Segal 2005). The INR is a clotting-time measurement; its link to bleeding is best established in people taking warfarin.

Point-of-care analyzers measure the INR in a drop of fingerstick blood with a dried thromboplastin reagent and a device-specific calibration. The ACCP/CHEST review describes greater differences from laboratory plasma-based methods as the INR rises above the therapeutic range, and cases of incorrect ISI calibration of point-of-care instruments (Ageno 2012). See Point-of-Care INR Testing below.

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Normal and Therapeutic Ranges

MedlinePlus gives these general values for people who are not taking a blood thinner, and notes that normal ranges vary among laboratories:

For people taking warfarin, MedlinePlus states that the INR is most often kept from 2.0 to 3.0. The 2012 ACCP/CHEST guidelines make targeting an INR of 2.0 to 3.0 for patients on vitamin K antagonists one of their two strong recommendations (Holbrook 2012), and set higher targets for some mechanical heart valves:

The 2020 American College of Cardiology/American Heart Association (ACC/AHA) valve guideline also addresses anticoagulation for prosthetic valves (Otto 2020); the full text was not available for this review, so its targets are not restated here.

A range bar chart on one INR axis: MedlinePlus gives 0.8 to 1.1 for someone not on a blood thinner; the 2012 CHEST guidelines target 2.0 to 3.0 for venous thromboembolism, atrial fibrillation and a mechanical aortic valve, and 2.5 to 3.5 for a mechanical mitral valve; the 2019 EULAR recommendations list 2 to 3 or 3 to 4 after a first arterial thrombosis in antiphospholipid syndrome; and a 1994 case-control study found subdural hemorrhage risk rose steeply above a PT ratio of 2.0, approximately an INR of 4.0. THE TARGET DEPENDS ON THE INDICATION INR bands from MedlinePlus, the 2012 CHEST guidelines and the 2019 EULAR APS recommendations not on a blood thinner (MedlinePlus) 0.8–1.1 venous thromboembolism · atrial fibrillation mechanical aortic valve (target 2.5) 2.0–3.0 mechanical mitral valve, or valves in both positions (target 3.0) 2.5–3.5 antiphospholipid syndrome, first arterial thrombosis EULAR lists either band (dashed: 2–3) 3–4 subdural risk rose steeply above about 4.0 1.02.03.0 4.05.0 INR → READ IT AGAINST THE REASON the indication sets the band one result can sit inside the band for a mechanical mitral valve and above it after a DVT 0.8–1.1 off a blood thinner warfarin holds the INR above this range on purpose higher INR, more intracranial bleeding 1994 case-control study: each 0.5 rise in the PT ratio doubled intracerebral hemorrhage risk (odds ratio 2.1) an INR is not high or low on its own — only against the target its indication sets. the INR corrects the PT for reagent differences; the PT in seconds (MedlinePlus 11–13.5) stays laboratory-specific

How guidelines handle an INR above the target range (2012 ACCP/CHEST, Holbrook 2012):

The 2020 American College of Cardiology (ACC) Expert Consensus Decision Pathway covers the management of bleeding in people taking oral anticoagulants, including warfarin and the direct oral anticoagulants (Tomaselli 2020). Decisions about anticoagulant doses and reversal belong with the treating clinician.

The INR level is tied to bleeding risk. In a 1994 case-control study of 121 people taking warfarin who were hospitalized with intracranial hemorrhage, each 0.5 increase in the prothrombin time ratio (the study used the PT ratio, not the INR) doubled the risk of intracerebral hemorrhage (odds ratio 2.1), and the risk of subdural hemorrhage was unchanged across a PT ratio of 1.0 to 2.0 but rose steeply above a PT ratio of 2.0, approximately an INR of 4.0 (Hylek 1994).

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Pre-Procedure INR Thresholds

An INR above 1.5 is a threshold often cited before invasive procedures. It is a convention rather than a validated cut-off. A 2005 systematic review of one trial and 24 observational studies found insufficient evidence to conclude that abnormal coagulation test results predict bleeding during invasive diagnostic procedures such as bronchoscopy, central vein cannulation, arteriography and liver biopsy; in the largest arteriography study, bleeding rates were the same with and without abnormal tests (Segal 2005).

Thresholds for specific procedures are set by specialty societies and by individual hospitals, and they differ. In cirrhosis, the 2019 American Gastroenterological Association (AGA) clinical practice update states that commonly used thresholds for INR correction are not supported by evidence, and that in general coagulopathy is not routinely corrected before low-risk paracentesis, thoracentesis or routine endoscopic variceal ligation (O'Leary 2019).

For people with atrial fibrillation whose warfarin is stopped for an elective procedure, the BRIDGE trial of 1,884 patients found that forgoing heparin "bridging" was noninferior to bridging with low-molecular-weight heparin for arterial thromboembolism (0.4% vs 0.3%) and reduced major bleeding (1.3% vs 3.2%) (Douketis 2015). Urgent reversal before emergency surgery is covered under Urgent Reversal Strategies.

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Warfarin Therapy and INR Monitoring

Warfarin, a vitamin K antagonist, has been used as an oral anticoagulant for decades. It inhibits vitamin K epoxide reductase (VKORC1), the enzyme that recycles vitamin K to its active reduced form. Without active vitamin K, the gamma-carboxylation of Factors II, VII, IX and X — and of the natural anticoagulant proteins C and S — is impaired, and the under-carboxylated proteins (called PIVKA, proteins induced by vitamin K absence or antagonism) do not function normally in coagulation.

How fast warfarin works depends on how quickly the existing factors are cleared. The ACCP/CHEST review gives a half-life of about 6 hours for Factor VII, 6 to 24 hours for the other vitamin K-dependent factors, and 60 to 72 hours for prothrombin, and states that the antithrombotic effect of warfarin requires about 6 days of treatment, because it depends on lowering prothrombin (Ageno 2012). The INR can therefore rise, through the fall in Factor VII, before full antithrombotic effect is reached.

Protein C, a natural anticoagulant, is also vitamin K-dependent. Warfarin-induced skin necrosis has been reported in association with protein C and protein S deficiency, typically on the third to eighth day of therapy, with extensive thrombosis of small skin vessels (Ageno 2012).

Drug interactions with warfarin are common. The S-enantiomer of warfarin, 2.7 to 3.8 times more potent than the R-enantiomer, is cleared mainly by the liver enzyme CYP2C9; the R-enantiomer mainly by CYP1A2 and CYP3A4 (Ageno 2012). The ACCP/CHEST review describes:

Among herbs, St John's wort increased the clearance of both warfarin enantiomers and reduced warfarin's effect on the INR in a study in healthy volunteers, while ginseng did not (Jiang 2004).

Dietary vitamin K changes the response to warfarin. The ACCP/CHEST review states that people on long-term warfarin are sensitive to fluctuating dietary vitamin K, that green vegetables or vitamin K-containing supplements can supply enough to reduce the anticoagulant response, and that a consistent intake of vitamin K-containing foods is advisable, while neither specific restrictions nor additions seem necessary in people with stable control (Ageno 2012). The 2012 guideline suggests against routine vitamin K supplementation for people taking vitamin K antagonists (Holbrook 2012). A study in healthy volunteers found that menaquinone-7 supplements at 50 micrograms a day or more may interfere with oral anticoagulant treatment (Schurgers 2007).

Acute illness can change the INR. The ACCP/CHEST review notes that fever and hyperthyroidism increase the response to warfarin, probably by speeding the breakdown of vitamin K-dependent clotting factors (Ageno 2012).

Pharmacogenomics of warfarin: two genes account for much of the person-to-person difference in warfarin dose:

Trials of genotype-guided dosing have had mixed results. In the European EU-PACT trial of 455 patients, genotype-guided dosing gave a higher time in therapeutic range over the first 12 weeks (67.4% vs 60.3%) (Pirmohamed 2013); in the US COAG trial, time in range at 4 weeks was the same with a genotype-guided or a clinical algorithm (45.2% vs 45.4%) (Kimmel 2013); and in the GIFT trial in hip or knee replacement, genotype-guided dosing reduced a combined outcome of major bleeding, INR of 4 or greater, venous thromboembolism or death (10.8% vs 14.7%) (Gage 2017). The 2012 ACCP/CHEST guideline recommends against routine pharmacogenetic testing to guide warfarin doses (Holbrook 2012).

Time in therapeutic range (TTR) is the share of time a person's INR spends inside the target range, commonly calculated by the Rosendaal linear-interpolation method, which assumes the INR changes in a straight line between two measurements (Rosendaal 1993). The 2018 CHEST atrial fibrillation guideline states that, when a vitamin K antagonist is used, it is important to aim for good anticoagulation control with a TTR above 70% (Lip 2018).

Direct oral anticoagulants (DOACs) — the factor Xa inhibitors rivaroxaban, apixaban and edoxaban and the thrombin inhibitor dabigatran — do not need routine INR monitoring. For atrial fibrillation where oral anticoagulation is indicated, the 2018 CHEST guideline suggests a DOAC rather than adjusted-dose warfarin (Lip 2018), and for venous thromboembolism without cancer the 2016 CHEST guideline suggests a DOAC over a vitamin K antagonist (Kearon 2016). Warfarin keeps a role in some settings. A trial of dabigatran in people with mechanical heart valves was stopped early because of an excess of thromboembolic and bleeding events with dabigatran (Eikelboom 2013), and in triple-positive antiphospholipid syndrome the TRAPS trial found more events with rivaroxaban than with warfarin (Pengo 2018).

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PT as a Liver Function Test

The liver makes most clotting factors; Factor VIII and von Willebrand factor are notable exceptions, produced largely by the lining of blood vessels. Impaired liver synthesis — in acute liver failure, decompensated cirrhosis or severe hepatitis — lowers clotting factors and prolongs the PT, which is why the PT/INR is used as a measure of liver synthetic function.

The PT/INR is part of two prognostic scores in liver disease:

Because the INR is calibrated with plasma from people on vitamin K antagonists, it behaves differently in cirrhosis. In a study of 57 patients with cirrhosis whose PT was measured with 7 thromboplastins, the mean INR and MELD values differed significantly by reagent; when the thromboplastins were recalibrated with plasma from patients with cirrhosis (an "INRliver"), the differences disappeared (Tripodi 2007). The ACCP/CHEST review notes that a liver-specific INR has been proposed for this reason (Ageno 2012).

In cirrhosis the INR also misses part of the picture of clotting. Liver disease lowers natural anticoagulants as well as clotting factors, a "rebalanced" state described in a review of the coagulopathy of chronic liver disease (Tripodi 2011). Von Willebrand factor is usually elevated in cirrhosis (O'Leary 2019), and levels of the enzyme that cleaves von Willebrand factor (ADAMTS13) were low in people with decompensated cirrhosis (Mannucci 2001). Global tests of clot formation such as thromboelastography and rotational thromboelastometry may eventually have a role in this setting, but the 2019 AGA update states that they currently lack validated target levels (O'Leary 2019).

The same AGA update states that blood products are used sparingly in cirrhosis because they increase portal pressure and carry risks including transfusion-associated circulatory overload and transfusion-related acute lung injury; that the large volume of fresh frozen plasma needed to reach an arbitrary INR target, its small effect on thrombin generation and its effect on portal pressure limit its usefulness; and that for active bleeding or high-risk procedures in advanced liver disease, thresholds of a hematocrit of 25% or more, a platelet count above 50,000 and fibrinogen above 120 mg/dL may optimize clot formation (O'Leary 2019).

In acute liver failure, the PT is one of the King's College prognostic indicators. In a study of 588 patients, poor prognosis in acetaminophen-induced liver failure was indicated by an arterial pH below 7.30, a prothrombin time above 100 seconds and a creatinine above 300 µmol/L; in viral hepatitis and drug reactions, the indicators included a prothrombin time above 50 seconds and a bilirubin above 300 µmol/L, along with cause, age and the time from jaundice to encephalopathy (O'Grady 1989). Factor V, which is not vitamin K-dependent, was an independent predictor of survival in fulminant hepatitis B (Bernuau 1986).

Newborns have low vitamin K stores at birth, which is the background to the vitamin K given at birth (see Vitamin K and PT/INR) (Shearer 2009).

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Vitamin K and PT/INR

Vitamin K takes its letter from the German and Scandinavian spelling Koagulation, for its role in blood clotting; the Danish biochemist Henrik Dam identified it in the 1930s. It occurs in the diet in two main forms (Booth 2012):

Vitamin K is the cofactor for gamma-glutamyl carboxylase, the enzyme that adds carboxyl groups to specific glutamate residues on vitamin K-dependent proteins. In the process the active reduced form of vitamin K (the hydroquinone) is converted to vitamin K epoxide, which vitamin K epoxide reductase (VKORC1) recycles back to vitamin K. Warfarin blocks this recycling step.

Vitamin K deficiency affects the PT first because Factor VII has the shortest half-life of the vitamin K-dependent factors (about 6 hours, compared with 60 to 72 hours for prothrombin) (Ageno 2012). Early or mild deficiency can therefore prolong the PT while the aPTT is still normal; as Factors IX, X and prothrombin fall, the aPTT also lengthens. Fibrinogen and Factor V are not vitamin K-dependent, which helps distinguish vitamin K deficiency from DIC, where fibrinogen is consumed. Heparin, by contrast, prolongs the aPTT much more than the PT.

Recognized causes of vitamin K deficiency include:

On the treatment side, the ACCP/CHEST review reports that oral and low-dose intravenous phytonadione (vitamin K1) produce similar INR reductions at 24 hours, that subcutaneous phytonadione appears less effective than low-dose oral phytonadione, and that after vitamin K is given for bleeding, the INR begins to fall within 2 hours and generally returns to normal within 24 hours if liver function is normal (Ageno 2012).

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Factor V Leiden and Coagulation Genetics

Factor V Leiden is a single change in the Factor V gene at nucleotide 1691 (G to A) that produces a Factor V molecule (FV Q506) that activated protein C does not inactivate properly. Activated protein C normally limits clotting by breaking down Factors Va and VIIIa; with the Leiden variant, Factor Va resists this, and clotting is less well switched off. In the original 1994 report, the allele frequency in the Dutch population was about 2%, at least ten times higher than all other genetic risk factors for thrombosis then known combined, and the resistance to activated protein C that it causes was associated with a sevenfold increase in the risk of deep vein thrombosis (Bertina 1994).

The risk adds to other risks. In a study of premenopausal women aged 15 to 49, oral contraceptive use increased the risk of venous thrombosis about 4-fold (relative risk 3.8), carrying Factor V Leiden about 8-fold (7.9), and both together more than 30-fold (34.7) compared with women with neither (Vandenbroucke 1994). Decisions about testing and treatment in people with inherited thrombophilia belong with a clinician.

The PT and INR are typically normal in people with Factor V Leiden, because the variant affects how clotting is switched off rather than how fast a clot forms. It is identified with a functional activated protein C resistance assay, based on the aPTT with and without added activated protein C, or by genetic testing.

Other inherited thrombophilias relevant to coagulation testing:

Timing matters when these proteins are measured. Acute thrombosis, pregnancy, hormonal contraception, acute illness and anticoagulants can all change protein C, protein S and antithrombin levels; warfarin lowers protein C and protein S along with the vitamin K-dependent clotting factors. Genetic tests for Factor V Leiden and prothrombin G20210A are not affected by these conditions.

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Point-of-Care INR Testing

Point-of-care INR monitors let people taking warfarin test at home or in a clinic from a fingerstick drop of whole blood. A test strip carries dried thromboplastin reagent; the device detects clot formation in the blood sample and converts the clotting time to an INR using a device-specific calibration.

Accuracy: the ACCP/CHEST review reports greater differences from laboratory plasma-based methods as the INR rises above the therapeutic range, and problems from incorrect ISI calibration. In a proficiency-testing study of three monitors over 6 years in more than 10 centers, INR results in 10% to 11% of centers were more than 15% different from results in other centers using the same monitors, and a European evaluation of 523 monitors of one widely used model at nine clinics found significant deviations in 20.3% (Ageno 2012). In people with antiphospholipid antibodies, most point-of-care results were close to the laboratory method, but in a subset of patients with both lupus anticoagulant and elevated anti-β2-glycoprotein I antibodies, one device gave no measurable INR and another gave higher INR values than the plasma-based method (Perry 2005).

Self-testing and self-management — testing at home, with or without adjusting the dose by a set schedule — have been studied in randomized trials:

For motivated patients who demonstrate competency in self-management and with the equipment, the 2012 ACCP/CHEST guideline suggests patient self-management rather than usual outpatient INR monitoring (Holbrook 2012).

Limits of point-of-care INR testing:

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Causes of Prolonged PT/INR

A prolonged PT/INR is read together with the aPTT, the fibrinogen level and the clinical setting:

Isolated PT prolongation (aPTT normal, fibrinogen normal):

Both PT and aPTT prolonged (fibrinogen normal or high):

Both PT and aPTT prolonged with low fibrinogen and high D-dimer:

Falsely prolonged PT (sample problems):

The mixing study: when the cause of a prolonged PT or aPTT is unclear, the patient's plasma is mixed 1:1 with normal pooled plasma and the test is repeated. If the result corrects, a factor deficiency is likely; if it stays prolonged, an inhibitor or other interference is likely (Kershaw 2013). Time-dependent inhibitors such as acquired Factor VIII antibodies may correct on the immediate mix and fail to correct after incubation. The aPTT page describes the procedure in detail.

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aPTT: The Intrinsic Pathway Partner Test

The PT is rarely read alone. Its partner, the activated partial thromboplastin time (aPTT), measures the intrinsic pathway (Factors XII, XI, IX and VIII) together with the same common pathway the PT covers (X, V, II and fibrinogen). Reading the two together turns a prolonged clotting time into a short list of causes.

The aPTT adds three things to citrated plasma: a contact activator (such as kaolin, ellagic acid or silica — the "activated" in the name), phospholipid without tissue factor (the "partial thromboplastin"), and calcium. MedlinePlus gives a general aPTT range of 25 to 35 seconds and notes that normal value ranges vary among laboratories.

A prolonged aPTT with a normal PT points to the intrinsic pathway:

Direct oral anticoagulants affect both tests unpredictably. For dabigatran, the aPTT and PT are less sensitive than drug-specific tests and may be normal at trough levels; for rivaroxaban and apixaban, the PT is less sensitive (especially for apixaban), and a normal PT may not exclude clinically relevant drug levels (Cuker 2014). Drug-specific anti-Xa or anti-IIa assays are the preferred methods when a quantitative result is useful (Douxfils 2018).

Taken together: a prolonged PT with a normal aPTT points to Factor VII (warfarin, early vitamin K deficiency, early liver disease); a prolonged aPTT with a normal PT points to the intrinsic factors, heparin or lupus anticoagulant; both prolonged point to the common pathway, advanced liver failure, severe vitamin K deficiency, a direct oral anticoagulant or — when fibrinogen is low and D-dimer high — DIC. When a result is unexpected, heparin contamination of the sample and other sample problems are usually considered before a mixing study.

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Urgent Reversal Strategies

When warfarin-associated coagulopathy causes life-threatening bleeding, or an operation cannot wait, the effect of the anticoagulant has to be reversed quickly. The 2012 ACCP/CHEST guidelines (Holbrook 2012) and the 2020 ACC Expert Consensus Decision Pathway on bleeding in people taking oral anticoagulants (Tomaselli 2020) address these situations.

4-Factor Prothrombin Complex Concentrate (4F-PCC)

4-factor prothrombin complex concentrate is a plasma-derived concentrate of the four vitamin K-dependent clotting factors (II, VII, IX and X) together with the anticoagulant proteins C and S. The ACCP/CHEST review notes that prothrombin complex concentrates do not need cross-matching, are virally inactivated, do not pose a risk of volume overload, and can be infused in 15 to 30 minutes (Ageno 2012). In a randomized trial of 202 people on vitamin K antagonists with major bleeding (median starting INR 3.90), effective hemostasis at 24 hours occurred in 72.4% with 4F-PCC and 65.4% with plasma, meeting the test for noninferiority, and rapid INR correction (to 1.3 or below 30 minutes after the infusion) in 62.2% versus 9.6%; adverse events, thromboembolic events and deaths were similar between groups (Sarode 2013). For major bleeding on a vitamin K antagonist, the 2012 guideline suggests rapid reversal with 4F-PCC rather than plasma, together with vitamin K 5 to 10 mg by slow intravenous injection (Holbrook 2012); the infused factors are cleared over time while warfarin's effect persists, and vitamin K restores the liver's own production of the factors.

Fresh Frozen Plasma (FFP)

Plasma contains every clotting factor, including Factor V, which prothrombin complex concentrate does not, but at ordinary rather than concentrated levels. It has to be thawed and blood-group compatible, and correcting the INR takes a large volume, which brings the risks of volume overload and transfusion-related acute lung injury. In the Sarode trial, rapid INR correction occurred in 9.6% of people given plasma, compared with 62.2% with 4F-PCC (Sarode 2013). Plasma is also part of balanced transfusion in massive bleeding (Holcomb 2015).

Vitamin K1 (Phytonadione)

Vitamin K1 does not replace clotting factors; it lets the liver make working factors again, so its effect is delayed but lasting. The ACCP/CHEST review reports that when oral phytonadione is given while warfarin is held, about 1.4 days are needed for an INR between 6 and 10 to fall below 4.0; that low-dose intravenous and oral phytonadione give similar INR reductions at 24 hours; that subcutaneous phytonadione appears less effective than low-dose oral phytonadione; and that after vitamin K for bleeding, the INR begins to fall within 2 hours and generally normalizes within 24 hours if liver function is normal (Ageno 2012). The 2012 guideline's suggestions by INR level and bleeding are listed under Normal and Therapeutic Ranges (Holbrook 2012).

Reversal of Direct Oral Anticoagulants

The INR is not a measure of direct oral anticoagulant effect. Rivaroxaban, apixaban and edoxaban (factor Xa inhibitors) and dabigatran (a thrombin inhibitor) prolong the PT and aPTT to a variable degree, and a normal PT may not exclude clinically relevant levels of rivaroxaban or apixaban (Cuker 2014; Douxfils 2018). Agent-specific reversal drugs exist:

An expert guidance document from 2012, written when there was no established antidote, suggested supportive care, activated charcoal if the drug had been taken within a couple of hours, and hemodialysis if feasible for dabigatran, and discussed the possible use of 4F-PCC (Kaatz 2012).

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Key Research Papers

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  3. Kearon C, Akl EA, Comerota AJ, et al. Antithrombotic therapy for VTE disease. Chest. 2012;141(2 Suppl):e419S–e496S. — PubMed PMID: 22315268
  4. Kaatz S, Kouides PA, Garcia DA, et al. Guidance on the emergent reversal of oral thrombin and factor Xa inhibitors. Am J Hematol. 2012;87(Suppl 1):S141–S145. — PubMed PMID: 22473649
  5. Tripodi A, Mannucci PM. The coagulopathy of chronic liver disease. N Engl J Med. 2011;365(2):147–156. — PubMed PMID: 21751907
  6. Schulman S, Kearon C, Kakkar AK, et al. Dabigatran versus warfarin in the treatment of acute venous thromboembolism. N Engl J Med. 2009;361(24):2342–2352. — PubMed PMID: 19966341
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  9. Fihn SD, Gardin JM, Abrams J, et al. 2012 ACCF/AHA/ACP/AATS/PCNA/SCAI/STS guideline for the diagnosis and management of patients with stable ischemic heart disease. J Am Coll Cardiol. 2012;60(24):e44–e164. Reference for warfarin AF management: Wann LS et al. 2011 ACCF/AHA/HRS focused update. J Am Coll Cardiol. 2011;57(2):223–242. — PubMed PMID: 23182125
  10. Connolly SJ, Ezekowitz MD, Yusuf S, et al. Dabigatran versus warfarin in patients with atrial fibrillation. N Engl J Med. 2009;361(12):1139–1151. — PubMed PMID: 19717844
  11. Seligsohn U, Lubetsky A. Genetic susceptibility to venous thrombosis. N Engl J Med. 2001;344(16):1222–1231. — PubMed PMID: 11309638
  12. Tripodi A. The international normalized ratio. Clin Chem. 2007;53(7):1260–1261. — Search PubMed (Editorial note: no record matching this citation was found in PubMed or Crossref in 2026; for the INR in liver disease see Tripodi 2007, Hepatology, below.)
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