Abstract

Severe deficiency of vitamin K–dependent proteins in patients not maintained on vitamin K antagonists is most commonly associated with poisoning by or surreptitious ingestion of warfarin, warfarin-like anticoagulants, or potent rodenticides (“superwarfarins”), such as brodifacoum. Serious bleeding manifestations are common. Superwarfarins are 2 orders of magnitude more potent than warfarin and have a half-life measured in weeks. These rodenticides are readily available household environmental hazards and are sometimes consumed accidentally or as manifestations of psychiatric disease. Immediate diagnosis and proper therapy is critically important to minimize morbidity and mortality because this condition, affecting thousands of patients annually, is reversible. Treatment with large doses of oral vitamin K1, often over months to years, to maintain a near-normal prothrombin time can reverse the coagulopathy associated with superwarfarins. Although these patients initially present to various medical specialties, the hematologist is often consulted to offer the definitive diagnosis and proper therapy.

Introduction

The onset of severe bleeding in a previously well patient or the identification of laboratory abnormalities indicating a high risk of severe bleeding can be a challenging presentation to the hematologist. Although the differential diagnosis can be extensive, the rapid determination of the presence of a prolonged prothrombin time (PT) and partial thromboplastin time (PTT) followed by demonstration of specific deficiency of vitamin K–dependent blood coagulation protein activities greatly narrows the diagnostic possibilities. Given the life-threatening nature of this disorder and its complete resolution with appropriate therapy, rapid and accurate diagnosis is critical.

Case 1

A 67-year-old man with a history of coronary artery disease previously treated with coronary artery stenting and for hypertension, hyperlipidemia, and depression presented with 2 days of epistaxis and hematuria. Medications included aspirin, clopidogrel, atorvastatin, labetalol, hydrochlorothiazide, lisinopril, isosorbide mononitrate, and venlafaxine. Clinical examination was notable only for blood in both nares and the absence of petechiae or purpura. His initial PTT was 92.2 seconds and PT and international normalized ratio (INR) were prolonged beyond the measured limit of 160 seconds and 14, respectively. The complete blood count was normal and no schistocytes were evident. His nares were cauterized and packed, and hemostasis was achieved with fresh frozen plasma (6 U) and vitamin K (30 mg). At transfer, his coagulopathy persisted with a PTT of 129.8 seconds, a PT of 135.8 seconds, and an INR of 17.9. Inhibitor screens were negative, thrombin time was normal, and specific coagulation factor assays were notable for prothrombin (0.11 IU/mL), factor VII (<0.01 IU/mL), factor IX (0.04 IU/mL), and factor X (0.10 IU/mL). Factor V (1.33 IU/mL), factor VIII (1.66 IU/mL), and antithrombin III (1.1 IU/mL) were normal. His PT and PTT were normal within the preceding year.

Case 2

A 48-year-old woman with a history of photosensitivity and fibromyalgia noted the onset of hematuria, ecchymoses, and hematomas on her wrist, face, lower back, and ankles. She complained of pain, weakness, and dizziness. She had no prior bleeding history, including after surgical procedures and dental extractions. Medications included hydroxychloroquine, gabapentin, and duloxetine. Her hemoglobin level was 7.4, hematocrit 22, PT >160 seconds, INR 15, PTT >200 seconds, and a platelet count of 332 000. Inhibitor screens were negative, her thrombin time was 14.2 seconds, and specific coagulation factor assays were notable for prothrombin (0.02 IU/mL), factor VII (0.07 IU/mL), factor IX (0.04 IU/mL), and factor X (0.05 IU/mL). Factor V (1.34 IU/mL), factor VIII (1.57 IU/mL), and fibrinogen (382 mg/dL) were normal.

Comments

Both cases demonstrate a potentially life-threatening coagulation disorder associated with deficiencies of vitamin K–dependent proteins. The reduced hydroquinone form of vitamin K1 is required for the posttranslational γ-carboxylation1,2  of select glutamic acid residues in the precursor forms of the 4 procoagulant vitamin K–dependent proteins: prothrombin, factor VII, factor IX, and factor X.3  Modification of these proteins enables them to chelate calcium and bind to anionic membranes exposed at sites of tissue injury.4  Vitamin K hydroquinone is oxidized to vitamin K epoxide during γ-carboxylation.5  Because mammals are unable to synthesize vitamin K de novo, vitamin K epoxide must be recycled to its hydroquinone form to complete the vitamin K cycle (Figure 1). The reductive branch of the cycle in hepatocytes is predominantly catalyzed by the vitamin K epoxide reductase (VKOR) encoded by VKORC1.6,7  The electrons to reduce vitamin K are supplied by the reduced dithiol motif within thioredoxin-like proteins.8-11  Vitamin K antagonists such as warfarin inhibit VKOR to disrupt the vitamin K cycle, resulting in undercarboxylation of vitamin K–dependent proteins and lowering the functional levels of vitamin K–dependent blood coagulation proteins.

Figure 1

Vitamin K cycle. Vitamin K hydroquinone (KH2) is oxidized to vitamin K epoxide (KO) by the vitamin K-dependent γ-glutamyl carboxylase (GGCX) with concomitant carboxylation of glutamic acid to γ-carboxyglutamic acid (GLA) in PT and other vitamin K–dependent proteins. VKOR uses electrons from a thioredoxin-like protein dithiol to reduce vitamin K epoxide to vitamin K quinone (K) and then vitamin K hydroquinone to complete the vitamin K cycle. VKOR is sensitive to inhibition by warfarin, brodifacoum, and other vitamin K antagonists. A NADPH-dependent pathway catalyzed by NQO1 and related enzymes may help to rescue blood coagulation after coumarin poisoning. NAD(P)+, NAD phosphate.

Figure 1

Vitamin K cycle. Vitamin K hydroquinone (KH2) is oxidized to vitamin K epoxide (KO) by the vitamin K-dependent γ-glutamyl carboxylase (GGCX) with concomitant carboxylation of glutamic acid to γ-carboxyglutamic acid (GLA) in PT and other vitamin K–dependent proteins. VKOR uses electrons from a thioredoxin-like protein dithiol to reduce vitamin K epoxide to vitamin K quinone (K) and then vitamin K hydroquinone to complete the vitamin K cycle. VKOR is sensitive to inhibition by warfarin, brodifacoum, and other vitamin K antagonists. A NADPH-dependent pathway catalyzed by NQO1 and related enzymes may help to rescue blood coagulation after coumarin poisoning. NAD(P)+, NAD phosphate.

The differential diagnosis at this stage in both cases is limited to: (1) vitamin K deficiency; (2) hereditary deficiency of either the vitamin K–dependent carboxylase or the vitamin K epoxide reductase; or (3) accidental or surreptitious ingestion of a vitamin K antagonist, either pharmacologic (eg, warfarin, phenprocoumon) or a potent rodenticide (“superwarfarin”).

Vitamin K deficiency from inadequate intake or malabsorption

Vitamin K deficiency is usually obvious from a patient’s clinical history. Bleeding from vitamin K deficiency during days 2 to 7 after birth represents the classic hemorrhagic disease of the newborn for which it is now routine practice to administer prophylactic vitamin K at birth.12  In the developed world, frank malnutrition is seen in vulnerable populations including children and the elderly, but clinically significant vitamin K deficiency presenting with bleeding is rare because the daily requirement for vitamin K is very low (only about 100 μg). Healthy subjects who maintained a diet devoid of vitamin K for up to 40 days had subtle accumulation of des-γ-carboxyprothrombin, an undercarboxylated nonfunctional form of prothrombin,13,14  but no prolongation of the prothrombin time despite depletion of serum vitamin K.15,16  A variety of gastrointestinal malabsorption disorders, including pancreatic insufficiency, sprue, and short gut syndrome, may result in malabsorption of vitamin K, a fat-soluble vitamin, despite adequate nutrition. Prolongation of the prothrombin time may be seen in chronically ill and hospitalized patients with nutritional deficiency who are concurrently treated with antibiotics. Broad-spectrum antibiotics decrease production of menaquinone (vitamin K2) by the commensal bacteria of the gut17  and contribute to significant hypoprothrombinemia.18,19  Decreased activity of vitamin K–dependent proteins is observed in liver disease resulting from decreased protein synthesis. Additional coagulation protein activities such as factor V are also depressed.20  An accompanying minor defect in γ-carboxylation of the vitamin K–dependent proteins is of no hemostatic significance.13 

Hereditary deficiency of either vitamin K–dependent carboxylase or VKOR

Inherited defects must be considered, but can usually be eliminated by prior history of normal coagulation studies or surgical challenge (including tooth extraction) without bleeding or the absence of a significant bleeding history. Although deficiency of individual blood clotting factors may result in prolongation of routine coagulation assays, these defects do not result in deficiency of all vitamin K–dependent proteins. Combined vitamin K–dependent clotting factor deficiency, a rare genetic disorder, is not characterized by a familial bleeding history because it is an autosomal recessive disorder. Congenital deficiency exists in 2 complementation groups: those with mutations in the gene GGCX encoding the vitamin K–dependent γ-carboxylase (VKDCFD type I)21  and those with mutations in the VKORC1 gene encoding the vitamin K epoxide reductase paralog most highly expressed in liver (VKDCFD type II).6,7 

Overdose, accidental, or surreptitious ingestion of a vitamin K antagonist

The most common cause of isolated deficiency of vitamin K–dependent coagulation proteins is the presence of a vitamin K antagonist. There are 2 classes of vitamin K antagonists: (1) pharmacologic agents used as oral antithrombotics and (2) rodenticides used to control mouse and rat populations. Both groups carry risks to human health. Pharmacologic agents such as warfarin are widely used for chronic anticoagulation of patients with a variety of cardiovascular diseases. Either from difficulties in compliance, errors in INR monitoring, exacerbation of heart failure with concomitant hepatic congestion, or concurrent ingestion of alcohol or drugs that enhance the pharmacologic action of oral anticoagulants, patients taking warfarin or other oral vitamin K antagonist medications can demonstrate excessive levels of the INR above that considered therapeutic. This results in marked deficiency of vitamin K–dependent blood coagulation proteins. Rates of major bleeding are estimated at about 1% annually in all patients taking warfarin therapeutically,22  and those with an excessively high INR have a much higher rate of bleeding.23  Given the broad availability of warfarin, accidental ingestion among children and the elderly is also common. “Factitious purpura” is a clinical variant of Munchausen’s syndrome resulting from intentional ingestion of coumarin agents for secondary gain, and warfarin is frequently ingested by psychiatric patients attempting suicide.24  Poisoning with warfarin itself accounted for 3777 reports to poison control centers in 2012, although these ingestions are underreported because of the delay between exposure and presentation.25 

Warfarin is also used as a rodenticide. Because rodent populations have developed increasing resistance to warfarin through mutations in VKORC1, “superwarfarins” have been developed that are potent VKOR inhibitors with longer half-lives.26,27  Brodifacoum (the active ingredient of D-Con) is the most commonly encountered rodenticide in the United States, but there are many superwarfarins including coumatetralyl and a growing family of indanediones (eg, bromadiolone, diphenadione). These superwarfarins are not for human use and have the potential to induce a profound and sustained coagulopathy when ingested. Nevertheless, there were 9555 reported superwarfarin poisonings in 2012.25 

Differential diagnosis of vitamin K–dependent protein deficiency

Reviewing cases 1 and 2, we can consider the various diagnostic possibilities and focus on the most likely issue. Given their age and absence of malabsorption, recent hospitalization, and medication history, it is apparent that neither of these patients is deficient in vitamin K. The absence of a bleeding history despite surgical intervention argues strongly against a hereditary basis for the deficiency of vitamin K–dependent proteins. This leads to a focus on the ingestion of a vitamin K antagonist. Neither patient had been prescribed an anticoagulant, so the ingestion must be either accidental or surreptitious. In case 1, given the deficiency of the vitamin K–dependent coagulation factors and persistence of coagulopathy despite 30 mg of vitamin K, the presence of a long-acting vitamin K antagonist rodenticide was presumed. This patient was then started empirically on high-dose oral vitamin K of 75 mg by mouth twice daily, with partial correction to an INR <2 after 1 week and a normal INR of 1.1 after 4 months of oral vitamin K therapy. Toxicologic studies subsequently confirmed the presence of serum brodifacoum. Despite extensive investigation and psychiatric evaluation, the basis of brodifacoum ingestion was never identified.

In case 2, supportive care involved transfusion of fresh frozen plasma given the severe anemia and significant bleeding. Superwarfarin ingestion was again suspected, and the patient was treated with vitamin K1, 100 mg orally daily, with correction of PT and PTT within 2 days. The results of an anticoagulant poisoning panel were positive for brodifacoum and negative for warfarin, dicumarol, chlorophacinone, difenacoum, and bromadiolone. The basis of brodifacoum ingestion was also never identified.

Treatment of isolated deficiency of vitamin K–dependent proteins

Step 1: Initiate steps toward making the diagnosis.

With knowledge of the prolonged PT and PTT, mixing studies to rule out an inhibitor of blood coagulation and a screen for disseminated intravascular coagulation (fibrinogen, d-dimer, complete blood count including platelet count) should be followed sequentially by an assay of prothrombin, factor X, factor VII, factor IX, and then factor V. With deficiency of only vitamin K–dependent proteins and preservation of factor V and fibrinogen, the diagnosis of specific deficiency of vitamin K–dependent proteins is secure. Fibrinogen and its degradation products will be normal, distinguishing disruption of the vitamin K cycle from liver failure. The thrombin time is also not affected. Mixing studies will not demonstrate an inhibitor. Complete blood counts and peripheral blood smear will be normal unless there is anemia to parallel significant bleeding. It is helpful to know the vitamin K antagonist used, but it is rarely possible to elicit this information from the patient. A serum warfarin level should be sent to a reference laboratory and an additional sample should be frozen for later evaluation with an anticoagulant poisoning panel. If warfarin is undetectable, it should be assumed that a superwarfarin is responsible.

Step 2: Manage severe bleeding.

If clinically significant major bleeding is present,28  then 10 mg of vitamin K should be administered intravenously in conjunction with prothrombin complex concentrate, fresh frozen plasma, or frozen plasma because even modest elevation of vitamin K–dependent proteins may control hemorrhage. However, even partial correction of the prothrombin time requires de novo synthesis of vitamin K–dependent proteins, with alterations of the prothrombin time measured in days even in the absence of vitamin K antagonists. If a superwarfarin is suspected, then empiric high-dose oral vitamin K should also be initiated (refer to step 3). In those with severe or life-threatening bleeding, a 4-factor prothrombin complex concentrate such as Kcentra (Kcentra in the United States; Beriplex in other jurisdictions; Octaplex is another similar prothrombin complex concentrate with Food and Drug Administration approval) should be administered.29  Frozen plasma is an alternative if the prothrombin complex concentrates are not readily available. There is little evidence to support the use of recombinant factor VIIa for this indication.30  In the absence of bleeding, a prolonged prothrombin time is not an indication for fresh frozen plasma or frozen plasma. In the bleeding patient, these agents will help to achieve hemostasis and provide time for vitamin K to reverse the effect of the antagonist.

Step 3: Reverse the effect of the vitamin K antagonist with vitamin K.

The treatment of vitamin K–dependent protein deficiency resulting from warfarin and that resulting from brodifacoum or superwarfarins is dramatically different. Unless there is a clear history of specific ingestion or access to a rapid toxicology screen, one must usually initiate treatment on suspicion of the diagnosis even before it is confirmed.

Several guidelines are available to guide reversal of the supratherapeutic INR resulting from warfarin intoxication.29,31,32  The approach to reversal should depend upon the urgency and the degree of INR elevation. In the absence of clinically significant bleeding and an elevated INR between 5 and 9, discontinuing the vitamin K antagonist with or without administration of 1 to 2.5 mg oral vitamin K is generally sufficient. Oral vitamin K (5 to 10 mg) may be considered if the INR is >9. The administration of vitamin K, however, will interfere with getting the patient back into a stable therapeutic range. In the case of accidental or surreptitious ingestion, high-dose oral therapy is preferable because there is no subsequent need to establish a therapeutic INR. Although 5 mg oral vitamin K and 1 mg intravenous vitamin K are generally equivalent at 24 hours,33  intravenous vitamin K has a more rapid effect, making it preferable when bleeding is present or urgent intervention is required. There is a risk of anaphylaxis from intravenous vitamin K, but this risk has been mitigated in modern preparations and can be further reduced by diluted administration over 30 minutes.34 

Superwarfarins are far more resistant to reversal of their effects. Furthermore, the actual agent and the amount ingested are rarely known on presentation. The goal is to administer vitamin K orally and at the lowest dose possible to maintain a PT in the normal range. However, the long-term effects of large doses of vitamin K are unknown. Often, in the case of serious bleeding manifestations, after control of the bleeding with prothrombin complex concentrate or frozen plasma, intravenous vitamin K is administered at doses of 50 mg/day or higher divided into 4 infusions over 30 min/day. After obtaining a response, with significant shortening of the prothrombin time, intravenous vitamin K1 can be replaced with oral vitamin K1. The dose of oral vitamin K necessary to rescue blockade of VKOR will vary by case and must be empirically determined in each patient via titration based on the prothrombin time. A low total daily dose of oral vitamin K1 may be 25 mg daily, a typical dose on the order of 100 mg daily35 ; to our knowledge the highest reported dose is 400 mg daily.36  Measurements of the plasma half-life of brodifacoum in humans vary, but estimates range from 16 to 36 days.35,37  Because brodifacoum is fat-soluble, has a very large volume of distribution, is concentrated in hepatocytes, and is 2 orders of magnitude more potent than warfarin,27,38,39  disruption of the vitamin K cycle can exist far beyond the detection of serum levels, with hemostatic defects extending in some cases beyond a year of ingestion of the superwarfarin.35  Serial serum brodifacoum levels can be used to approximate elimination kinetics,36  but these levels do not necessarily correlate with tissue levels. Based on a longitudinal study of 3 cases in which the prothrombin time and vitamin K epoxide:vitamin K1 ratio were measured for months to years until a “cure” was achieved,35  we favor initiating a slow taper of vitamin K dose with close monitoring of the PT to keep the PT normal or near normal—to eliminate increased bleeding risk. Vitamin K can be discontinued as soon as it is no longer required to maintain a near-normal PT, but typically treatment will extend from 3 to 6 months and sometimes more than a year.

Step 4: Long-term evaluation.

With the serum samples obtained at presentation, the evaluation can be completed by assay for superwarfarins, specifically brodifacoum. The finding of brodifacoum in the serum establishes the diagnosis. When warfarin, brodifacoum, and other superwarfarins are undetectable, one of the less commonly ingested superwarfarins must be assumed to be the culprit.

Additional testing is desirable for academic interest, but not required. An especially sensitive measure of vitamin K deficiency or the presence of a vitamin K antagonist is the accumulation of the nonfunctional, abnormal prothrombin des-γ-carboxy prothrombin, also known as protein induced by vitamin K absence or PIVKA-II.13,14  γ-Carboxyglutamic acid–containing peptides are stoichiometrically excreted in urine, and a decrease in γ-carboxyglutamic acid–containing peptides can be readily detected when a vitamin K antagonist is present.40 

Determination of serum vitamin K1 quinone levels by high-performance liquid chromatography is available in reference laboratories to discriminate states of vitamin K deficiency (eg, malnutrition, malabsorption, antibiotic use) from genetic or chemical blockade of the vitamin K cycle. The reference range for serum vitamin K is 0.10 to 2.20 ng/mL, but it remains unclear what lower limit should be recommended.41  Disruption of the vitamin K cycle by vitamin K antagonists results in a dramatically increased vitamin K1 epoxide to vitamin K1 quinone ratio in serum, but currently there is no commercially available assay for measurement of the epoxide form.

Step 5: Psychosocial evaluation.

Evaluation of a patient with suspected vitamin K antagonist ingestion must include a thorough psychosocial assessment. Psychiatry should be consulted because attempted suicide and factitious purpura must be considered. Social services or law enforcement should also be involved to evaluate abuse, intentional poisoning, home living conditions, home safety for both young children and the elderly, and other potential sources of exposure.

Biochemistry of vitamin K rescue

Vitamin K is an effective antidote for poisoning with a vitamin K antagonist.42,43  There are 2 distinct enzymatic activities capable of reducing vitamin K1 quinone to the hydroquinone form.44,45  Pathway I is the dithiol-driven activity now known to be catalyzed by VKOR,6,7  whereas pathway II is a reduced NAD phosphate (NADPH)-dependent activity contributed in part by the flavoprotein NADPH:quinone oxidoreductase 1 (NQO1).46-48  NQO1 is inhibited by dicoumarol49  and is much less sensitive to warfarin or superwarfarin inhibition than is VKOR.47,50,51  Although pathway II does not appear to play a significant role in vitamin K metabolism under physiologic conditions, it remains a critical mechanism by which hemostasis is rescued by high-dose vitamin K therapy in the setting of vitamin K antagonist poisoning.47  NQO1-deficient mice have no hemostatic defect and warfarin-induced coagulopathy in these animals can be corrected with vitamin K,52  suggesting that the actual rescue pathway to generate vitamin K hydroquinone may be partially catalyzed by a still unknown coumarin-resistant, NADPH-dependent vitamin K reductase.48 

Conclusion

Coagulopathy resulting from ingestion of potent vitamin K antagonist rodenticides is a common and reversible condition. Clinicians and in particular hematologists must be aware and consider this diagnosis early to prevent inadequate treatment that exposes the patient to serious bleeding risk and potentially death.

Acknowledgments

The authors thank Drs Judith Ratzan and Daniel Dammrich (University of Miami) and Dr Maureen Okam (Brigham & Women’s Hospital) for bringing these cases to our attention and Drs Judith Ratzan and Gregg Furie for critical comments on the manuscript.

Authorship

Contribution: S.S. and B.F. wrote and edited the manuscript.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Bruce Furie, Beth Israel Deaconess Medical Center, Harvard Medical School, Center for Life Science, Room 903, 3 Blackfan Circle, Boston, MA 02115; e-mail: bfurie@bidmc.harvard.edu.

References

References
1
Stenflo
J
Fernlund
P
Egan
W
Roepstorff
P
Vitamin K dependent modifications of glutamic acid residues in prothrombin.
Proc Natl Acad Sci USA
1974
, vol. 
71
 
7
(pg. 
2730
-
2733
)
2
Nelsestuen
GL
Zytkovicz
TH
Howard
JB
The mode of action of vitamin K. Identification of gamma-carboxyglutamic acid as a component of prothrombin.
J Biol Chem
1974
, vol. 
249
 
19
(pg. 
6347
-
6350
)
3
Furie
B
Bouchard
BA
Furie
BC
Vitamin K-dependent biosynthesis of gamma-carboxyglutamic acid.
Blood
1999
, vol. 
93
 
6
(pg. 
1798
-
1808
)
4
Soriano-Garcia
M
Padmanabhan
K
de Vos
AM
Tulinsky
A
The Ca2+ ion and membrane binding structure of the Gla domain of Ca-prothrombin fragment 1.
Biochemistry
1992
, vol. 
31
 
9
(pg. 
2554
-
2566
)
5
Wood
GM
Suttie
JW
Vitamin K-dependent carboxylase. Stoichiometry of vitamin K epoxide formation, gamma-carboxyglutamyl formation, and gamma-glutamyl-3H cleavage.
J Biol Chem
1988
, vol. 
263
 
7
(pg. 
3234
-
3239
)
6
Li
T
Chang
CY
Jin
DY
Lin
PJ
Khvorova
A
Stafford
DW
Identification of the gene for vitamin K epoxide reductase.
Nature
2004
, vol. 
427
 
6974
(pg. 
541
-
544
)
7
Rost
S
Fregin
A
Ivaskevicius
V
, et al. 
Mutations in VKORC1 cause warfarin resistance and multiple coagulation factor deficiency type 2.
Nature
2004
, vol. 
427
 
6974
(pg. 
537
-
541
)
8
Silverman
RB
Nandi
DL
Reduced thioredoxin: a possible physiological cofactor for vitamin K epoxide reductase. Further support for an active site disulfide.
Biochem Biophys Res Commun
1988
, vol. 
155
 
3
(pg. 
1248
-
1254
)
9
Wajih
N
Hutson
SM
Wallin
R
Disulfide-dependent protein folding is linked to operation of the vitamin K cycle in the endoplasmic reticulum. A protein disulfide isomerase-VKORC1 redox enzyme complex appears to be responsible for vitamin K1 2,3-epoxide reduction.
J Biol Chem
2007
, vol. 
282
 
4
(pg. 
2626
-
2635
)
10
Li
W
Schulman
S
Dutton
RJ
Boyd
D
Beckwith
J
Rapoport
TA
Structure of a bacterial homologue of vitamin K epoxide reductase.
Nature
2010
, vol. 
463
 
7280
(pg. 
507
-
512
)
11
Schulman
S
Wang
B
Li
W
Rapoport
TA
Vitamin K epoxide reductase prefers ER membrane-anchored thioredoxin-like redox partners.
Proc Natl Acad Sci USA
2010
, vol. 
107
 
34
(pg. 
15027
-
15032
)
12
Zipursky
A
Prevention of vitamin K deficiency bleeding in newborns.
Br J Haematol
1999
, vol. 
104
 
3
(pg. 
430
-
437
)
13
Blanchard
RA
Furie
BC
Jorgensen
M
Kruger
SF
Furie
B
Acquired vitamin K-dependent carboxylation deficiency in liver disease.
N Engl J Med
1981
, vol. 
305
 
5
(pg. 
242
-
248
)
14
Liebman
HA
Furie
BC
Furie
B
Hepatic vitamin K-dependent carboxylation of blood-clotting proteins.
Hepatology
1982
, vol. 
2
 
4
(pg. 
488
-
494
)
15
Suttie
JW
Mummah-Schendel
LL
Shah
DV
Lyle
BJ
Greger
JL
Vitamin K deficiency from dietary vitamin K restriction in humans.
Am J Clin Nutr
1988
, vol. 
47
 
3
(pg. 
475
-
480
)
16
Ferland
G
Sadowski
JA
O’Brien
ME
Dietary induced subclinical vitamin K deficiency in normal human subjects.
J Clin Invest
1993
, vol. 
91
 
4
(pg. 
1761
-
1768
)
17
Conly
J
Stein
K
Reduction of vitamin K2 concentrations in human liver associated with the use of broad spectrum antimicrobials.
Clin Invest Med
1994
, vol. 
17
 
6
(pg. 
531
-
539
)
18
Conly
JM
Stein
KE
The absorption and bioactivity of bacterially synthesized menaquinones.
Clin Invest Med
1993
, vol. 
16
 
1
(pg. 
45
-
57
)
19
Suttie
JW
The importance of menaquinones in human nutrition.
Annu Rev Nutr
1995
, vol. 
15
 (pg. 
399
-
417
)
20
Deitcher
SR
Interpretation of the international normalised ratio in patients with liver disease.
Lancet
2002
, vol. 
359
 
9300
(pg. 
47
-
48
)
21
Wu
SM
Cheung
WF
Frazier
D
Stafford
DW
Cloning and expression of the cDNA for human gamma-glutamyl carboxylase.
Science
1991
, vol. 
254
 
5038
(pg. 
1634
-
1636
)
22
Libby
EN
Garcia
DA
A survey of oral vitamin K use by anticoagulation clinics.
Arch Intern Med
2002
, vol. 
162
 
16
(pg. 
1893
-
1896
)
23
White
HD
Gruber
M
Feyzi
J
, et al. 
Comparison of outcomes among patients randomized to warfarin therapy according to anticoagulant control: results from SPORTIF III and V.
Arch Intern Med
2007
, vol. 
167
 
3
(pg. 
239
-
245
)
24
O’Reilly
RA
Aggeler
PM
Covert anticoagulant ingestion: study of 25 patients and review of world literature.
Medicine (Baltimore)
1976
, vol. 
55
 
5
(pg. 
389
-
399
)
25
Mowry
JB
Spyker
DA
Cantilena
LR
Jr
Bailey
JE
Ford
M
2012 Annual Report of the American Association of Poison Control Centers’ National Poison Data System (NPDS): 30th Annual Report.
Clin Toxicol (Phila)
2013
, vol. 
51
 
10
(pg. 
949
-
1229
)
26
Hadler
MR
Shadbolt
RS
Novel 4-hydroxycoumarin anticoagulants active against resistant rats.
Nature
1975
, vol. 
253
 
5489
(pg. 
275
-
277
)
27
Bachmann
KA
Sullivan
TJ
Dispositional and pharmacodynamic characteristics of brodifacoum in warfarin-sensitive rats.
Pharmacology
1983
, vol. 
27
 
5
(pg. 
281
-
288
)
28
Schulman
S
Kearon
C
Subcommittee on Control of Anticoagulation of the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis
Definition of major bleeding in clinical investigations of antihemostatic medicinal products in non-surgical patients.
J Thromb Haemost
2005
, vol. 
3
 
4
(pg. 
692
-
694
)
29
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
, vol. 
141
 
2 Suppl
(pg. 
e44S
-
88S
)
30
Rosovsky
RP
Crowther
MA
What is the evidence for the off-label use of recombinant factor VIIa (rFVIIa) in the acute reversal of warfarin? ASH evidence-based review 2008.
Hematology (Am Soc Hematol Educ Program)
2008
(pg. 
36
-
38
)
31
Goodnough
LT
Shander
A
How I treat warfarin-associated coagulopathy in patients with intracerebral hemorrhage.
Blood
2011
, vol. 
117
 
23
(pg. 
6091
-
6099
)
32
Ageno
W
Garcia
D
Aguilar
MI
, et al. 
Prevention and treatment of bleeding complications in patients receiving vitamin K antagonists, part 2: treatment.
Am J Hematol
2009
, vol. 
84
 
9
(pg. 
584
-
588
)
33
Lubetsky
A
Yonath
H
Olchovsky
D
Loebstein
R
Halkin
H
Ezra
D
Comparison of oral vs intravenous phytonadione (vitamin K1) in patients with excessive anticoagulation: a prospective randomized controlled study.
Arch Intern Med
2003
, vol. 
163
 
20
(pg. 
2469
-
2473
)
34
Fiore
LD
Scola
MA
Cantillon
CE
Brophy
MT
Anaphylactoid reactions to vitamin K.
J Thromb Thrombolysis
2001
, vol. 
11
 
2
(pg. 
175
-
183
)
35
Weitzel
JN
Sadowski
JA
Furie
BC
, et al. 
Surreptitious ingestion of a long-acting vitamin K antagonist/rodenticide, brodifacoum: clinical and metabolic studies of three cases.
Blood
1990
, vol. 
76
 
12
(pg. 
2555
-
2559
)
36
Spahr
JE
Maul
JS
Rodgers
GM
Superwarfarin poisoning: a report of two cases and review of the literature.
Am J Hematol
2007
, vol. 
82
 
7
(pg. 
656
-
660
)
37
Breckenridge
AM
Cholerton
S
Hart
JA
Park
BK
Scott
AK
A study of the relationship between the pharmacokinetics and the pharmacodynamics of the 4-hydroxycoumarin anticoagulants warfarin, difenacoum and brodifacoum in the rabbit.
Br J Pharmacol
1985
, vol. 
84
 
1
(pg. 
81
-
91
)
38
Palmer
RB
Alakija
P
de Baca
JE
Nolte
KB
Fatal brodifacoum rodenticide poisoning: autopsy and toxicologic findings.
J Forensic Sci
1999
, vol. 
44
 
4
(pg. 
851
-
855
)
39
Hollinger
BR
Pastoor
TP
Case management and plasma half-life in a case of brodifacoum poisoning.
Arch Intern Med
1993
, vol. 
153
 
16
(pg. 
1925
-
1928
)
40
Kuwada
M
Katayama
K
An improved method for the determination of gamma-carboxyglutamic acid in proteins, bone, and urine.
Anal Biochem
1983
, vol. 
131
 
1
(pg. 
173
-
179
)
41
Shearer
MJ
Fu
X
Booth
SL
Vitamin K nutrition, metabolism, and requirements: current concepts and future research.
Adv Nutr
2012
, vol. 
3
 
2
(pg. 
182
-
195
)
42
Park
BK
Scott
AK
Wilson
AC
Haynes
BP
Breckenridge
AM
Plasma disposition of vitamin K1 in relation to anticoagulant poisoning.
Br J Clin Pharmacol
1984
, vol. 
18
 
5
(pg. 
655
-
662
)
43
Bjornsson
TD
Blaschke
TF
Vitamin K1 disposition and therapy of warfarin overdose.
Lancet
1978
, vol. 
2
 
8094
(pg. 
846
-
847
)
44
Suttie
JW
Mechanism of action of vitamin K: synthesis of gamma-carboxyglutamic acid.
CRC Crit Rev Biochem
1980
, vol. 
8
 
2
(pg. 
191
-
223
)
45
Wallin
R
Vitamin K antagonism of coumarin anticoagulation. A dehydrogenase pathway in rat liver is responsible for the antagonistic effect.
Biochem J
1986
, vol. 
236
 
3
(pg. 
685
-
693
)
46
Wallin
R
Gebhardt
O
Prydz
H
NAD(P)H dehydrogenase and its role in the vitamin K (2-methyl-3-phytyl-1,4-naphthaquinone)-dependent carboxylation reaction.
Biochem J
1978
, vol. 
169
 
1
(pg. 
95
-
101
)
47
Wallin
R
Martin
LF
Vitamin K-dependent carboxylation and vitamin K metabolism in liver. Effects of warfarin.
J Clin Invest
1985
, vol. 
76
 
5
(pg. 
1879
-
1884
)
48
Wallin
R
Hutson
S
Vitamin K-dependent carboxylation. Evidence that at least two microsomal dehydrogenases reduce vitamin K1 to support carboxylation.
J Biol Chem
1982
, vol. 
257
 
4
(pg. 
1583
-
1586
)
49
Asher
G
Dym
O
Tsvetkov
P
Adler
J
Shaul
Y
The crystal structure of NAD(P)H quinone oxidoreductase 1 in complex with its potent inhibitor dicoumarol.
Biochemistry
2006
, vol. 
45
 
20
(pg. 
6372
-
6378
)
50
Wallin
R
Martin
LF
Warfarin poisoning and vitamin K antagonism in rat and human liver. Design of a system in vitro that mimics the situation in vivo.
Biochem J
1987
, vol. 
241
 
2
(pg. 
389
-
396
)
51
Fasco
MJ
Principe
LM
R- and S-warfarin inhibition of vitamin K and vitamin K 2,3-epoxide reductase activities in the rat.
J Biol Chem
1982
, vol. 
257
 
9
(pg. 
4894
-
4901
)
52
Ingram
BO
Turbyfill
JL
Bledsoe
PJ
Jaiswal
AK
Stafford
DW
Assessment of the contribution of NAD(P)H-dependent quinone oxidoreductase 1 (NQO1) to the reduction of vitamin K in wild-type and NQO1-deficient mice.
Biochem J
2013
, vol. 
456
 
1
(pg. 
47
-
54
)