Warfarin


Article Author:
Shivali Patel
Ravneet Singh
Charles Preuss


Article Editor:
Neepa Patel


Editors In Chief:
Sherri Murrell


Managing Editors:
Avais Raja
Orawan Chaigasame
Khalid Alsayouri
Kyle Blair
Radia Jamil
Erin Hughes
Patrick Le
Anoosh Zafar Gondal
Saad Nazir
William Gossman
Hassam Zulfiqar
Navid Mahabadi
Hussain Sajjad
Steve Bhimji
Muhammad Hashmi
John Shell
Matthew Varacallo
Heba Mahdy
Ahmad Malik
Abbey Smiley
Sarosh Vaqar
Mark Pellegrini
James Hughes
Beenish Sohail
Hajira Basit
Phillip Hynes
Sandeep Sekhon


Updated:
10/29/2019 11:18:01 AM

Indications

Warfarin is an oral anticoagulant that is commonly used to treat and prevent blood clots. Warfarin has multiple FDA-approved and off-label clinical uses.[1][2][3]

FDA-approved clinical uses for warfarin:

  • Prophylaxis and treatment of venous thrombosis and arising pulmonary embolisms
  • Prophylaxis and treatment of thromboembolic complications from atrial fibrillation or cardiac valve replacement
  • Reduction in the risk of death, recurrent myocardial infarction, and thromboembolic events (e.g., stroke, systemic embolization) after myocardial infarction

Off-Label clinical uses of warfarin include:

  • Secondary prevention of recurrent stroke and transient ischemic attacks

Mechanism of Action

Warfarin competitively inhibits the vitamin K epoxide reductase complex 1 (VKORC1), which is an essential enzyme for activating the vitamin K available in the body. Through this mechanism, warfarin can deplete functional vitamin K reserves and therefore reduce the synthesis of active clotting factors. The hepatic synthesis of coagulation factors II, VII, IX, and X, as well as coagulation regulatory factors protein C and protein S, require the presence of vitamin K. Vitamin K is an essential cofactor for the synthesis of all of these vitamin K-dependent clotting factors.

Administration

Warfarin is a once-daily oral medication. Warfarin administration can be at any time during the day, but recommendations are for administration in the afternoon or evening. By instructing patients to take warfarin later in the day, healthcare providers can have the opportunity to individualize a patient's warfarin dose the same day based on their most current lab values. The dose-response of warfarin among patients is highly variable and depends on interpatient differences. Patient-specific factors such as drug metabolism, the presence of a vitamin K enriched diet, genetics, quantity of vitamin K-dependent clotting factors, concurrent disease states, binding proteins, concomitant drug interactions, laboratory testing, and medication adherence requires assessment when dosing warfarin.[4][5][6]

Pharmacodynamics/Kinetics:

Route of Administration: Oral

Drug Composition: Warfarin is a racemic mixture composed of R and S enantiomers of the drug. Each enantiomer gets metabolized through unique pathways, and the S-enantiomer is approximately three to five times more potent that than the R-enantiomer.

Absorption: Rapid and complete absorption

The onset of action: The onset of action is typically 24 to 72 hours. A peak therapeutic effect is seen 5 to 7 days after initiation. However, the patient’s international normalized ratio (INR) may increase within 36 to 72 hours after initiating treatment.

Duration of Action:  2 to 5 days

Distribution: Small volume of distribution (0.14 L/kg)

Protein Binding: 99%

Metabolism: Hepatic metabolism, primarily through the CYP2C9 enzyme. Other minor enzymatic pathways for metabolism include CYP2C8, 2C18, 2C19, 1A2, and 3A4.

Genomic Variants: Genetic variations in CYP2C9 have been shown to affect an individual’s clearance of warfarin. Patients who are heterozygous for the 2C9 (*1/*2 or *1/*3) can experience an approximately 37% reduction in clearance of S-warfarin. Patients who are homozygous for reduced function alleles (*2/*2, *2/*3, or *3/*3) may experience a nearly 70% reduction in clearance of S-warfarin.

Elimination Half-life: The half-life of warfarin is generally 20 to 60 hours (mean: 40 hours). However, it is highly variable among individuals.

Time to peak (plasma concentration): approximately 4 hours

Excretion: Warfarin is primarily eliminated as metabolites by glomerular filtration in the kidney (92% via urine).

Adverse Effects

Serious adverse effects of warfarin include bleeding and significant hemorrhage. A major hemorrhage (e.g., intracranial hemorrhage, gastrointestinal (GI) bleed, hematemesis, intraocular bleeding, hemarthrosis) can occur at virtually any site on the body. Patients should receive education about the easy bleeding or bruising that is a common adverse effect. A physician should also counsel patients about the proper management of cuts, bruises, and nosebleeds. The risk of bleeding and hemorrhage is dependent on multiple variables, including the intensity of anticoagulation and patient susceptibility. Individuals should undergo assessment for their risk, with appropriate adjustments to their treatment plan made accordingly.

Other adverse effects include nausea, vomiting, abdominal pain, bloating, flatulence, and altered sense of taste.

Rare cases of purple toe syndrome, warfarin-induced skin necrosis, and there are reports of calciphylaxis with warfarin therapy. Purple toe syndrome is a complication characterized by cholesterol microembolization that causes purple lesions to develop on the toes and sides of the feet. Purple toe syndrome usually develops 3 to 8 weeks after the initiation of warfarin therapy. Warfarin-induced skin necrosis is a serious condition in which the subcutaneous tissue necrosis occurs due to an acquired protein C deficiency following treatment with warfarin. The risk of necrosis increases in patients with protein C or protein S deficiency. The skin necrosis usually occurs within the first week of therapy and management strategies, including discontinuing treatment with warfarin, administering fresh frozen plasma and vitamin K, and initiating anticoagulation therapy with either unfractionated heparin or low molecular weight heparin. Calciphylaxis or calcium uremic arteriolopathy is another rare adverse effect that can occur in patients with or without end-stage renal disease.

Contraindications

Warfarin has several contraindications and clinical considerations to its use.[7][8][9][10]

Warfarin is contraindicated in patients with:

  • Hypersensitivity to warfarin or any component of the formulation
  • Hemorrhagic tendencies (e.g., active GI ulceration, patients bleeding from the GI, respiratory, or GU tract; a cerebral aneurysm; central nervous system (CNS) hemorrhage; dissecting aortic aneurysm; spinal puncture and other diagnostic or therapeutic procedures with potential for significant bleeding)
  • Recent or contemplated surgery of the eye, central nervous system, or traumatic surgery resulting in large open surfaces
  • Bleeding tendencies associated with any of the following:
  1. Active ulceration or overt bleeding of the gastrointestinal, genitourinary, or respiratory tract
  2. Central nervous system hemorrhage
  3. Cerebral aneurysms, dissecting aorta
  4. Pericarditis and pericardial effusions
  5. Bacterial endocarditis
  • Threatened abortion, eclampsia, or preeclampsia
  • Unsupervised patients with conditions associated with a high potential of non-adherence
  • Spinal puncture and other diagnostic or therapeutic procedures with potential for uncontrollable bleeding
  • Major regional or lumbar block anesthesia
  • Malignant hypertension
  • Pregnancy (exception: women with mechanical heart valves at high risk for thromboembolism)

Clinical Considerations

Geriatric Considerations:

Elderly patients are at an increased risk of bleeding complications secondary to falls, concomitant drug interactions, cognitive status, and living situation. The risk of bleeding complications has been associated with increased age. These patients need to be monitored closely and may require a more conservative dosage regimen.

Pregnancy Considerations:

Warfarin has two different pregnancy categories depending on the presence of a mechanical heart valve. Warfarin is currently listed as a Category D drug for pregnant women with a mechanical heart valve and Category X for all other indications in pregnant women. Warfarin crosses the placental barrier, causing fetal plasma levels similar to maternal values. Warfarin can cause bleeding in the fetus, and use during pregnancy is commonly associated with spontaneous abortion, stillbirth, preterm birth, and neonatal death. In pregnant women with high-risk for thromboembolism from mechanical heart valves, the benefits and risks of warfarin therapy should be evaluated. Some experts recommend avoiding warfarin use during the first trimester and close to delivery.

Breast-Feeding Considerations:

Warfarin is not excreted in breast milk. Therefore, breastfeeding women may be treated with warfarin. However, the drug manufacturer recommends close monitoring of breastfeeding infants for bruising or bleeding.

Renal Impairment:

A reduced renal function can cause warfarin to accumulate in the body, thereby increasing the risk of bleeding. However, dosing adjustments are not necessary for renal impairment. Close monitoring of the patient’s INR is recommended. It is also important to note that warfarin is not dialyzable through hemodialysis.

Hepatic Impairment:

According to the manufacturer’s drug labeling, there are no dosage adjustments necessary for hepatic impairment. However, the response to oral anticoagulants may be enhanced in patients with obstructive jaundice, hepatitis, and cirrhosis. The INR should be closely monitored in these patients.

Dietary Considerations:

While there is no specific diet recommended for patients on warfarin, certain foods and beverages can either limit or enhance the anticoagulant effect of warfarin. Vitamin K decreases the effectiveness of warfarin in the body. Therefore, the patient should be educated on maintaining a consistent dietary intake of vitamin K containing foods. Examples of foods that are high in vitamin K include kale, green tea leaves, Brussel sprouts, and spinach. Other dietary recommendations include limiting the consumption of cranberry juice and alcohol while on warfarin therapy. Grapefruit juice and alcohol can enhance the anticoagulant effect of warfarin, thereby leading to increased bleeding complications.

Drug Interactions:

Warfarin has numerous drug interactions that can either increase the risk of adverse effects or decrease the anticoagulant effect of warfarin. Therefore dosing adjustments, close monitoring, and use of alternative agents should be considered when combining warfarin with certain medications. Caution is advised when administering warfarin with antiplatelet agents, fibrinolytics, nonsteroidal anti-inflammatory agents (NSAIDs), antimicrobials, anti-arrhythmic medications, and other anticoagulant agents. It should be noted that the S-enantiomer of warfarin is approximately three to five times more potent that than the R-enantiomer. Thus, drug interactions involving the inhibition of the S-enantiomer are considered more severe and may require pre-emptive dose adjustments or therapeutic interchange of alternative anticoagulants. A comprehensive drug interaction database should be referenced when necessary.

Monitoring

Patients receiving treatment with warfarin should be closely monitored to ensure the safety and efficacy of the medication. Periodic blood testing is recommended to assess the patient’s prothrombin time (PT) and the international normalized ratio (INR).

The laboratory parameter utilized to monitor warfarin therapy is the PT/INR. The PT is the number of seconds it takes the blood to clot, and the INR allows for the standardization of the PT measurement depending on the thromboplastin reagent used by a laboratory. Therefore, monitoring a patient’s INR while on warfarin is strongly preferable over PT because it allows for a standardized measurement without variations due to different laboratory sites. Routine assessment of INR is essential in the management of patients receiving warfarin therapy. The INR of a patient who is not on anticoagulation therapy is approximately 1.0. If a patient has an INR of 2.0 or 3.0, that would indicate that it takes two or three times longer for that individual’s blood to clot than someone who is not taking any anticoagulants The therapeutic INR goal for patients on warfarin therapy is dependent on the indication, but may vary based on the patient’s clinical presentation and provider preference. Most patients on warfarin have an INR goal of 2 to 3. However, specific indications, such as a mechanical mitral valve, require an INR goal of 2.5 to 3.5.

Close monitoring of a patient’s INR is strongly recommended when initiating warfarin. The INR should be monitored more frequently when starting warfarin. For hospitalized patients, INR monitoring commonly occurs daily. Once a patient has reached the maintenance phase of therapy, the INR assessment is typically at least every four weeks but up to the discretion of the provider. More frequent monitoring is necessary for patients with supratherapeutic or subtherapeutic INR to evaluate safety and efficacy. Also, the INR requires assessment when initiating, discontinuing, or changing doses of medications that are known to interact with warfarin.

Patients also require close monitoring for signs and symptoms of active bleeding throughout their treatment. Close monitoring for signs and symptoms of bleeding, such as dark tarry stools, nosebleeds, and hematomas, is necessary. The patient’s hemoglobin and hematocrit level should undergo an assessment before initiating warfarin and approximately every six months while on therapy. Other laboratory tests may be recommended based on the patient’s clinical presentation and INR result. Monitoring liver function, renal function, and occult blood may be indicated in certain patient populations.

Toxicity

Warfarin toxicity is assessable through signs and symptoms of bleeding, as well as the determination of a supratherapeutic INR level. The risk of bleeding is significantly greater with increased INR, especially above 5.0. When managing warfarin toxicity, the initial step would be to discontinue warfarin and then administer vitamin K (phytonadione). The vitamin K may administration can be either via the oral, intravenous, or subcutaneous route. However, the initial administration of oral vitamin K is often preferable in patients without major bleeding or extremely elevated INR. A reduction in INR is expected to occur within 24 hours of administration. After that, intravenous vitamin K administration is recommended if necessary. Subcutaneous vitamin K is often not recommended for warfarin toxicity or reversal due to erratic and unpredictable absorption. Administering agents like prothrombin complex concentrate (PCC), fresh frozen plasma (FFP), and activated Factor VII may be considered for cases of major bleeding.

Enhancing Healthcare Team Outcomes

All healthcare workers, including the primary care provider, nurse practitioner, and emergency department physician, need to know how to manage to bleed associated with warfarin. After prescribing warfarin, patients need to be educated about the importance of regular follow up, foods that can interact with warfarin, and when to seek medical help. The prescriber should enlist the services of a pharmacist to help "dial in" the warfarin dose, and inpatients often have their INR and dose adjustment handled by a pharmacist. Nurses must be aware of the signs of warfarin toxicity so that they can alert the attending/prescribing clinician. More importantly, healthcare workers need to be aware that today, there are other options instead of warfarin, including the novel oral anticoagulant drugs, which are deemed to be safer and require less intense monitoring. Given the potential severity of warfarin toxicity, there needs to be an interprofessional team approach to prescribing and managing warfarin, including clinicians, specialists, pharmacists, and nursing staff, so positive therapeutic benefit has the greatest chance with minimal adverse effects. [Level V]


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When administering warfarin, how is therapy assessed?



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When starting a patient on warfarin, approximately how long does it take for anticoagulation?



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Which of the following anticoagulants has been associated with skin necrosis?



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A patient is on warfarin for a mechanical valve. Which anticoagulation test is used to monitor him?



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What is the most common complication of warfarin therapy?



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A female from Southeast Asia traveled by air to Toronto. Four days later, she noticed swelling in her right leg and was started on heparin followed by warfarin. Three months later she was found to have acid-fast bacteria. She was started on isoniazid, rifampin, pyrazinamide, and ethambutol. She expired from a pulmonary embolism. What is the most likely reason for her expiration?



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A patient is being seen in the emergency department for ingesting a "rat poison" he found in the gardening shed. What agent can cause severe hemorrhage when ingested and is used as a rat poison?



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Which is true regarding warfarin?



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What is the mechanism of action of warfarin?



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Which of the following food items can potentiate the actions of warfarin?



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What are the commonly recommended INR values for warfarin treatment?



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How often should the INR be checked for a patient who is on a stable warfarin regimen?



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Which of the following inhibits the enzyme epoxide reductase?



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A 35-year-old woman is six weeks pregnant. Which of the following should be avoided in this patient?



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What is the mechanism of action of warfarin?



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Which of the following is an early complication of warfarin therapy?



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Which of the following medications works by inhibiting the enzyme epoxide reductase?



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Which of the following would be recommended in a patient taking a combination of warfarin and cholestyramine?



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A 68-year-old male with a past medical history of hypertension and recently diagnosed atrial fibrillation presents to the clinic for pain in his abdomen. He is currently taking lisinopril, carvedilol, and warfarin. On examination, a black leathery eschar is noted on the right lower quadrant of the abdomen, which is tender to touch. His vital signs are unremarkable. What is the likely diagnosis?



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Which of the following drugs' effects may be diminished when used with oral contraceptives?



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What would be a therapeutic INR in a patient on warfarin for atrial fibrillation?



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A 61-year-old female presents to her cardiologist for her monthly check-up. Her vital signs are blood pressure of 132/90 mmHg, temperature of 98.5 F, and pulse of 105 beats per minute. The patient has a history of atrial fibrillation. The physical exam is unremarkable. The patient is currently prescribed several medications, including warfarin. The patient's international normalized ratio (INR) has changed from her last visit from 2.3 to now 1.2. Which of the following most likely caused the change in the patient’s INR?



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Which foods might affect the INR in the patient on warfarin?



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How often should PT/INR be checked in reliable patients on warfarin with a stable INR who have a good understanding of the medication and dietary implications?



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Which is a contraindication to warfarin use?



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Which of the following about duration of treatment with warfarin is correct?



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Why are patients with atrial fibrillation prescribed warfarin?



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A patient is started on warfarin for atrial fibrillation. The patient returns in 2 weeks. Which of the following statements by the patient indicates teaching is needed?



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Which of following medications has the slowest onset of action?



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Which of the following medications should be avoided in patients on warfarin?



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Which of the following are statements that should be used when counseling a patient who has been prescribed warfarin for the first time?



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Which drug in this list has the longest elimination half-life?



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A patient has started warfarin therapy for atrial fibrillation. Which of the following is a counseling point for this medication?



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A 33-year-old woman presents to her obstetrician with pain and swelling in her left calf muscle. Her vital signs are blood pressure of 110/75 mmHg, temperature of 98.8 F, and pulse of 77 beats per minute. She is in her 30th week of pregnancy. Now, she is about to deliver. Physical exam reveals erythema, warmth, and swelling of the left calf muscle with pain upon palpation. Which of the following is the best management for this patient?



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A patient comes in asking for a recommendation for headache medicine. In looking at her medication record in the pharmacy, you see that she is currently taking warfarin. Which of the following is you recommendation?



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A client with a pulmonary embolism was started on warfarin therapy 2 days after receiving intravenous heparin. Which of the following is true regarding warfarin use? Select all that apply.



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A 71-year-old man presents to the clinic for his monthly check-up. His vital signs show blood pressure of 133/92 mmHg, temperature of 98.7 F, and pulse of 104/minute. The patient has a history of atrial fibrillation. Physical exam is unremarkable. He is currently prescribed several medications including warfarin. The patient's international normalized ration (INR) has changed from his last visit from 2.3 to currently 6.2. Which of the following is the next best step in the management of this patient?



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A 69-year-old woman presents to the clinic for her monthly check-up. Her vital signs show a blood pressure of 130/90 mmHg, temperature 98.5 F, and pulse 105/min. The patient has a history of atrial flutter, hypertension, and hypercholesterolemia. A physical exam is unremarkable. The patient is currently prescribed several medications, including warfarin. The patient's international normalized ration (INR) has changed from her last visit from 2.4 to 1.2 now. A medication interaction is suspected. Which of the following is most likely responsible for the change in the patient’s INR?



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Warfarin - References

References

Doliner B,Jaller JA,Lopez AJ,Lev-Tov H, Treatments to prevent primary venous ulceration after deep venous thrombosis. Journal of vascular surgery. Venous and lymphatic disorders. 2019 Jan 16;     [PubMed]
Sharp CR,deLaforcade AM,Koenigshof AM,Lynch AM,Thomason JM, Consensus on the Rational Use of Antithrombotics in Veterinary Critical Care (CURATIVE): Domain 4-Refining and monitoring antithrombotic therapies. Journal of veterinary emergency and critical care (San Antonio, Tex. : 2001). 2019 Jan;     [PubMed]
Badjatiya A,Rao SV, Advances in Antiplatelet and Anticoagulant Therapies for NSTE-ACS. Current cardiology reports. 2019 Jan 12;     [PubMed]
Mckenzie JA,Wilson-Clarke C,Prout J,Campbell J,Douglas RD,Gossell-Williams M, Improving warfarin therapy through implementation of a hospital-based pharmacist managed clinic in Jamaica. Pharmacy practice. 2018 Oct-Dec;     [PubMed]
Lee S,Han J,Park RW,Kim GJ,Rim JH,Cho J,Lee KH,Lee J,Kim S,Kim JH, Development of a Controlled Vocabulary-Based Adverse Drug Reaction Signal Dictionary for Multicenter Electronic Health Record-Based Pharmacovigilance. Drug safety. 2019 Jan 16;     [PubMed]
Chokesuwattanaskul R,Thongprayoon C,Bathini T,Torres-Ortiz A,O'Corragain OA,Watthanasuntorn K,Lertjitbanjong P,Sharma K,Preechawat S,Ungprasert P,Kröner PT,Wijarnpreecha K,Cheungpasitporn W, Efficacy and safety of anticoagulation for atrial fibrillation in patients with cirrhosis: A systematic review and meta-analysis. Digestive and liver disease : official journal of the Italian Society of Gastroenterology and the Italian Association for the Study of the Liver. 2018 Dec 13;     [PubMed]
Trujillo TC,Dobesh PP,Crossley GH,Finks SW, Contemporary Management of Direct Oral Anticoagulants During Cardioversion and Ablation for Nonvalvular Atrial Fibrillation. Pharmacotherapy. 2018 Dec 12;     [PubMed]
Divito A,Kerr K,Wilkerson C,Shepard S,Choi A,Kitagawa RS, Use of Anticoagulation Agents After Traumatic Intracranial Hemorrhage. World neurosurgery. 2018 Dec 6;     [PubMed]
Garlo KG,Steele DJR,Nigwekar SU,Chan KE, Demystifying the Benefits and Harms of Anticoagulation for Atrial Fibrillation in Chronic Kidney Disease. Clinical journal of the American Society of Nephrology : CJASN. 2019 Jan 7;     [PubMed]
Clark NP, Role of the anticoagulant monitoring service in 2018: beyond warfarin. Hematology. American Society of Hematology. Education Program. 2018 Nov 30;     [PubMed]

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