Risk of warfarin-related bleeding events and supratherapeutic international normalized ratios associated with complementary and alternative medicine: a longitudinal analysis

Warfarin and acetaminophen interaction: a summary of the evidence and biologic plausibility

Ms TS is a 66-year-old woman who receives warfarin for prevention of systemic embolization in the setting of hypertension, diabetes, and atrial fibrillation. She had a transient ischemic attack about 4 years ago when she was receiving aspirin. Her INR control was excellent; however, over the past few months it has become erratic, and her average dose required to maintain an INR of 2.0 to 3.0 appears to have decreased. She has had back pain over this same period and has been taking acetaminophen at doses at large as 650 mg four times daily, with her dose varying based on her symptoms. You recall a potential interaction and wonder if (1) her acetaminophen use is contributing to her loss of INR control, and (2) does this interaction place her at increased risk of warfarin-related complications?

Intensity of Anticoagulation with Warfarin and Risk of Adverse Events in Patients Presenting to the Emergency Department

Background:

The ability of patients receiving warfarin to maintain an international normalized ratio (INR) within the desired therapeutic range is important for both efficacy and risk of adverse events. It is unclear whether the desired INR is maintained in patients receiving warfarin who present to the emergency department (ED) and whether they have a higher rate of adverse events.

Objective:

To evaluate the intensity of anticoagulation with warfarin and the risk of bleeding and thromboembolic complications in patients in the ED.

Methods:

A prospective observational study was performed using a convenience sample of patients receiving warfarin and presenting to the ED over an 18-week period. Data were collected using a standardized form that included chief complaint, history of present illness, past medical history, medication history, and allergy status. Information from the physical examination, laboratory results, and other diagnostic tests obtained as part of routine assessment in the ED, was used as necessary. The primary outcome was the proportion of patients whose INR was within, above, or below the desired therapeutic range. Bleeding complications and thromboembolic events were recorded in an attempt to determine the relationship between the intensity of anticoagulation and adverse outcomes.

Results:

Two hundred one patients were included, with a mean (SD) age of 74.0 (13.2) years; 53.7% were female. Primary indications for warfarin were atrial fibrillation (75.6%) and venous thromboembolic disease (14.9%). A therapeutic INR was observed in 88 patients (43.8%; 95% CI 37.1 to 50.7), while 45 patients (22.4%; 95% CI 17.2 to 26.7) and 68 patients (33.8%; 95% CI 27.6 to 40.6) had subtherapeutic and supratherapeutic INRs, respectively. Overall, there were 28 (18 major and 10 minor) bleeding complications (13.9%; 95% CI 9.8 to 19.4) and 4 thromboembolic events (2.0%; 95% CI 0.6 to 5.2). Among patients with a bleeding complication, 14 (50.0%) had a supratherapeutic INR, while 2 patients who experienced a thromboembolic event (50.0%) had a subtherapeutic INR.

Conclusions:

The majority of patients receiving warfarin on presentation to the ED had INRs outside the desired therapeutic range. By establishing the impact of warfarin-related adverse events in this population, focused interventions can be established in this setting to address factors that can be targeted to reduce these events.

Are the risk factors listed in warfarin prescribing information associated with anticoagulation-related bleeding? A systematic literature review

Warfarin significantly reduces thromboembolic risk, but perceptions of associated bleeding risk limit its use. The evidence supporting the association between bleeding and individual patient risks factors is unclear. This systematic review aims to determine the strength of evidence supporting an accentuated bleeding risk when patients with risk factors listed in the warfarin prescribing information are prescribed the drug. A systematic literature search of MEDLINE and Cochrane CENTRAL was conducted to identify studies reporting multivariate relationships between prespecified covariates and the risk of bleeding in patients receiving warfarin. The prespecified covariates were identified based on patient characteristics for bleeding listed in the warfarin package insert. Each covariate was evaluated for its association with specific types of bleeding. The quality of individual evaluations was rated as 'good', 'fair' or 'poor' using methods consistent with those recommended by the Agency for Healthcare Research and Quality (AHRQ). Overall strength of evidence was determined using the Grading of Recommendations Assessment, Development (GRADE) criteria and categorised as 'insufficient', 'very low', 'low', 'moderate' or 'high'. Thirty-four studies, reporting 134 multivariate evaluations of the association between a covariate and bleeding risk were identified. The majority of evaluations had a low strength of evidence for the association between covariates and bleeding and none had a high strength of evidence. Malignancy and renal insufficiency were the only two covariates that had a moderate strength of evidence for their association with major and minor bleeding respectively. The associations between covariates listed in the warfarin prescribing information and increased bleeding risk are not well supported by the medical literature.

Association between CHADS₂risk factors and anticoagulation-related bleeding: a systematic literature review

OBJECTIVE

To determine the strength of evidence supporting an accentuated bleeding risk when patients with CHADS(2) risk factors (chronic heart failure, hypertension, advanced age, diabetes, and prior stroke/transient ischemic attack) receive warfarin.

METHODS

A systematic literature search of MEDLINE (January 1, 1950, through December 22, 2009) and Cochrane CENTRAL (through December 22, 2009) was conducted to identify studies that reported multivariate results on the association between CHADS(2) covariates and risk of bleeding in patients receiving warfarin. Each covariate was evaluated for its association with a specific type of bleeding. Individual evaluations were rated as good, fair, or poor using methods consistent with those recommended by the Agency for Healthcare Research and Quality. The strength of the associations between each CHADS(2) covariate and a specific type of bleeding was determined using Grading of Recommendations Assessment, Development and Evaluation criteria as insufficient, very low, low, moderate, or high for the entire body of evidence.

RESULTS

Forty-one studies were identified, reporting 127 multivariate evaluations of the association between a CHADS(2) covariate and bleeding risk. No CHADS(2) covariate had a high strength of evidence for association with any bleeding type. For the vast majority of evaluations, the strength of evidence between covariates and bleeding was low. Advanced age was the only covariate that had a moderate strength of evidence for association; this was the strongest independent positive predictor for major bleeding. Similar findings were observed regardless of whether all included studies, or only those evaluating patients with atrial fibrillation, were assessed.

CONCLUSION

The associations between CHADS(2) covariates and increased bleeding risk were weak, with the exception of age. Given the known association of the CHADS(2) score and stroke risk, the decision to prescribe warfarin should be driven more by patients' risk of stroke than by the risk of bleeding.

Warfarin and Supplement Interactions: Survey of Published Literature

Objective:

To review published literature related to potential interactions between warfarin and common dietary supplement products.

Data Sources:

Tertiary databases including Micromedex, LexiComp, and Natural Medicines Comprehensive Database were used to assess drug interactions with warfarin. Searches of literature from database inception through July 2010 were conducted in MEDLINE, International Pharmaceutical Abstracts, and Iowa Drug Information Service and were restricted to the English language. The following search terms were used: warfarin, ginkgo biloba, St. John's wort, garlic, coenzyme Q10, ginger, ginseng, red clover, fish oil, dong quai, cranberry, green tea, saw palmetto, bilberry, soy, chamomile, glucosamine, chondroitin, echinacea, interactions, anticoagulation/antiplatelet, bleeding, herbals, and pharmacokinetics/pharmacodynamics.

Study Selection and Data Extraction:

Thirty-two English language publications were identified and analyzed. Reference lists of each of the included articles were reviewed to obtain related articles for further analysis.

Data Synthesis:

Quality of existing data for interactions between warfarin and commonly available dietary supplements varies greatly. The majority of information available is derived from case reports, although for some products pharmacokinetic studies have been performed to assess the effect of supplement use in patients concurrently using warfarin. Some of the data that suggest interactions with warfarin were gleaned from case reports or from an understanding of supplement mechanisms of action that would indicate such an interaction. The strength of evidence for the majority of the herbal products studied seems to be lacking and therefore it is difficult to draw firm conclusions.

Conclusions:

Based on the narrow therapeutic window of warfarin therapy, practitioners should be encouraged to document any potential drug interaction. However, it seems inappropriate to suggest absolute avoidance of all dietary supplements in all patients using warfarin therapy. Instead, diligent monitoring and reporting should be implemented to detect such events.

Patient Factors That Influence Warfarin Dose Response

Warfarin has long been the mainstay of oral anticoagulation therapy for the treatment and prevention of venous and arterial thrombosis. The narrow therapeutic index of warfarin, and the complex number of factors that influence international normalized ratio (INR) response, makes optimization of warfarin therapy challenging. Determination of the appropriate warfarin dose during initiation and maintenance therapy requires an understanding of patient factors that influence dose response: age, body weight, nutritional status, acute and chronic disease states, and changes in concomitant drug therapy and diet. This review will examine specific clinical factors that can affect the pharmacokinetics and pharmacodynamics of warfarin, as well as the role of pharmacogenetics in optimizing warfarin therapy.

Adverse interaction of warfarin and paracetamol: evidence from a post-mortem study

PURPOSE

The aim of the study was to establish the prevalence and nature of potential adverse drug combinations of warfarin in a large post-mortem toxicology database. The concomitant use of warfarin and non-steroidal anti-inflammatory drugs (NSAIDs) was of interest as these drugs have been associated with internal bleeding both in clinical and post-mortem study settings. Another purpose was to obtain facts related to the questioned safety of warfarin-paracetamol and warfarin-tramadol combinations.

METHODS

The post-mortem database was searched for a 1-year period. All warfarin-positive cases and cases containing interacting drugs, as defined by the SFINX interaction database (Swedish, Finnish, Interaction X-referencing), were included. For controls, all cases containing paracetamol or tramadol were also included, and for each warfarin-positive case, an age-, sex- and alcohol-matched control case was sourced. The contribution of anticoagulant use to the deaths was evaluated from the death certificates based on medico-legal autopsies.

RESULTS

In 33% of the 328 warfarin-positive cases, at least one interacting drug was present, and paracetamol was the most abundant, accounting for 49% (n = 53). When paracetamol and warfarin were detected simultaneously, the number of fatal bleeds was 4.6 and 2.7 times higher compared to paracetamol or warfarin use alone respectively. The presence of an NSAID in combination with warfarin was rare, as only six cases were identified. A majority (66%) of the post-mortem blood samples had a warfarin concentration below 0.5 mg/l, and for the rest of the cases, the mean concentration was 0.70 mg/l.

CONCLUSIONS

This study supports the clinical evidence suggesting that warfarin-paracetamol interactions may create significant life-threatening conditions. It also accentuates the significant role post-mortem database research can have in improving drug safety.

Prevalence of use and the risk of adverse effects associated with complementary and alternative medicine in a cohort of patients receiving warfarin

BACKGROUND

The use of complementary and alternative medicine (CAM), including orally administered herbals, botanicals, vitamins, and supplements, may pose a risk to patients on warfarin therapy.

OBJECTIVE

To estimate the prevalence of CAM use among patients taking warfarin and evaluate the impact of CAM exposure on the risk of warfarin-related adverse effects.

METHODS

A survey was administered to hospital inpatients and clinic outpatients on drug exposure (including CAM) over the previous month, self-reported bleeding events, use of alcohol and vitamin K-rich foods, and medical conditions. Prescription medication use was verified, and laboratory records were checked for out-of-range international normalized ratios (INRs) (defined as INR >4 or <2). The use of CAM, including products with reported or theoretical interactions with warfarin, was compared between patients with and without self-reported bleeding or out-of-range INR.

RESULTS

Among the 314 patients who completed the survey, 44.3% reported using CAM at least weekly. Potentially interacting CAM was used by 34.1% of all patients, or 18.2% if vitamin E was excluded as an interacting CAM. Vitamin E was used by 24.2% of all patients and 71.0% of those who used potentially interacting CAM. There was no significant difference in CAM use or consumption of vitamin K-rich foods between patients with and without INRs greater than 4 or for patients with and without INRs less than 2.

CONCLUSIONS

The use of potentially interacting CAM in this cohort was higher than the use previously reported among patients on warfarin therapy. However, exposure to CAM was not associated with an increase in the risk of self-reported bleeding or out-of-range INR.

Metabolomics: a tool for personalizing medicine?

The concept of personalized medicine is based upon using personal genetic information to predict individual responses to drug therapy. However, environmental factors such as diet, energy status, gut microbiota, health status and age will influence the expression of one’s genetic potential. Metabolomics data from biofluid and tissue sample analysis hold information regarding a patient’s genotype and phenotype. Metabolomics data can be rapidly collected from biofluid samples over time, providing temporal metabolic analyses of patient samples. In addition to metabolic markers of a patient’s phenotype, metabolomics can provide markers of drug efficacy, toxicity and clearance.

Genetic Testing for Warfarin Therapy Initiation

Objective

To review the literature regarding the efficacy of genetic testing for determining the appropriate initial dose of warfarin and the effect that this testing has on the safety and efficacy of therapy.

Data Sources:

Searches of MEDLINE (1966–May 2008) and Cochrane Database (1993–May 2008) were conducted using the search terms warfarin, anticoagulation, pharmacogenomics, pharmacogenetics, CYP2C9, VKORC1, and interindividual variability. Limits included articles written in English with human subjects. Additional data were identified through bibliographic reviews.

Study Selection And Data Extraction:

All English-language articles identified from the data sources were evaluated. Studies were eligible for inclusion if they evaluated the efficacy of pharmacogenomic testing to improve outcomes with initiation of warfarin therapy.

Data Synthesis:

The use of warfarin presents numerous challenges in clinical practice. Four studies (N = 38, 48, 200, 297) evaluating the efficacy of genetic testing for determining the initial dose of warfarin therapy have been published. Results show that time to therapeutic international normalized ratio (INR) and time to stable warfarin dose are similar regardless of genotype. When conventional warfarin dosing was compared with pharmacogenomic-based dosing, no significant difference was seen between groups in terms of time spent within the target INR range (41.5% vs 41.7%; no p value reported). Similar results were found in a subsequent study in which patients receiving conventional dosing were outside their target INR range 33.1% of the time compared with 30.7% of the lime for patients whose dose was guided by pharmacogenomics (p = 0.47).

Conclusions:

There is growing evidence that variant alleles for CYP2C9 and VKORC1 genotypes account for a proportion of the variability seen in warfarin dosing. The currently available literature related to the use of pharmacogenomic testing in the initiation of warfarin therapy does not show improved outcomes in either safety or efficacy with warfarin therapy and therefore does not support the routine use of pharmacogenomic testing when initiating warfarin therapy.