Plasma Warfarin Levels Research Articles

Plasma Warfarin Level and International Normalized Ratio do not Correlate with Bleeding Events in Indonesian Patients of Minangkabau Ethnicity with Atrial Fibrillation

BACKGROUND: Warfarin is the mainstay of anticoagulant therapy to prevent thromboembolism in atrial fibrillation (AF). It has a narrow therapeutic window, rendering monitoring prothrombin time necessary using the international normalized ratio (INR). However, INR value is not always correlated with the clinical risk of bleeding.
 AIM: We aimed to monitor plasma warfarin concentration and to analyze its correlation with bleeding events in Indonesian patients of Minangkabau ethnicity with AF.
 METHODS: We consecutively recruited outpatients with AF from January to November 2017 at a tertiary hospital in West Sumatera, Indonesia. At the time of the study, patients had received at least 5 weeks of warfarin. Their characteristics were obtained from medical records, and INR data were collected. Warfarin plasma concentration was analyzed using high-performance liquid chromatography.
 RESULTS: There were a total of 45 patients (25 males and 20 females; mean age 54.6 years). The number of patients with INR value lower than, within, and higher than target value (2.0–3.0) was 25, 12, and 8, respectively. Half of the patients (n = 23; 51.1%) had subtherapeutic plasma warfarin levels and nearly half (n = 20; 44.4%) of the patients had therapeutic plasma warfarin levels. INR value was not significantly correlated with plasma warfarin level (r = 0.273; p = 0.07). Bleeding events occurred in 14 patients. INR value was not significantly different (p = 0.12), while the plasma warfarin level was marginally significantly different (p = 0.05) between those with bleeding and no bleeding events.
 CONCLUSION: Neither warfarin plasma concentration nor INR was correlated with bleeding events in Indonesian patients of Minangkabau ethnicity with AF.

LC–MS/MS assay for assessing medical adherence in patients under warfarin maintenance therapy

Warfarin (WRN) use is plagued by response fluctuation which is managed by dose adjustment. The plasma WRN level had always been an abandoned prerequisite for dose adjustment. However, recent reports had correlated WRN level to the consumed dose. More importantly, the intra-individual alteration in WRN level was found proportional to response fluctuation. This renewed the interest to use WRN level prior to dose adjustment to account for dispensing errors or inadequate adherence. Detection of possibly interacting drugs may further assess compliance and enable a wiser dose adjustment.In this study, an LC–MS/MS assay was developed and validated for the determination of the WRN and the simultaneous detection of interacting drugs in citrated plasma samples normally collected for monitoring international normalized ration (INR). Samples were prepared by liquid-liquid extraction. Separation of WRN and interacting drugs (n = 19) was achieved on an Inertsil®C18 column using a gradient elution program. The assay was applied in 96 plasma samples from patients.The developed method was linear, accurate and precise for WRN determination with average recovery of 99.5 ± 8.4% and no relative matrix effect. WRN plasma level was measured in all samples and possible interacting drugs were detected. An improved WRN-response correlation was observed inter-individually when patient variability was reduced.Our results support the significance of WRN monitoring and suggest the additional monitoring of other interacting drugs for integrated evaluation of patient adherence. The developed method enables ruling-out inadequate adherence prior to unnecessary dose adjustment and highlights the need to strengthen patient education procedures.

Does cessation of the use of tobacco products affect the INR in patients receiving warfarin?

Evidence-Based Answer Smoking cessation is associated with higher plasma warfarin levels (SOR: C, bench research) and perhaps a 12% decreased warfarin dosing requirement (SOR: C, conflicting cross-sectional studies).

Determination of Plasma Warfarin Concentrations in Korean Patients and Its Potential for Clinical Application

Warfarin is a widely used oral anticoagulant with broad within- and between-individual dose requirements. Warfarin concentrations can be monitored by assessing its pharmacologic effects on International Normalized Ratio (INR). However, this approach has not been applied in the routine clinical management of patients receiving warfarin therapy. We performed a plasma warfarin assay using high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS) to determine if such an assay can be utilized in routine clinical practice. We included a total of 105 patients with atrial fibrillation, and who were receiving warfarin for more than 1 yr. The plasma concentrations of total warfarin and 7-hydroxywarfarin were determined by HPLC-MS/MS (Waters, UK). We assessed the association between warfarin dose, concentration, and INR as well as the effects of these factors on warfarin concentrations. The mean maintenance dose of warfarin in 105 patients was 4.1 +/-1.3 mg/day (range, 1.7-8.0 mg/day) and their mean plasma warfarin concentration was 1.3+/-0.5 mg/L. We defined a concentration range of 0.6-2.6 mg/L (corresponding to the 2.5th to 97.5th percentile range of the Plasma warfarin levels in the 74 patients showing INR within target range) as the therapeutic range for warfarin. The correlation of warfarin dose with warfarin concentration (r(2)=0.259, P<0.001) was higher than that with INR (r(2)=0.029, P=0.072). There was a significant correlation between warfarin dose and plasma warfarin concentrations in Korean patients with atrial fibrillation. Hence, plasma warfarin monitoring can help determine dose adjustments and improve our understanding of individual patient response to warfarin treatment.

A Randomized, Double-Blind Trial of the Interaction between Cranberry Juice and Warfarin.

Therapeutic warfarin therapy is highly influenced by concomitant drug therapy or dietary factors by effecting warfarin metabolism or vitamin K status. Cranberry juice (CJ) was recently implicated to influence the INR in patients on warfarin based on a number of brief anecdotal or case reports. This study was designed to determine what influence, if any, daily cranberry juice consumption has on INR levels and plasma warfarin concentrations in patients stably anticoagulated with warfarin. Patients on warfarin for a variety of indications from a population of anticoagulation clinic patients were selected because of their past history of stable anticoagulation and willingness to participate in the study. Major inclusion criteria required a therapeutic INR range of 2.0 – 3.0, no hepatic or renal dysfunction, a stable INR (1.7 – 3.3) on at least 2 measurements within 8 weeks prior to study entry, and informed consent. The study consisted of a 2 week lead-in phase (with weekly INRs in range); a 2 week intervention phase (double-blind daily consumption of an 8 oz glass of cranberry juice or placebo); and a one week follow-up phase (35 day total). Any patient with an out of range INR during the intervention phase ceased the intervention but continued through follow-up. 30 patients were randomized, 14 to CJ and 16 to placebo. 28.6% (4/14) in the CJ group developed a minimally elevated INR and 7.1%(1/14) developed a low INR during the intervention phase. 25% (4/16) in the control group developed a minimally elevated INR during the intervention. There was no significant difference in the mean INRs from each group on each day of measurement except for day 11 of the intervention phase when the CJ group was significantly higher than the placebo group (p<0.02), although on day 15 (end of intervention) the mean INRs from both groups were identical. Plasma warfarin levels are currently pending and should be helpful in identifying any effect on warfarin clearance or warfarin plasma protein binding. A daily 8 oz glass of cranberry juice appears to have no consistent effect on the INR in patients on warfarin therapy compared to placebo.

Brief Communication: American Ginseng Reduces Warfarin's Effect in Healthy Patients

People using prescription medication often concurrently take herbal supplements. In a case report, the anticoagulant effect of warfarin decreased after patients consumed ginseng. To evaluate the interactions between American ginseng and warfarin. Randomized, double-blind, placebo-controlled trial. General Clinical Research Center, University of Chicago, Chicago, Illinois. 20 healthy patients. In this 4-week study, 20 patients received warfarin for 3 days during weeks 1 and 4. Beginning in week 2, patients were assigned to receive either American ginseng or placebo. International normalized ratio (INR) and plasma warfarin level. The peak INR statistically significantly decreased after 2 weeks of ginseng administration compared with placebo (difference between ginseng and placebo, -0.19 [95% CI, -0.36 to -0.07]; P = 0.0012). The INR area under the curve (AUC), peak plasma warfarin level, and warfarin AUC were also statistically significantly reduced in the ginseng group as compared with the placebo group. Peak INR and peak plasma warfarin level were positively correlated. The study sample consisted of young, healthy volunteers in a research setting rather than patients taking therapeutic doses of warfarin. American ginseng reduces warfarin's anticoagulant effect. When prescribing warfarin, physicians should ask patients about ginseng use.

Changes in plasma warfarin levels and variations in steady-state prothrombin times.

To determine the relative contribution of changes in the plasma warfarin level to variation in the serial steady-state prothrombin times. This was a prospective observational cohort study performed at two outpatient anticoagulation clinics. Serial prothrombin times and paired plasma total warfarin levels were determined in a convenience sample of otherwise healthy patients who required long-term oral anticoagulation therapy with warfarin. Serial measurements were obtained from 129 patients, 60 of whom provided three or more serial samples. Analysis of covariance showed a highly significant (p = 0.0001) relationship between the anticoagulant effect and the logarithm of the warfarin concentration (R2 = 0.75), with 15.3% of the total variance attributable to the effect of warfarin and 31.1% attributable to individual variation in sensitivity to warfarin. In an analysis of the subjects who had three or more serial measurements, the mean weighted correlation coefficient for the relationship between the logarithm of the warfarin concentration and the anticoagulant response varied widely, from strongly negative to strongly positive, and as the range of observed prothrombin times increased, stronger positive correlation was observed. In this cohort, the plasma warfarin level was a strong predictor of observed changes in serial prothrombin time measurements. However, the correlation between clotting times and warfarin levels varied widely among subjects, particularly when the range of observed prothrombin times was moderate. This suggests that in these subjects, other factors, such as measurement error or pharmacodynamic changes, played a major role.

Hereditary Warfarin Resistance

We report on a case of hereditary warfarin resistance in a 27-year-old black man. Hereditary warfarin resistance is a rare disorder transmitted as autosomal dominant. It should be suspected in patients who require high doses of warfarin, achieve high plasma warfarin levels, and have a normal plasma warfarin clearance. Treatment includes either high dose warfarin or heparin (subcutaneous).

Hereditary Warfarin Resistance

A 57-year-old black woman required a daily dosage of 50 mg of warfarin sodium to maintain her prothrombin time in a therapeutic range. The central volume of distribution and clearance of warfarin were normal for this patient. These findings, combined with the patient's requirement for plasma warfarin levels four times greater than those usually required to achieve adequate anticoagulation, indicated that the relative resistance was due to altered pharmacodynamics of warfarin. The only child of the propositus, a daughter, showed a similar relative resistance, confirming that this family is the third to be reported with hereditary resistance to warfarin.

Anti-warfarin antibody preparation and its characterization for radioimmunoassay.

Anti-warfarin antiserum was prepared in rabbits by immunization with a synthesized warfarin antigen, 4'-azo-warfarin human serum albumin, which possesses two enantiomorphic haptenic sites of warfarin on the molecule. The antiserum recognized both R- and S-warfarin to the same degree, 50% cross reactivities of racemic warfarin, respectively. One of the warfarin metabolites, racemic warfarin alcohol, showed 1% cross reactivity, and R- or S-warfarin alcohol have half the reactivity of the racemic alcohol. Rabbit plasma warfarin levels were determined by radioimmunoassay using this antiserum and racemic [14C]warfarin and by fluorometric assay after isolation by thin layer chromatography. After a single administration of warfarin (2 mg kg-1 orally or 500 microgram kg-1 i.v.), the plasma levels determined by both assay methods showed a good correlation (r = 0.97, P less than 0.001, Y = 1.04-0.09). The results show that the radioimmunoassay can determine total plasma warfarin without interference of plasma metabolite. The applicability and limitation of the radioimmunoassay for pharmacokinetic study are discussed.

Influence of dietary protein on the response of rats receiving toxic levels of warfarin

Toxic doses of racemic [ 14C]warfarin (3-(α-acetonylbenzyl)-4-hydroxycoumarin) were administered (0.8 mg/kg, po) for 5 days to rats fed one of three different purified diets, 25% casein, 8% casein, and casein-free. Four rats from each dietary group were bled at 6, 30, 54, 78, and 102 hr after the initial dose. Rats consuming a casein-free diet maintained higher total plasma warfarin levels than rats fed a 25 or 8% casein diet. Additionally, percentage free warfarin levels of 0% casein rats were greater than those of 25 and 8% groups through the first 78 hr. An increase in percentage unbound drug was therefore most likely associated with a decrease in total plasma clearance. The protein-free rats had a more rapid increase in prothrombin time and lower plasma albumin levels than rats fed either a 25 or 8% casein diet. A significant correlation was found between the percentage free fraction of warfarin and the prothrombin response in each of the three dietary groups. A negative correlation was found between percentage free plasma warfarin and plasma albumin concentration in each of the three dietary groups. A correlation was found between total plasma warfarin and prothrombin time for the rats on the 0% protein diet. It was concluded that the quantity of dietary protein was an important factor in determining the response of rats to toxic levels of warfarin.

Quantitation of plasma warfarin levels by gas chromatography chemical ionization mass spectrometry

A quantitative assay for plasma concentrations of warfarin has been developed using a deuterated warfarin analogue as internal standard. The method employs methylation (diazomethane) of warfarin to its 4'-methoxy derivative and gas chromatography chemical ionization mass spectrometry. Selected ion monitoring was used to relate the response of warfarin methyl ether and the internal standard. This technique can measure warfarin plasma concentrations five days after a single oral (25 mg) dose. Warfarin half-lives were determined in 12 healthy male volunteers. Each plasma analysis required six minutes of instrument time.

Coagulopathy Associated With Vitamin E Ingestion

Although hypervitaminosis E causes a prolonged prothrombin time and a hemorrhagic state in animals, excessive vitamin E has not previously been found to cause bleeding in normal humans. We have seen a prolonged prothrombin time and ecchymoses develop in a patient who was taking warfarin sodium and clofibrate concomitant with self-administration of vitamin E. The prothrombin time returned to base line after vitamin E ingestion was stopped while warfarin and clofibrate treatment was continued. Coagulation studies demonstrated enhanced reduction of the levels of vitamin K-dependent coagulation factors during the period of vitamin E ingestion, which returned to base-line levels after the patient stopped taking the vitamin. Plasma warfarin levels did not change and platelet function remained normal. The data suggest that patients with vitamin K deficiency may risk hemorrhage if they take vitamin E. (JAMA230:1300-1301, 1974)

Coagulopathy associated with vitamin E ingestion

Although hypervitaminosis E causes a prolonged prothrombin time and a hemorrhagic state in animals, excessive vitamin E has not previously been found to cause bleeding in normal humans. We have seen a prolonged prothrombin time and ecchymoses develop in a patient who was taking warfarin sodium and clofibrate concomitant with self-administration of vitamin E. The prothrombin time returned to base line after vitamin E ingestion was stopped while warfarin and clofibrate treatment was continued. Coagulation studies demonstrated enhanced reduction of the levels of vitamin K-dependent coagulation factors during the period of vitamin E ingestion, which returned to base-line levels after the patient stopped taking the vitamin. Plasma warfarin levels did not change and platelet function remained normal. The data suggest that patients with vitamin K deficiency may risk hemorrhage if they take vitamin E. (<i>JAMA</i>230:1300-1301, 1974)

Determinants of the response to coumarin anticoagulants in patients with acute myocardial infarction.

Abstract Determinants of the response to coumarin anticoagulant, warfarin, have been analysed in a series of 19 patients with acute myocardial infarction. Cardiac failure developing in the acute phase of the disease seemed to be without any great influence on the plasma half‐life of warfarin, with the exception of patients in whom serious circulatory catastrophes occurred. In patients who had received other drugs regularly before the hospital admission the plasma half‐life of warfarin showed a significant prolongation within 3 weeks—a phenomenon apparently caused by a decreasing induction of microsomal drug‐metabolizing enzymes in the liver. The plasma half‐life of warfarin was found to explain 44% of the variance in the daily requirement of warfarin during maintenance treatment. The steady state “therapeutic” plasma warfarin level showed a wide range of individual variation—from 0.8 to 4.1 mg/l, with a median value of 1.5 mg/l. The results indicate that in addition to the rate of coumarin anticoagulant metabolism other factors have a marked influence on the response to coumarin drugs in patients with myocardial infarction.

Studies on the Interaction of Warfarin and Clofibrate in Man

SummaryTo evaluate the basis for the hemorrhagic diathesis associated with oral anticoagulant therapy plus the hypolipidemic agent clofibrate, this interaction was studied in 8 normal subjects. Administration of clofibrate 2.0 g/day alone for 14 days had no effect on the platelet count, bleeding time, platelet aggregation, plasma adenosine diphosphatase, platelet release of adenosine diphosphate and adenosine triphosphate, platelet-collagen adhesion, one-stage prothrombin time, or vitamin K-dependent clotting factors (II, VII, IX and X) but significantly reduced platelet adhesiveness and epinephrine-induced platelet aggregation. After addition of large single doses of sodium warfarin, 1.5 mg/kg body weight, to the clofibrate regimen, all values remained within the normal range, including platelet adhesiveness, except epinephrine-induced platelet aggregation. The one-stage prothrombin activity and clotting factors II and X were significantly lower with warfarin plus clofibrate than with warfarin alone, but the plasma level of warfarin was unchanged. Co-administration of sodium warfarin and clofibrate for 21 days augmented the hypoprothrombinemia observed in long-term therapy with warfarin alone but caused no significant change in the plasma warfarin level. It is concluded that the hemorrhagic complications of therapy with warfarin plus clofibrate result primarily from the more rapid decline in the activities of clotting factors II and X and perhaps also from the reduced platelet aggregation.

Interaction of warfarin and nonsystemic gastrointestinal drugs.

The absorption and pharmacologic action of warfarin may be influenced by such factors as gut motility, pH, solubility, and chemical binding. To investigate the possible interaction of warfarin with nonsystemic gastrointestinal drugs in clinical use, (1) cholestyramine, (2) an antacid mixture containing magnesium and aluminum hydroxide, and (3) psyllium hydrophyllic mucilloid were studied in 6 normal subjects. In a series of experiments average therapeutic doses of each drug were administered concurrently with a single oral dose of warfarin 40 mg. Cholestyramine was found to decrease significantly mean plasma warfarin levels and the hypoprothrombinemic effect of warfarin even after a 3 hour interval between ingestion of the two drugs. Neither the antacid nor psyllium colloid was found to alter plasma concentrations or pharmacologic action of warfarin. In vitro experiments demonstrated significant binding of warfarin to cholestyramine at a pH above its pKa of 5.5.

Coumarin anticogulant requirement in relation to serum cholesterol and triglyceride level.

Abstract The relationship of warfarin requirement to serum cholesterol and triglyceride level was studied in 91 patients, who were on a well stabilized long‐term anticoagulant therapy. In male patients a significant positive correlation was observed between warfarin requirement and serum cholesterol level. A similar trend was observed in female patients, although the correlation did not reach statistical significance. No correlation was present between warfarin requirement and serum triglyceride level.The concentration of warfarin in the plasma 14–15 hours after the preceding warfarin dose was determined in the 91 patients on long‐term warfarin therapy. The plasma warfarin level varied from 0.5 to 6.2 mg per liter with a median value of 2.0 mg per liter. When a serum cholesterol value of 300 mg/100 ml was chosen as a dividing line between normocholesteremia and hypercholesteremia, the median “therapeutic” plasma warfarin level was found to be somewhat higher in hypercholesteremic patients than in normocholesteremic patients.The wide range of individual variation in the “therapeutic” plasma warfarin level indicates that, in addition to the individual variation in the rate of warfarin metabolism, there are considerable individual differences acting at the level of the receptor site for coumarin drugs.The plasma warfarin half‐life was determined in 37 subjects. The mean half‐life of warfarin was 35.6 hours (S.D. 19.1 hours). The range of individual variation was from 10 to 90 hours. There was no correlation between the half‐life of warfarin and serum cholesterol or triglyceride level.The duration of the depression of the prothrombin complex clotting factors after a single warfarin dose was shorter in hypercholesteremic than in normocholesteremic subjects, when related to the half‐life of warfarin in the plasma. A possible explanation of this phenomenon is that a greater amount of vitamin K is available at the receptor site for coumarin drugs in hypercholesteremic subjects. The results support the hypothesis that hypercholesteremia may be associated with increased levels of vitamin K in the plasma and liver.