Laboratory and clinical outcomes of pharmacogenetic vs. clinical protocols for warfarin initiation in orthopedic patients

BACKGROUND

Warfarin is commonly prescribed for prophylaxis and treatment of thromboembolism after orthopedic surgery. During warfarin initiation, out-of-range International Normalized Ratio (INR) values and adverse events are common.

METHODS

In orthopedic patients beginning warfarin therapy, we developed and prospectively validated pharmacogenetic and clinical dose refinement algorithms to revise the estimated therapeutic dose after 4 days of therapy.

RESULTS

The pharmacogenetic algorithm used the cytochrome P450 (CYP) 2C9 genotype, smoking status, peri-operative blood loss, liver disease, INR values and dose history to predict the therapeutic dose. The R(2) was 82% in a derivation cohort (n = 86) and 70% when used prospectively (n = 146). The R(2) of the clinical algorithm that used INR values and dose history to predict the therapeutic dose was 57% in a derivation cohort (n = 178) and 48% in a prospective validation cohort (n = 146). In 1 month of prospective follow-up, the percent time spent in the therapeutic range was 7% higher (95% CI: 2.7-11.7) in the pharmacogenetic cohort. The risk of a laboratory or clinical adverse event was also significantly reduced in the pharmacogenetic cohort (Hazard Ratio 0.54; 95% CI: 0.30-0.97).

CONCLUSIONS

Warfarin dose adjustments that incorporate genotype and clinical variables available after four warfarin doses are accurate. In this non-randomized, prospective study, pharmacogenetic dose refinements were associated with more time spent in the therapeutic range and fewer laboratory or clinical adverse events. To facilitate gene-guided warfarin dosing we created a non-profit website, http://www.WarfarinDosing.org.

Optimal initial dose adjustment of warfarin in orthopedic patients

BACKGROUND

Warfarin sodium is commonly prescribed for the prophylaxis and treatment of venous thromboembolism. Dosing algorithms have not been widely adopted because they require a fixed initial warfarin dose (eg, 5 mg) and are not tailored to other factors that may affect the international normalized ratio (INR).

OBJECTIVE

To develop an algorithm that could predict a therapeutic warfarin dose based on drug interactions, INR response after the initial warfarin doses, and other clinical factors.

METHODS

We used stepwise regression to quantify the relationship between these factors in patients beginning prophylactic warfarin therapy immediately prior to joint replacement. In the derivation cohort (n = 271), we separately modeled the therapeutic dose after 2 and 3 initial doses. We prospectively validated these 2 models in an independent cohort (n = 105).

RESULTS

About half of the therapeutic dose variability was predictable after 3 days of therapy: R2 was 53% in the derivation cohort and 42% in the validation cohort. INR response after 3 warfarin doses (INR3) inversely correlated with therapeutic dose (p < 0.001). Intraoperative blood loss transiently, but significantly, elevated the postoperative INR values. Other significant (p < 0.03) predictors were the first and second warfarin doses (+7% and +6%, respectively, per 1 mg), and statin use (-15.0%). The model derived after 2 warfarin doses explained 32% of the variability in therapeutic dose.

CONCLUSIONS

We developed and validated algorithms that estimate therapeutic warfarin doses based on clinical factors and INR response available after 2-3 days of warfarin therapy. The algorithms are implemented online at www.WarfarinDosing.org.

Prospective dosing of warfarin based on cytochrome P-450 2C9 genotype

Cytochrome P-450 2C9 (CYP2C9) polymorphisms (CYP2C9*2 and CYP2C9*3) reduce the clearance of warfarin, increase the risk of bleeding, and prolong the time to stable dosing. Whether prospective use of a retrospectively developed algorithm that incorporates CYP2C9 genotype and nongenetic factors can ameliorate the propensity to bleeding and delay in achieving a stable warfarin dose is unknown. We initiated warfarin therapy in 48 orthopedic patients tailored to the following variables: CYP2C9 genotype, age, weight, height, gender, race, and use of simvastatin or amiodarone. By using pharmacogenetics-based dosing, patients with a CYP2C9 variant achieved a stable, therapeutic warfarin dose without excessive delay. However compared to those without a CYP2C9 variant, patients with a variant continued to be at increased risk (hazard ratio 3.6, 95% confidence interval 1.4-9.5, p = 0.01) for an adverse outcome (principally INR > 4), despite pharmacogenetics-based dosing. There was a linear relationship (R(2) = 0.42, p < 0.001) between the pharmacogenetics-predicted warfarin doses and the warfarin maintenance doses, prospectively validating the dosing algorithm. Prospective, perioperative pharmacogenetics-based dosing of warfarin is feasible; however, further evaluation in a randomized, controlled study is recommended.

Effect of warfarin nonadherence on control of the International Normalized Ratio

OBJECTIVE

The frequency and causes of ab- errant International Normalized Ratios (INRs) in warfarin recipients and the percentage explainable by warfarin nonadherence were studied.

METHODS

The medical records of patients whose warfarin therapy was monitored by a telephone-based anticoagulation service in a Midwestern urban hospital between March 2000 and March 2001 were reviewed for causes of out-of-range INRs, the percentage of out-of-range INRs attributable to warfarin nonadherence, and demographic and clinical variables predictive of nonadherence.

RESULTS

Data from 347 patients were studied. The cohort yielded 4305 INRs, of which 1002 (23%) were out of range (lower than 1.8 or higher than 3.4). Thirty-six percent of the out-of-range INRs were due to warfarin or dietary nonadherence, 9% were due to medical systems problems (including drug interactions), 18% were due to a change in clinical status, and 38% were idiopathic. Age younger than 65 years, age greater than 80 years, and living closer to the laboratory were predictive of warfarin nonadherence.

CONCLUSION

Warfarin nonadherence was the most common cause of explainable aberrant INRs in patients taking warfarin. Age younger than 65 years, age greater than 80 years, and living close to the laboratory were predictive of warfarin nonadherence.

Warfarin dose reduction vs watchful waiting for mild elevations in the international normalized ratio

BACKGROUND

Whether clinicians should decrease the warfarin dose in response to a mild, asymptomatic elevation in the international normalized ratio (INR) is unknown.

OBJECTIVES

The study objectives were as follows: (1) to evaluate the safety of an anticoagulation service (ACS) policy advocating that the warfarin dose not be changed for isolated, asymptomatic INRs of < or = 3.4; (2) to compare the dosing strategies of an ACS and primary care providers (PCPs); and (3) to quantify the relationship between reduction of the warfarin dose and the subsequent fall in the INR.

DESIGN AND SETTING

Randomized controlled study of health maintenance organization outpatients who were receiving warfarin.

PATIENTS

We identified 231 patients with a target INR of 2.5 and an isolated, asymptomatic INR between 3.2 and 3.4. Our ACS monitored 103 of the patients; PCPs monitored the remaining 128 patients.

MEASUREMENTS

From all 231 patients, we obtained INRs and warfarin dosing history. From the 103 ACS enrollees, we also recorded adverse events.

RESULTS

One ACS patient had epistaxis in the 30 days after the elevated INR. Twenty-three percent of ACS enrollees and 47% of PCP patients reduced their warfarin dose (p < 0.001). The median follow-up INRs were similar in both cohorts: 2.7 in the ACS enrollees and 2.6 in the PCP patients. However, in a subgroup analysis of 190 patients who presented with an INR of 3.2 or 3.3, ACS enrollees were more likely to have a follow-up INR in the range of 2 to 3 (p = 0.03). The median follow-up INR was 2.7 in 148 patients who maintained their warfarin dose, 2.5 in 77 patients who decreased their dose by 1 to 20%, and 1.7 in 6 patients who decreased their dose by 21 to 43% (p < 0.001).

CONCLUSIONS

These findings support maintaining the same warfarin dose in asymptomatic patients with an INR of < or = 3.3, and reducing the dose for patients who have a greater INR or an increased risk of hemorrhage. Warfarin dose reductions > 20% should be avoided for mildly elevated INRs.

Substitution of generic warfarin for Coumadin in an HMO setting

BACKGROUND

Substitution of generic warfarin for Coumadin presents safety concerns due to warfarin's narrow therapeutic index and because a prior generic formulation was removed from the US market after it was associated with adverse events.

OBJECTIVE

To determine whether a health maintenance organization (HMO) can add generic warfarin to its formulary without adversely affecting warfarin management or increasing adverse events.

DESIGN

In a prospective, observational study, an HMO that formerly dispensed only Coumadin added a generic warfarin preparation (Barr Laboratories, Pomona, NY) to its formulary.

SETTING

An anticoagulation service (ACS) affiliated with an HMO that was based in St. Louis, MO.

PARTICIPANTS

The cohort consisted of 182 enrollees in the ACS as of May 1, 1999. At the start of the study, these participants were taking Coumadin; by October 31, 2000, all had switched to Barr warfarin.

MEASUREMENTS AND MAIN RESULTS

We collected data 8 months prior to and 10 months after the introduction of generic warfarin for the following endpoints: international normalized ratio (INR) control, frequency of INR monitoring, number of dose changes, and rate of thrombotic and hemorrhagic events. Statistical process control charts were used to differentiate between random variation in the endpoints and changes due to different warfarin formulations, and we used the Wilcoxon signed-rank test to look for a change in any endpoint after patients changed to generic warfarin. No significant differences were found in any endpoint.

CONCLUSIONS

Substitution of Barr warfarin for Coumadin did not significantly affect INR control, warfarin management, or adverse events. Our findings suggest that HMOs can safely substitute at least 1 generic formulation of warfarin without extra monitoring.

Establishing and running an effective telephone-based anticoagulation service

Observational studies and randomized controlled trials have revealed improvement in international normalized ratio (INR) control and reduced thrombotic and hemorrhagic events in patients taking warfarin who are managed by an anticoagulation service (ACS) compared with traditional physician care. In this article, we describe how to establish a multidisciplinary telephone-based ACS to monitor INRs, dose warfarin, and heparin therapy, and to educate patients by telephone. We address how to improve ACS efficiency by using an electronic medical record, charting by exception, holding group-based education, communicating by telephone, and conducting quality assurance. We also make recommendations for improving the quality of care of patients taking anticoagulants that can be implemented in any setting and we discuss how to apply these guidelines to other remote disease-state management programs (eg, diabetes).