Relationship of age, weight and body surface area to warfarin maintenance dose requirements

Evaluation of the impact of body mass index on warfarin requirements in hospitalized patients

BACKGROUND

Despite well established empiric dose adjustments for drug and disease-state interactions, the impact of body mass index (BM) on warfarin remains unclear. The objective of this study is to evaluate warfarin requirements in hospitalized patients, stratified by BMI.

METHODS

This retrospective review included two cohorts of patients: cohort A (patients admitted with a therapeutic international normalized ratio (INR)) and cohort B (newly initiated on warfarin during hospitalization). Exclusion criteria included: age under 18 years, pregnancy, INR (goal 2.5-3.5), and warfarin thromboprophylaxis post orthopedic surgery. The primary outcome was mean total weekly dose (TWD) of warfarin based on weight classification: underweight (BMI <18 kg/m²), normal/overweight (BMI 18-29.9 kg/m²), obese (BMI 30-39.9 kg/m²), and morbidly obese (BMI ⩾ 40 kg/m²). Data were extracted from two community hospitals in reverse chronologic order during July 2015-June 2013 until both study institutions evaluated 100 patients per cohort in each BMI classification or until all patients had been evaluated within the prespecified timeframe.

RESULTS

A total of 585 patients were included in cohort A (26 underweight, 200 normal/overweight, 200 obese, 159 morbidly obese). There was a statistically significant difference in TWD as determined by one-way analysis of variance ( p < 0.05). A Tukey post hoc test revealed a statistically significantly higher TWD in morbidly obese (41.5 mg) compared with underweight (25.6 mg, p < 0.05), normal/overweight (28.8 mg, p < 0.05) and obese patients (32.4 mg, p < 0.05). In cohort B, 379 patients were evaluated (9 underweight, 166 normal/overweight, 152 obese, 52 morbidly obese). Overall, 191 patients had a therapeutic INR on discharge (88.9% underweight, 52.4% normal/overweight, 44.1% obese, 55.8% morbidly obese, p = 0.035). Of those, there was a statistically significant difference in TWD ( p = 0.021) with a higher TWD in the morbidly obese (41 mg) compared with underweight patients (24.4 mg, p = 0.017).

CONCLUSIONS

Based on the results of this study, morbidly obese patients may require higher TWD to obtain and maintain a therapeutic INR.

Clinical pharmacokinetics and pharmacodynamics of warfarin. Understanding the dose-effect relationship

The simplest complete system accounting for the time-course of changes in the prothrombin time induced by warfarin requires the combination of 4 independent models: A pharmacokinetic model for the absorption, distribution, and elimination of warfarin. Warfarin is essentially completely absorbed, reaching a maximum plasma concentration between 2 and 6 hours. It distributes into a small volume of distribution (10 L/70kg) and is eliminated by hepatic metabolism with a very small clearance (0.2 L/h/70kg). The elimination half-life is about 35 hours. A pharmacodynamic model for the effect of warfarin on the synthesis of clotting factors (prothrombin complex). Prothrombin complex synthesis is inhibited 50% at a warfarin concentration of about 1.5 mg/L. Warfarin concentrations associated with therapeutic anticoagulation are of similar magnitude. A physiological model for the synthesis and degradation of the prothrombin complex. The synthesis rate is about 5%/h/70kg and the elimination half-life estimated from changes in prothrombin time is approximately 17 hours. On average it will take 3 days for the anticoagulant effect of warfarin to reach a stable value when warfarin concentrations are constant. A model for the relationship between the activity of prothrombin complex and the prothrombin time. In general there is a hyperbolic relationship between these quantities. Its exact shape depends upon the method used for measuring the prothrombin time. Attempts to integrate these models into a single system have used essentially the same pharmacokinetic, physiological, and prothrombin activity models. Four distinct pharmacodynamic models have been proposed: linear, log-linear, power and Emax. One might be preferred on theoretical grounds (Emax) but its performance is not clearly different from the others. Empirical methods for warfarin dose prediction as well as those based on the combined pharmacokinetic-pharmacodynamic-physiological-prothrombin system have been proposed. Only one (which was also the first) [Sheiner 1969] has been adequately described and compared with the performance of an unaided physician. The programme compared favourably with decisions made by those physicians normally responsible for adjusting warfarin dose, but was not tested prospectively. A sizeable body of theoretical and experimental observations has contributed to our understanding of the warfarin dose-effect relationship. It remains to be demonstrated that any alternative method is superior to the traditional empirical approach to warfarin dose adjustment.