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Impact of Population-Specific Phenytoin Correction Factors on Estimation of Free Phenytoin Concentrations in a
Abriana M Palumbo1, Andrew Soliman1, Kaysey Cloud2
1Department of Pharmacy, Texas Health Harris Methodist Hospital Fort Worth, USA.
Insights
Population-specific correction factors (CFs) improve the accuracy of estimated free phenytoin concentrations (fPHTe) compared to traditional methods. This study highlights the need for tailored CFs in specific patient groups for better therapeutic drug monitoring.
Area of Science:
- Pharmacokinetics and Therapeutic Drug Monitoring
- Clinical Pharmacy Practice
- Pharmacometrics
Background:
- Phenytoin is a critical antiepileptic drug with a narrow therapeutic index, necessitating precise monitoring.
- Accurate measurement of free phenytoin serum concentrations (fPHTm) is crucial but often limited by availability.
- Traditional methods for estimating fPHTm may lack accuracy in diverse patient populations.
Purpose of the Study:
- To assess the efficacy of population-specific correction factors (CFs) in enhancing the accuracy of estimated free phenytoin serum concentrations (fPHTe).
- To compare the performance of novel CFs against the traditional Winter-Tozer CF for fPHTe estimation.
- To identify optimal CFs for specific patient subgroups, including those with seizures, undergoing hemodialysis, or within certain age demographics.
Main Methods:
- A retrospective analysis of adult patients with available data on measured total phenytoin (mPHT), fPHTm, albumin, and blood urea nitrogen (BUN).
- Stratification of patients into groups based on seizure activity (active/breakthrough), intermittent hemodialysis (IHD), and age (<65 years).
- Utilized patient-specific fPHTm percentages to estimate fPHTe and evaluate various correction factors.
Main Results:
- Population-specific CFs demonstrated superior precision and reduced bias compared to traditional methods.
- In patients with active seizures and BUN <50 mg/dL, CF 0.29 significantly improved accuracy over CF 0.2 (MPE -59.59 vs -111.99, RMSE 1.79 vs 2.91).
- Preliminary data suggests CF 0.275 is more accurate for adults <65 years, and CF 0.201 outperforms CF 0.1 in the IHD cohort.
Conclusions:
- Population-specific correction factors offer a more accurate and precise method for estimating free phenytoin concentrations.
- The findings suggest that tailored CFs are beneficial for therapeutic drug monitoring of phenytoin.
- Further research with larger, diverse cohorts is warranted to refine and validate these population-specific CFs for broader clinical application.
Background:
Phenytoin is a highly protein-bound medication and monitored due to its narrow therapeutic range. Free phenytoin serum concentratinos (fPHTm) are not readily available in many institutions.
Objective:
To evaluate whether population-specific correction factors (CFs) improve estimated free phenytoin serum concentrations (fPHTe) compared with traditional Winter-Tozer CF.
Methods:
This retrospective review included adults with same-day measured total phenytoin serum concentrations (mPHT), fPHTm, albumin, and blood urea nitrogen (BUN) values. Patients were stratified by active/breakthrough seizures, intermittent hemodialysis (IHD), or adults younger than 65. Patient-specific fPHTm percentages were used in the estimation of fPHTe.
Results:
Of 126 phenytoin levels reviewed, 66 met inclusion criteria. Population-specific CFs demonstrated lower bias and greater precision. In patients with active/breakthrough seizures and BUN <50 mg/dL, CF 0.29 improved accuracy compared with CF 0.2 (reduced mean percentage error [MPE] -59.59 vs -111.99 - indicating overprediction, root mean square error [RMSE] 1.79 vs 2.91), with similar trends when BUN exceeded 50 g/dL. A preliminary finding among adult patients younger than 65 showed better performance with a CF 0.275 than CF 0.2, with lower MPE (-40.43 vs -78.98) and RMSE (1.32 vs 2.16). In the IHD cohort, CF 0.201 outperformed CF 0.1 (MPE -74.40 vs -197.53 and RMSE 2.07 vs 5.32).
Conclusion And Relevance:
Population-specific CF estimated fPHTe more accurately and precisely than traditional CF. Findings remain preliminary due to limited representation, and further research is needed to improve CF selection across various subgroups.
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