Kelli Keats, BCCCP, PharmD, MPA
Augusta University Medical Center
Augusta, Georgia
Disclosure information not submitted.
Rebecca Powell, BS
Pharmacy Student
Augusta University Medical Center, United States
Disclosure information not submitted.
Jody Rocker, PharmD, BCCCP
Clinical Pharmacist - Longterm Care
Augusta University Medical Center, United States
Disclosure information not submitted.
Lindsey Sellers, PharmD, BCCCP
Clinical Pharmacist - Neurosciences ICU
Augusta University Medical Center, United States
Disclosure information not submitted.
Title: Phenytoin Loading Doses in Overweight Patients Using Actual Versus Adjusted Body Weight
INTRODUCTION/HYPOTHESIS: The appropriate loading dose strategy for phenytoin/fosphenytoin in overweight patients is unknown. A small pharmacokinetic study indicated that overweight patients have a higher volume of distribution and potentially would benefit from using adjusted body weight (AdjBW) instead of actual body weight (ABW) to calculate the loading dose. The purpose of this study was to determine the optimal loading dose strategy of phenytoin in patients whose ABW is greater than 120% of their ideal body weight (IBW) using either ABW or AdjBW for calculation of the loading dose.
Methods: This was a single center, retrospective study which included patients who received a loading dose of phenytoin of at least 10mg/kg based on ABW, had a phenytoin level drawn < 6 hours after the end of the dose, and weighed ≥120% of their IBW. Patients were excluded if they received intramuscular phenytoin or were prescribed phenytoin prior to the loading dose. Patients were divided into two groups, those who were dosed using their AdjBW versus those dosed using ABW. The primary outcome was achievement of therapeutic phenytoin level of 10-20mcg/mL. Secondary outcomes included achievement of a subtherapeutic (< 10mcg/mL) or supratherapeutic ( >20mcg/mL) level.
Results: A total of 195 patients (128 in AdjBW group and 67 in ABW group) met criteria for inclusion. Patients in the AdjBW group weighed more (96.2kg vs. 91.2kg, p=0.04) and received a lower dose in milligrams (1364 vs. 1760, p< 0.0001) and in mg/kg of ABW (14.2 vs. 19.3, p< 0.0001). The primary outcome was achieved in 74% of patients in the AdjBW group and 57% of patients in the ABW group (p=0.02). Patients in the ABW group were more likely to have a supratherapeutic level (43% vs. 22%, p=0.003), although adverse reactions (hypotension, bradycardia, nystagmus, or ataxia) did not differ between the groups.
Conclusions: Patients weighing >120% of their IBW who received a 20mg/kg loading dose based on AdjBW were more likely to achieve a therapeutic phenytoin concentration compared to those dosed based on ABW. Further research is needed to correlate this finding with clinical outcomes, such as resolution of status epilepticus.