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Phenytoin Dosing Calculator

This initial program provides some general dosage guidelines based on population averages for the Michaelis-Menten parameters (Km and Vmax). The recommendations do not take into account the following: (1) existence of interacting drugs (3) inter-patient variability (3) existing disease states which may significantly alter the eventual therapeutic dose.

(1) Always remember that because phenytoin’s elimination is dose-dependent (“capacity limited”), that small increases in dosage can produce disproportionate increases in serum levels (possibly 3 to 4 fold).

(2) Never assume a linear relationship exists between steady state concentrations and the dosage given.

(3) Changes in the daily maintenance dose should be made in small increments (30-100mg maximum). Sample serum levels 7 to 10 days following each dosage change to assess the trend. Steady state is usually achieved after 10-14 days, however, it may be much longer than this in some patients.

(4) Once therapeutic steady state levels are achieved, periodic levels based on clinical judgment should be obtained. Some studies have found fluctuations in steady state serum concentrations > 150% in patients receiving the same daily maintenance dose. All factors must be considered: addition of interacting drugs, changes in absorption (eg enteral feeding + oral administration of phenytoin), concomitant disease state(s) which may alter phenytoin kinetics, etc.
Patient Name:      Floor:
Age    Weight    Height
Gender      Current albumin level: (g/dl)
What is the current phenytoin level if available? (mcg/ml)
Dosage form used for maintenance dose:
Estimated creatinine clearance:
Read the following:
     -    Program background information


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Phenytoin Dosing Calculator and analysis