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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.
CAUTIONS

(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:
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     -    Program background information

References

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Ther Drug Monit. 1985;7(1):74-80.
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10.  Gugler R, Manion CV, Azarnoff DL.  Phenytoin: pharmacokinetics and bioavailability. Clin Pharmacol Ther. 1976 Feb;19(2):135-42.
11.  Hayes MJ, Langman MJ, Short AH.  Changes in drug metabolism with increasing age: 2. phenytoin clearance and protein binding.  Br J Clin Pharmacol. 1975 Feb;2(1):73-9.
12.   Hudson SA, Farquhar DL, Thompson D, Smith RG. Phenytoin dosage individualization--five methods compared in the elderly.  J Clin Pharm Ther. 1990 Feb;15(1):25-34.
13.  Liponi DF, Winter ME, Tozer TN.  Renal function and therapeutic concentrations of phenytoin. Neurology. 1984 Mar;34(3):395-7.
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18.  Mauro LS, Mauro VF, Bachmann KA, Higgins JT.  Accuracy of two equations in determining normalized phenytoin concentrations.  DICP. 1989 Jan;23(1):64-8.
19.  McCauley DL, Tozer TN, Winter ME.  Time for phenytoin concentration to peak: consequences of first-order and zero-order absorption. Ther Drug Monit. 1989 Sep;11(5):540-2.
20.  Mlynarek ME, Peterson EL, Zarowitz BJ.  Predicting unbound phenytoin concentrations in the critically ill neurosurgical patient. Ann Pharmacother. 1996 Mar;30(3): 219-23.
21.  Peterson GM, Khoo BH, von Witt RJ.  Clinical response in epilepsy in relation to total and free serum levels of phenytoin. Ther Drug Monit. 1991 Sep;13(5):415-9.
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Phenytoin Dosing Calculator and analysis

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