Article

Nomogram for the prediction of unbound phenytoin concentrations in patients on a combined treatment of phenytoin and valproic acid.

Biochemical Department, Gesellschaft für Epilepsieforschung, Bielefeld, FRG.
European Neurology (impact factor: 1.81). 02/1991; 31(1):57-60. pp.57-60
Source: PubMed

ABSTRACT The interpretation of the total phenytoin (PT) concentration can be problematic if valproic acid (VPA) is given as a comedication, because VPA displaces PT from the protein binding sites and can increase the free fraction of PT. In order to estimate the free or unbound PT concentration (PTf) from the total PT concentration (PTt) and VPA concentration, a nomogram was constructed and evaluated. Data of 84 patients on combined therapy with PT and VPA were used in drawing up the regression equation (PTf = 0.0792.PTt + 0.000636.PTt.VPA) from which the nomogram was constructed. The predictions were checked using another 33 patients whose serum concentrations were determined in the morning and in addition several times during the day. The results show that using this method the PTf concentrations can be accurately estimated from the PTt and VPA concentrations.

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Keywords

33 patients
 
84 patients
 
comedication
 
free
 
free fraction
 
predictions
 
protein binding sites
 
PT
 
PTf
 
PTf concentrations
 
PTt
 
serum concentrations
 
total phenytoin
 
total PT concentration
 
unbound PT concentration
 
VPA
 
VPA concentration
 
VPA concentrations
 
VPA displaces PT