Phenytoin (PHT) and valproic acid (VPA) compete for the same binding sites on albumin. In patients taking both PHT and VPA, the free fraction of phenytoin will be increased, which causes a total PHT level to falsely represent a patient’s active PHT status.
To supplement the recent video on Total vs. Free Phenytoin and the existing correction calculator for hypoalbuminemia, a new correction calculator for concurrent valproic acid has been released.
This new phenytoin correction calculator reports an estimated free phenytoin level based on a total phenytoin level and a total valproic acid level. It uses two published equations (May 1991 and Haidukewych 1989) to generate an estimated correction value.
To try out the new calculator, please visit http://clincalc.com/Phenytoin/WithValproicAcid.aspx
