Phenytoin correction for concurrent valproic acid
Estimates free and normal-binding-equivalent phenytoin when valproate is displacing it.
Inputs
0 of 2When to use
- Total phenytoin is measured while the patient also takes valproate.
Pearls and pitfalls
- Derived with normal albumin; do not stack with the albumin correction.
- A measured free level is preferred when available.
Why use
Valproate displaces phenytoin from albumin and inhibits its metabolism, so totals mislead.
Free PHT = (0.095 + 0.001 × VPA) × total PHT; normal-binding equivalent = free ÷ 0.1