Phenytoin Adjustment

Phenytoin is highly protein-bound (~90%). In states of low albumin, the free (active) fraction increases while the reported total level appears misleadingly low or normal. Use the Winter-Tozer equation to estimate the true therapeutic level.

In ESRD, accumulated uremic toxins displace phenytoin from albumin, altering the constant.

Adjusted Total Phenytoin

mg/L

*Standard reference range is 10 - 20 mg/L.

Winter-Tozer Formula

Adjusted Phenytoin =
Reported Total Phenytoin
-------------------------
(X × (Albumin / 10)) + 0.1
  • Normal Renal Function (X = 0.2):
    Albumin in g/L must be converted to g/dL (divide by 10) for the standard formula.
  • ESRD (X = 0.1):
    Used for patients with CrCl < 20 mL/min due to altered binding affinity.
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