Corrections for phenytoin concentrations

Correction for serum albumin

Phenytoin is about 90% protein-bound. The level usually measured includes that bound to protein, but the un-bound phenytoin is the biologically active fraction. The most abundant protein in plasma is albumin, and the measured total phenytoin level can be "corrected" for the measured albumin concentration using this equation [1].

The phenytoin level corrected for albumin concentration is:

P_normal_binding = P / ( (1 - f) [ A' / A ] + f )


P is the measured phenytoin concentration,
f is the normal free fraction of phenytoin (a = 0.1),
A' is the patient's serum albumin in grams/dL,
A is the normal serum albumin concentration (4.4 grams/dL), and
P_normal_binding is the plasma concentration that would have been observed had serum albumin concentration been normal.

The equation is less accurate in the presence of other drugs that displace phenytoin from albumin, for example, valproate, and in the presence of renal failure. Default initial values for f and A are given but can be varied as desired.

Measured phenytoin concentration (mg/dL)..........
Normal free fraction of phenytoin (0.1)...........
Measured serum albumin concentration (grams/dL) ..
Normal serum albumin concentration (4.4 grams/dL).
Corrected phenytoin concentration (mg/dL).........


[1] Winter, Michael E. Basic Clinical Pharmacokinetics, 3rd ed., Applied Therapeutics Inc, Vancouver WA, 1994.

Last revised 12/19/2011
M. Steven Evans [ mail | home page ]