Phenytoin – serum

Last updated: Sunday, 21, March, 2004

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Item Process

5 mL blood in plain tube.

MethodChromatography, immunoassay.
Therapeutic Interval
40-80 µmol/L (10-20 mg/L).

Therapeutic drug monitoring.


Steady state is not achieved for several days. Re-testing after a change of dose should be delayed for 48 hours.

The half-life is 7-60 hours (dose dependent).

This drug exhibits Michaelis-Menten kinetics, that is, as serum levels rise a stage is reached when a small increase in dose causes a large increase in serum levels.

ReferenceGlauser TA and Pippenger CE. Epilepsia. 2000; 41(8): 6-15.