Phenytoin, Free

Alternate test name

Dilantin, Free

Lab area

Clinical Biochemistry - TDM & Toxicology

Method and equipment

HPLC

Expected turn-around time

Stat: 4 hours

Specimen type

Serum

Plasma (Heparin)

Specimen requirements

300 uL

Storage and transportation

4°C (transport with a cool pack if possible)

Background and clinical significance

Refer to the Phenytoin (Total) for a complete description of the clinical significance of Phenytoin.

Routine therapeutic drug monitoring generally measures the concentration of the total drug.

Considerations:

Drug which is bound to plasma proteins is pharmacologically inactive, whereas free drug is active.
An equilibrium exists between the bound and unbound drug that is based on the drug and protein concentrations.
A change in binding, due to the protein concentration, may alter the fraction of free drug and thus lead to pharmacological response at a given total concentration of the drug.

Measurement of the free drug concentration is not routinely required, but may be helpful in the following clinical situations:

The drug is bound to plasma proteins by > 85%. For drugs that are so highly bound, a relatively small decrease in the amount of protein available for drug binding may have a correspondingly large impact on the amount of free drug present.
The extent of binding of the drug is known to vary as a result of changes in protein or drug concentration, availability of/or competition for binding sites and binding affinity

Disease condition

Anticonvulsant