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Test Code PTN Phenytoin Level, Total, LAB31


Turbidimetric Inhibition Immunoassay

Performing Laboratory

St. Luke's Hospital Cedar Rapids

Specimen Requirements


Preferred: Plain red-top tube, serum physically removed from cells as soon as possible

Acceptable: Gold-top serum gel tube or Green-top (heparin) tube, must be removed within 2 hours from gel separator.
Specimen Volume: 0.2 mL of serum or plasma

Stability: Serum or plasma should be physically separated from cells as soon as possible

               and should be analyzed immediately or kept at 2 – 8°C and analyzed within 2 hours

               from the time of collection.  If the serum/plasma is removed from the SST/PST tubes it can

               be tested up to 72 hours when kept at 2 – 8°C or frozen for long term storage. Thawed

               samples should not be refrozen.

Collection Instructions: Peak level should be drawn 2 to 6 hours after oral dosing or

                                         3 to 9 hours post dose with sustained release formulations.

Note: 1. Phenytoin time to steady state is after 4 to 24 days of chronic oral dosing.

          2. If fosphenytoin is drug being given, specimen cannot be drawn until at least 2 hours

              after an intravenous dose or 4 hours after an intramuscular dose to allow enough time

              for fosphenytoin to be converted to phenytoin.

          3. Levels may be ordered at any time toxicity is suspected.


Reference Values

Therapeutic range: 10.0-20.0 mcg/mL

Critical value: >40.0 mcg/mL

Day(s) Test Set Up

Monday through Sunday

Test Classification and CPT Coding


Useful For

Phenytoin (Dilantin®) is a drug widely used and effective for all types of seizure disorders except absence seizures. It is of value in the treatment of elementary partial (focal) or complex partial epilepsy (psychomotor, temporal lobe) seizures, but ineffective in petit mal epilepsy. Occasionally, it is used in the treatment of cardiac arrhythmias.