Name: Phenytoin, Free
Test Name: Phenytoin, Free
Health Link Test Code: HCUPTN
LIS Test Code: UPTN
CPT Code(s): 80185, 80186
Total phenytoin and free phenytoin.
Methodology: Liquid Chromatography
Days Performed: Daily, dayshift.
In-Lab Turnaround Time: 1 day.
Stat In-Lab Turnaround Time: 3 hours.
Collection Container: Red top
Also Acceptable: Green top (with lithium heparin anticoagulant)
Collection Volume: 9 mL
Pediatric Collection Volume: 3 mL
Sample Analyzed: Serum
Testing Volume: 2 mL
Pediatric Testing Volume: 1 mL
Centrifuge. Transfer cell-free serum to plastic vial. Refrigerate.
Transport specimen to laboratory. Transport with coolant pack if coming from clinic location.
Specimens collected in red cap with yellow ring (SST) tubes are not acceptable.
The free level of phenytoin is expected to be approximately 10% of the total level. Free levels greater than 3 mcg/mL usually produce toxicity.
Generally effective therapeutic concentration:
18 years and up: 1 - 2 mcg/mL
Interpretation Type: Expected Results
Critical Calls: yes
Result of "BELOW DETECTION" indicates that result is below the method detection limit of 0.5 mcg/mL. The total concentration of a drug in the blood includes the protein-bound fraction. Only the free drug is considered to be pharmacologically active. Due to the fact that phenytoin is highly bound to plasma proteins (89-95%), any alteration in the protein binding of phenytoin due to uremia, hypoalbuminemia, ingestion of other drugs, or age can result in a significantly different clinical response to a total phenytoin concentration. When a patient's clinical response does not agree with the total phenytoin concentration, or the protein binding of a patient is believed to be abnormal, the free phenytoin level may correlate more accurately than the total level with the clinical effectiveness or toxicity of the drug. In such cases, the determination of the free level of phenytoin is essential. Total phenytoin, free phenytoin, and % free phenytoin will be reported. If total phenytoin is less than 5 mcg/mL, the free level will be run only upon the request of the ordering source.
This test was developed and its performance characteristics determined by the UWHC Clinical Laboratories. This test meets the standards for clinical testing. This test should not be regarded as investigational or for research use. The US Food and Drug Administration (FDA) has not approved or cleared this test; however, the FDA has determined that such clearance or approval is not necessary.