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HelpTest Code PNYG Phenytoin, Total and Phenobarbital Group, Serum
Useful For
Monitoring for appropriate therapeutic concentration of phenytoin and phenobarbital
Assessing compliance or toxicity
Profile Information
| Test ID | Reporting Name | Available Separately | Always Performed |
|---|---|---|---|
| PNYA | Phenytoin, Total, S | Yes | Yes |
| PBR | Phenobarbital, S | Yes | Yes |
Method Name
Kinetic Interaction of Microparticles in a Solution (KIMS)Reporting Name
Phenytoin, Tot and Phenobarbital, SSpecimen Type
SerumSerum Red
Specimen Required
One serum specimen (0.5 mL of serum) may be sent if using a red top tube.
Serum for Phenytoin:
Collection Container/Tube: Red top
Submission Container/Tube: Plastic vial
Specimen Volume: 0.5 mL
Collection Instructions: Centrifuge and aliquot serum into a plastic vial within 2 hours of collection.
Serum for Phenobarbital:
Container/Tube:
Preferred: Serum gel
Acceptable: Red top
Submission Container/Tube: Plastic vial
Specimen Volume: 0.5 mL
Collection Instructions:
1. Serum gel tubes should be centrifuged within 2 hours of collection.
2. Red-top tubes should be centrifuged, and the serum aliquoted into a plastic vial within 2 hours of collection.
Specimen Minimum Volume
0.25 mL for 2 specimens; 0.25 mL for 1 serum red top
Specimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| Serum | Refrigerated (preferred) | 14 days | |
| Frozen | 28 days | ||
| Ambient | 72 hours | ||
| Serum Red | Refrigerated (preferred) | 7 days | |
| Frozen | 14 days | ||
| Ambient | 7 days |
Reference Values
PHENYTOIN, TOTAL
Therapeutic: 10.0-20.0 mcg/mL
Critical value: ≥30.0 mcg/mL
PHENOBARBITAL
Therapeutic: 10.0-40.0 mcg/mL:
Critical value: ≥60.0 mcg/mL
Interpretation
The therapeutic ranges for adults taking phenytoin have been established at 10 to 20 mcg/mL for total phenytoin (bound plus unbound). The therapeutic range for phenobarbital is10 to 40 mcg/mL. Within these ranges, most people will respond to the drugs without symptoms of toxicity. However, response and side effects will be individual. Dosage determinations and adjustments must be evaluated on a case-by-case basis. A free (unbound) phenytoin level may also need to be ordered when a person has kidney failure, liver disease, hypoalbuminemia, or is taking other medications like aspirin, naproxen, or ibuprofen, in which situation the percentage of free (active) phenytoin may be increased.
Clinical Reference
Phenytoin, Total:
1. Richens A: Clinical pharmacokinetics of phenytoin. Clin Pharmacokinet 1979;4:153-169
2. Moyer TP: Therapeutic drug monitoring. In Tietz Textbook of Clinical Chemistry. Fourth edition. Edited by CA Burtis, ER Ashwood. WB Saunders Company, Philadelphia, 2005, pp 1237-1285
Phenobarbital:
Foero O, Kastrup KW, Nielsen EL, et al: Successful prophylaxis of febrile convulsions with phenobarbital. Epilepsia 1972;13:279-285
Day(s) Performed
Monday through Sunday
Report Available
Same day/1 dayPerforming Laboratory
Mayo Clinic Laboratories in Rochester
Test Classification
This test has been cleared or approved by the U.S. Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.CPT Code Information
80184-Phenobarbital
80185-Phenytoin, total
LOINC Code Information
| Test ID | Test Order Name | Order LOINC Value |
|---|---|---|
| PNYG | Phenytoin, Tot and Phenobarbital, S | 104596-2 |
| Result ID | Test Result Name | Result LOINC Value |
|---|---|---|
| PBR | Phenobarbital, S | 3948-7 |
| PNYA | Phenytoin, Total, S | 3968-5 |
Forms
If not ordering electronically, complete, print, and send a Therapeutics Test Request (T831) with the specimen.