Test ID: PHYF
Phenytoin, Free, Serum
NY State Approved 
Indicates the status of NY State approval and if the test is orderable for NY State clients.
Useful For Suggests clinical disorders or settings where the test may be helpful
Monitoring for appropriate therapeutic concentration: free phenytoin level is the best indicator of adequate therapy in renal failure
Assessing compliance and toxicity
Method Name A short description of the method used to perform the test
Membrane Separation/Immunoassay
Reporting Name A shorter/abbreviated version of the Published Name for a test; an abbreviated test name
Aliases Lists additional common names for a test, as an aid in searching
Dilantin, Total and Free ORDER 9993
Free Dilantin, Serum
Free Dilantin
Free Phenytoin
Specimen Type Describes the specimen type needed for testing
Specimen Required Defines the optimal specimen. This field describes the type of specimen required to perform the test and the preferred volume to complete testing. The volume allows automated processing, fastest throughput and, when indicated, repeat or reflex testing.
Container/Tube: Red top
Specimen Volume: 2 mL
Collection Instructions: Spin down within 2 hours of draw.
Specimen Minimum Volume Defines the amount of specimen required to perform an assay once, including instrument and container dead space. Submitting the minimum specimen volume makes it impossible to repeat the test or perform confirmatory or perform reflex testing. In some situations, a minimum specimen volume may result in a QNS (quantity not sufficient) result, requiring a second specimen to be collected.
Reject Due To Identifies specimen types and conditions that may cause the specimen to be rejected
| Hemolysis | Mild OK; Gross reject |
| Lipemia | Mild OK; Gross reject |
| Icterus | Mild OK; Gross reject |
| Other | NA |
Specimen Stability Information Provides a description of the temperatures required to transport a specimen to the laboratory. Alternate acceptable temperature(s) are also included.
| Specimen Type | Temperature | Time |
|---|---|---|
| Serum Red | Refrigerated (preferred) | 14 days |
| Ambient | 14 days | |
| Frozen | 14 days |
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Phenytoin is the drug of choice to treat and prevent tonic-clonic and psychomotor seizures. If phenytoin alone will not prevent seizure activity, coadministration with phenobarbital is usually effective.
Phenytoin is highly protein-bound (90%), mostly to albumin. Ten percent of the phenytoin circulates in the free, unbound form. Free phenytoin is the active form of the drug, available to cross biologic membranes and bind to receptors. Increased free phenytoin produces an enhanced pharmacologic effect. At the same time, the free fraction is more available to the liver to be metabolized, so it is cleared more quickly.
Valproic acid, an antiepileptic frequently coadministered with phenytoin, competes for the same binding sites on albumin as phenytoin. Valproic acid displaces phenytoin from albumin, reducing the bound fraction and increasing the free fraction of phenytoin. The overall effect of coadministration of a therapeutic dose of valproic acid is that the total concentration of phenytoin decreases due to increased clearance, but the concentration of free phenytoin remains virtually the same. Thus, no dosage adjustment is needed when valproic acid is added to maintain the same pharmacologic effect, but the total concentration of phenytoin should decrease.
In renal failure, the opportunity for the free phenytoin fraction to be cleared is significantly reduced. The end result is that both the total and free concentration of phenytoin increase, with the free concentration increasing faster than the total. Dosage must be reduced to avoid toxicity. Accordingly, the free phenytoin level is the best indicator of adequate therapy in renal failure.
Toxicity is a constant possibility because of the manner in which phenytoin is metabolized. Small increases in dose can lead to very large increases in blood concentration, resulting in early signs of toxicity such as nystagmus, ataxia, and dysarthria. Severe toxicity is typified by tremor, hyperreflexia, lethargy, and coma.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
Therapeutic concentration: 1.0-2.0 mcg/mL
Toxic concentration: > or =2.5 mcg/mL
Interpretation Provides information to assist in interpretation of the test results
Dose should be adjusted to achieve steady-state blood concentration of free phenytoin between 1 and 2 mcg/mL.
Severe toxicity occurs when the total blood concentration exceeds 30 mcg/mL.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
No significant cautionary statements.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
Richens A: Clinical pharmacokinetics of phenytoin. Clin Pharmacokinet 1979;4:153-169
Method Description Describes how the test is performed and provides a method-specific reference
Fresh serum subjected to ultra-filtration to generate a protein-free filtrate, which is then analyzed for free phenytoin by enzyme-multiplied immunoassay technique (EMIT) using the Olympus analyzer. EMIT offers an alternative to the traditional spectroscopic and chromatographic method for quantitating blood concentrations of drugs. The technique for drugs is based upon an enzymatic assay for glucose-6-phosphate dehydrogenase, using spectral properties at 340 nm, in which the reduction of nicotinamide adenine dinucleotide (NAD) substrate is monitored. The basis of the drug detection technique is an immunological reaction between the drug and a specific antibody. The reagent contains the enzyme (glucose-6-phosphate dehydrogenase) to which the drug is covalently bound and antibody-specific to the drug. The antibody binds most of the drug-bound enzyme, rendering the enzyme inactive. This results in a baseline enzymatic activity. In the presence of free drug, antibody equilibrates between free drug and enzyme-bound drug leaving some of the drug-bound enzyme uncomplexed and able to catalyze the reaction. If more free drug is introduced, either as standard or sample, then competition for the antibody takes place between the drug in the sample and the drug attached to the enzyme. This results in more drug-bound enzyme being left uncomplexed and able to catalyze the enzyme reaction at a greater rate as compared to the baseline activity. The observed enzyme activity increases with the amount of total free drug in the sample.(Package insert: Emit 2000 Phenytoin Assay. Olympus, Melville, NY. October 2005)
Day(s) and Time(s) Test Performed Outlines the days and times the test is performed. This field reflects the day and time the sample must be in the testing laboratory to begin the testing process and includes any specimen preparation and processing time required before the test is performed. Some tests are listed as continuously performed, which means assays are performed several times during the day.
Monday through Sunday; Varies
Analytic Time Defines the amount of time it takes the laboratory to setup and perform the test. This is defined in number of days. The shortest interval of time expressed is "same day/1 day," which means the results may be available the same day that the sample is received in the testing laboratory. One day means results are available 1 day after the sample is received in the laboratory.
Maximum Laboratory Time Defines the maximum time from specimen receipt at Mayo Medical Laboratories until the release of the test result
Performing Laboratory Location The location of the laboratory that performs the test
Test Classification Provides information regarding the medical device classification for laboratory test kits and reagents. Tests may be classified as cleared or approved by the US Food and Drug Administration (FDA) and used per manufacturer's instructions, or as products that do not undergo full FDA review and approval, and are then labeled as an Analyte Specific Reagent (ASR), Investigation Use Only (IUO) product, or a Research Use Only (RUO) product.
CPT Code Information Provides guidance in determining the appropriate Current Procedural Terminology (CPT) code(s) information for each test or profile. The listed CPT codes reflect Mayo Medical Laboratories interpretation of CPT coding requirements. It is the responsibility of each laboratory to determine correct CPT codes to use for billing.
80186
LOINC® Code Information Provides guidance in determining the Logical Observation Identifiers Names and Codes (LOINC) values for the result codes returned for this test or profile.
| Result ID | Reporting Name | LOINC Code |
|---|---|---|
| 9993 | Phenytoin, Free, S | 3969-3 |
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