Sorted By Test Name - Mayo Medical Laboratories

FPEMA
90102
PHYF
6794
Conversion Formulas:
Result in mg/dL x 55.6=result in nmol/mL
Result in nmol/mL x 0.0181=result in mg/dL
See Inborn Errors of Amino Acid Metabolism in Special Instructions.
Clinical References: 1. Mitchell GA, Grompe M, Tanguay RM: Hypertyrosinemia. In The
Metabolic and Molecular Bases of Inherited Disease. 8th edition. Edited by CR Scriver, AL Beaudet, WS
Sly, et al. New York, McGraw-Hill Book Company, 2001, pp 1777-1806 2. Scriver CR, Kaufman S:
Hyperphenylalaninemia: phenylalanine hydroxylase deficiency. In The Metabolic and Molecular Bases of
Inherited Disease. 8th edition. Edited by CR Scriver, AL Beaudet, WS Sly, et al. New York, McGraw-Hill
Book Company, 2001, pp 1667-1724 3. Burgard P, Luo X, Hoffmann GF: Phenylketonuria. In Pediatric
Endocrinology and Inborn Errors of Metabolism. Edited by K Sarafoglou, GF Hoffmann, KS Roth, New
York, NY,McGraw-Hill Medical Division, 2009, pp 163-168 4. Blau N, Thony B:
Hyperphenylalanemias: Disorders of Tetrahydrobiopterin Metabolism. In Pediatric Endocrinology and
Inborn Errors of Metabolism. Edited by K Sarafoglou, GF Hoffmann, KS Roth, New York, NY,
McGraw-Hill Medical Division, 2009, pp 169-175
Phenylethylmalonamide (PEMA)
Reference Values:
Reference Range: 1.5-10.0 ug/mL
Test Performed By
Medtox Laboratories, Inc.
402 W. County Road D
St. Paul, MN 55112
Phenytoin, Free, Serum
Clinical Information: 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.
Useful For: Monitoring for appropriate therapeutic concentration: free phenytoin level is the best
indicator of adequate therapy in renal failure Assessing compliance and toxicity
Interpretation: 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.
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