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Test ID: VPA    
Valproic Acid, Total, Serum

Useful For Suggests clinical disorders or settings where the test may be helpful

Monitoring therapy

 

Assessing compliance

 

Evaluating potential toxicity

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Valproic acid is used for treatment of simple and complex absence seizures and as combination therapy with other anticonvulsants for control of generalized seizures that include absence seizures. Valproic acid is initially dosed at 15 mg/kg/day, with dosage increases over time to a maximum of 60 mg/kg/day.

 

Hepatic failure and a Reyes-like syndrome associated with administration of valproic acid at therapeutic levels have been reported. Careful monitoring of liver function during the first 6 months of therapy is required.

 

The volume of distribution of valproic acid is 0.2 L/kg and its half-life is 10 to 14 hours in adults, and shorter in children. It is approximately 90% protein bound.

 

Analysis of free valproic acid levels may be useful in delineating the cause of toxicity when the total concentration is not excessive.

 

Valproic acid exhibits substantial effects on the pharmacology of phenytoin, whereas phenytoin exhibits only a limited effect on valproic acid. This is due to the relative abundance of the 2 drugs in the body. Valproic acid is present at a 2-fold to 3-fold mass excess and a 5-fold to 7-fold molar excess.

 

Major side effects such as central nervous system depression, thrombocytopenia, and hepatic dysfunction are likely to be experienced if the peak level regularly is >125 mcg/mL.

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: 40 (trough)-100 (peak) mcg/mL

Toxic concentration: > or =120 mcg/mL

Interpretation Provides information to assist in interpretation of the test results

Optimal response is usually observed when the trough level is >40 mcg/mL.

 

Peak levels should not be >100 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

1. Cotariu D, Zaidman JL: Valproic acid and the liver. Clin Chem 1988;34:890-897

2. Moyer TP: Therapeutic drug monitoring. In Tietz Textbook of Clinical Chemistry. Edited by CA Burtis, ER Ashwood. Fourth edition. WB Saunders Company. Philadelphia,  2005, pp 1237-1285