Valproate, Free, plasma or serum

Alternate test name

Valproic Acid, Depakene, Epival

Lab area

Clinical Biochemistry - TDM & Toxicology

Method and equipment

Equipment : Roche Cobas Pro c503

Method : The assay is based on a homogeneous enzyme immunoassay technique used for the quantitative analysis of valproic acid (free and protein‑bound)
in human serum or plasma. The assay is based on competition between drug in the sample and drug labeled with the enzyme glucose‑6‑phosphate dehydrogenase (G6PDH) for antibody binding sites. Enzyme activity decreases upon binding to the antibody, so the drug concentration in the sample can be measured in terms of enzyme activity. Active enzyme converts oxidized nicotinamide adenine dinucleotide (NAD) to NADH, resulting in an absorbance change that is measured spectrophotometrically. Endogenous serum G6PDH does not interfere because the coenzyme functions only with the bacterial (Leuconostoc mesenteroides) enzyme employed in the assay.

Expected turn-around time

STAT/Urgent: 3 hours Routine: 24 hours

Specimen type

Serum, Plasma (Heparin)

Specimen requirements

300 uL

Storage and transportation

4°C (transport with a cool pack if possible)

Background and clinical significance

Refer to the Valproic Acid (Total) for a complete description of the clinical significance of Valproic Acid. Routine therapeutic drug monitoring generally measures the concentration of the total drug.

Considerations:

Drug which is bound to plasma proteins is pharmcologically inactive, whereas free drug is active.
An equilibrium exists between the bound and unbound drug that is based on the drug and protein concentrations.
A change in binding, due to the protein concentration, may alter the fraction of free drug and thus lead to pharmacological response at a given total concentration of the drug.

Measurement of the free drug concentration is not routinely required, but may be helpful in the following clinical situations:

The drug is bound to plasma proteins by > 85%. For drugs that are so highly bound, a relatively small decrease in the amount of protein available for drug binding may have a correspondingly large impact on the amount of free drug present.
The extent of binding of the drug is known to vary as a result of changes in protein or drug concentration, availability of/or competition for binding sites and binding affinity.

Disease condition

Anticonvulsant