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Paediatric Laboratory Medicine

Valproic Acid (Free)

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.

Test Name

Valproic Acid (Free)

Alternate Name/ Synonym

Depakene (Free)

Test Code

VAPFR

Drug Class

Anticonvulsants

Division

Therapeutic Drug Monitoring - TDM

Method

Abbott Architect ci4100

External Proficiency Testing

CAP

Turn Around Time

Monday to Friday

Disease/Condition

Anticonvulsant

Specimen Type

Serum, Plasma (Heparin, Citrate or EDTA)

Minimum Specimen Requirements

300 uL

Optimal Collection Time

Trough: 0-30 minutes prior to dose

Storage/Transportation

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

Approval is not required

CPT Codes

80164

Shipping and Contact Information

The Hospital for Sick Children
Rapid Response Laboratory
170 Elizabeth Street, Room 3642
Toronto, ON
M5G 2G3
Canada
Phone: 416-813-7200
Phone: 1-855-381-3212

Therapeutic Range

35-70 umol/L

Critical Values

Not determined