Specimen Collection

Sample stability:

• 2 days at 20-25 ^o C
• 7 days at 4-8 ^o C
• 3 months at -20 ^o C

4.5 mL PST Blood (Preferred)

0.5 mL Paediatric Micro-PST Blood (Preferred)

5 mL Heparin Blood

4 mL Plain Blood

3.5 mL SST Blood

0.5 mL Paediatric Microsample Blood

0.6 mL Paediatric Micro-heparin Blood

0.5 mL Paediatric Micro-SST Serum
Reference Intervals

Units: umol/L

Therapeutic interval :	Poor control <350
.	Hepatotoxic >700

Uncertainty of Measurement: 10%

Conversion of units: ug/mL x 6.93 = umol/L

Turnaround Time: Within 3 hours
Assay Method

Principle: Homogenous enzyme immunoassay

Assay: Roche TDM Valproic Acid

Analyser: Cobas c502

Diagnostic Use and Interpretation

The recommended blood collection time is at trough - at least 8 hours from the most recent dose to immediately before the next dose. Time to reach serum steady state for valproate is around 2-4 days (longer in neonates).

The range of serum valproate (trough) which most patients on conventional dose tend to fall into is between 350 - 700 umol/L. This range however, cannot be considered as a true ?therapeutic interval? because of poor correlation between valproate dose and serum concentration. In addition, valproate concentration bears little relation with clinical efficacy of seizure control or with the magnitude of side effects.

Routine therapeutic drug monitoring for valproate level is not recommended. Titration of sodium valproate dose is better accomplished by clinical assessment and regular liver enzyme tests aim for minimal dose to achieve set therapeutic goal while maintaining minimum/no toxicity.

It is only under certain clinical circumstances when valproate level may be helpful:

1. assessment of toxicity

2. assessment of compliance

3. alteration of valproate dose in patients on polypharmacy with potential drug- drug interactions

References:

Rathmalgoda C et al. Serum sodium valproate testing ? is it appropriate ? MJA 2007; 187: 582-584 (see editorial by Vajda FJE. Monitoring anti-epileptic drug therapy with serum level measurement. MJA 2007; 187:581)

Tomson T, Dahl ML, Kimland E. Therapeutic monitoring of antiepileptic drugs for epilepsy. Cochrane Review 2007, issue 4

Scottish Intercollegiate Guidelines Network Guideline no.70. Diagnosis and management of epilepsy in adults. 2004

David R, Peters DH et al. Valproic acid: a reappraisal of its pharmacological properties and clinical efficacy in epilepsy. Drugs 1994; 47:332-72

Contact Information

Emails to chemicalpathologist@adhb.govt.nz will receive priority attention from the on-call chemical pathologist.

If the query concerns a specific patient please include the NHI number in your email.

If email is not a suitable option, please contact the on-call chemical pathologist via Lablink (Auckland City Hospital ext. 22000 or 09-3078995).

Individual chemical pathologists may be contacted but will not be available at all times.

After-hours : contact Lablink (Auckland City Hospital ext. 22000 or 09-3078995) or hospital operator for on duty staff after hours.

Dr Samarina Musaad (Clinical Lead) : SamarinaM@adhb.govt.nz ext. 22402

Dr Cam Kyle: CampbellK@adhb.govt.nz ext 22052

Dr Weldon Chiu: WeldonC@adhb.govt.nz ext. 23427

Dr Campbell Heron: CHeron@adhb.govt.nz ext. 23427

Dr Sakunthala Jayasinghe: Sakunthala@adhb.govt.nz ext. 23427

Last updated at 15:26:00 06/01/2025