Specimen Collection

Sample stability:

• 4 days at 15-25 ^o C
• 4 days at 2-8 ^o C
• 1-2 months at -20 ^o C

External Referrers: Please separate plasma/serum, freeze at -20 ^o C and transport frozen on ice.

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

The quoted therapeutic range is for trough concentrations; specimens should be collected shortly before administering the next dose of the drug.

Units : umol/L

Therapeutic interval : 40 - 80

Uncertainty of Measurement: 10%

Turnaround Time: Within 3 hours
Assay Method

Principle: Immunoturbidometric assay

Reagents: Roche TDM Phenytoin kit

Analyser: Cobas c502

Diagnostic Use and Interpretation

Adjustment of therapeutic interval in hypoalbuminemia:

Phenytoin is strongly (approx. 90%) bound to albumin. This test measures total phenytoin.

Free phenytoin is the active fraction, but cannot be measured at Labplus.

The therapeutic range given above assumes the patient has a normal serum albumin concentration. In patients with hypoalbuminaemia, a proportionally lower therapeutic range is appropriate, and toxicity may occur at lower drug concentrations.

Approximate therapeutic intervals for hypoalbuminemic patients:

Plasma albumin 30 g/L: 30 - 60 umol/L

Plasma albumin 20 g/L: 20 - 40 umol/L

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