Adult patient:
Heparin gel tube
Minimum blood volume: 5 mL
Paediatric patient:
Paediatric heparin tube
Number of vials: 1
Minimum blood volume: 1.3 mL
Authorisation code required
No
24 Hr available service
Yes
For suspected phenytoin overdose cases, please call Duty Biochemist for 'Urgent' phenytoin level testing.
Method
Immunoassay
Reference interval
| Therapeutic range: | 40 – 80 µmol/L |
| Nystagmus often appears: | 81 – 99 µmol/L |
| Ataxia may be observed: | 100 – 119 µmol/L |
| Dysarthria may present: | > 158 µmol/L |
(Source: from manufacturer's kit insert)
Clinical indication
Therapeutic drug monitoring
Result interpretation
Steady state is not achieved for several days. Re-testing after a change of dose should be delayed for 48 hours.
The half-life is 7 - 60 Hr (dose dependent).
This drug exhibits Michaelis-Menten kinetics, that is, as serum levels rise a stage is reached when a small increase in dose causes a large increase in serum levels.
Measurement of uncertainty
Frequency of measurement
Daily