Test Directory
Phenytoin
Containers - Adult
![]() Brown Cap, Gel Clotted
|
Volume Range
0.1 ml
Additive per Container
None |
![]() Orange Cap
|
Volume Range
0.1 ml
Additive per Container
Lithium-heparin |
Containers - Child
![]() Orange Cap - Paediatric
|
Volume Range
0.07 ml
Additive per Container
Lithium-heparin |
Reference ranges
Therapeutic range: 10-20 mg/L
Laboratory Site
RIE
51 Little France Crescent
Old Dalkeith Road
Edinburgh
EH16 4SA
Old Dalkeith Road
Edinburgh
EH16 4SA
Telephone (Biochemistry): 0131 242 6806
SJH
Howden Road West
Howden
Livingston
West Lothian
EH54 6PP
Howden
Livingston
West Lothian
EH54 6PP
Telephone (Biochemistry): 01506 52 3160/1
WGH
Crewe Road South
Edinburgh
EH4 2XU
Edinburgh
EH4 2XU
Telephone (Biochemistry): 0131 537 1910/1
Special instructions for collection
Time of sample collection in relation to dose not crucial due to long half life of phenytoin in plasma. Time to reach steady state is 5-14 days after initiation of therapy or dose adjustment.
Anticipated turnaround
1 day
General additional information
An Adjusted phenytoin level will be calculated where serum albumin levels are less than 40g/L. This is because as albumin levels fall the proportion of active “free” drug is increased, and the reported level therefore requires adjustment to take this effect into account. This adjusted level is a more accurate reflection of pharmacologically active phenytoin than the measured level if albumin is less than 40g/L.
Other factors may also affect phenytoin binding – and caution should be used when interpreting levels in the following patient groups:
- Patients with impaired renal function (seek pharmacy advice where creatinine clearance is less than 25ml/min).
- Patients requiring renal replacement therapy need their adjusted phenytoin levels further corrected (seek advice from a specialist clinical pharmacist).
- Patients prescribed concomitant highly protein-bound medicines (eg sodium valproate).
- Pregnant women (seek specialist advice).
- Children (seek specialist advice).
- Patients requiring renal replacement therapy need their adjusted phenytoin levels further corrected (seek advice from a specialist clinical pharmacist).
- Patients prescribed concomitant highly protein-bound medicines (eg sodium valproate).
- Pregnant women (seek specialist advice).
- Children (seek specialist advice).
Other factors to consider when interpreting phenytoin levels include:
- The time the sample was taken in relation to the dose (for routine sampling a trough level is recommended).
- Time for phenytoin levels to reach steady-state (takes 5 to 14 days for full effect of dose changes).
- Route of administration.
- The time the sample was taken in relation to the dose (for routine sampling a trough level is recommended).
- Time for phenytoin levels to reach steady-state (takes 5 to 14 days for full effect of dose changes).
- Route of administration.