Carbamazepine (Tegretol)

Clinical use

Steady state at 2 weeks trough level immediately before next dose. Peak level 4hrs post dose levels increased by liver disease. Steady state at 2 weeks after any change. Serum levels may be increased by hepatic disease – the drug is almost completely metabolised by the liver.
Low therapeutic – toxic ratio.
Aid to dosage change.
Recommended sampling time – Trough levels (consistently).

Background

Carbamazepine (5-H-dibenzoazepine-5-carboxamide) is regarded as the drug of choice in the treatment of epilepsy, except absence seizures. The complex pharmokinetics of carbamazepine are influenced by its limited solubility in aqueous solution and by a feed-back system involving its ability to induce hepatic oxidase enzymes and hence increase its own metabolism. This metabolism converts it first to a 10, 11-epoxide, which is still pharmacologically active. It is then deactivated by conjugation.

Carbamazepine metabolism is also influenced by other drugs being prescribed at the same time. For these reasons it is important to monitor serum carbamazepine as part of a TDM programme. Carbamazepine levels are also used in the diagnosis and treatment of carbamazepine overdose.

Also known as Tegretol

Reference ranges

Therapeutic range: 4 – 12 mg/L

Specimen requirements

Sample type:

• Serum and Plasma (Lithium Heparin)

Sample identification:

• Three patient identifiers from
  • Name
  • D.O.B.
  • Address
  • N.H.S. number
  • Unit Number
  should match on the specimen and request form. This check may be performed prior to centrifugation by the Central Sort department.

Turnaround time

2 hours