Guideline for Therapeutic Drug Monitoring of Antiepileptic Drugs - Version 1.0 - Key sampling times and reference ranges

Sep 4, 2026

Guideline for Therapeutic Drug Monitoring of Antiepileptic Drugs (Version 1.0) specifies ideal sampling timing, required sample volumes, time to steady state, and reference ranges for commonly monitored antiepileptic drugs. It highlights special considerations such as extended time to steady state for initial carbamazepine use and the impact of diurnal variation on sampling relative to dosing. It also clarifies recommended metabolite monitoring during primidone therapy and outlines available specimen types (blood or saliva) and assay reporting formats for total versus free concentrations.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages3

Reading Time8 min

AccessFree to download

Therapeutic Drug Monitoring guideline

Drug-by-drug sampling and reference ranges

Notes for sampling considerations

Specimen types and assay reporting

Guideline for Therapeutic Drug Monitoring of Antiepileptic Drugs-Version: 1.0. Index: TD M-FR M-SER-010. Printed: 27-Feb-2026 11:25 Therapeutic Drug Monitoring Unit Title: Guideline for Therapeutic Drug Monitoring of Antiepileptic Drugs Code: TDM-FRM-SER-010 Version: 1.0 Authors: Sharon Allen-Phillips Authorised By: Cerian Dance and Sharon Allen-Phillips Authorised Date: 27-Feb-2026 Review Date: 27-Feb-2028 Document Type: Document Document Status: Authorised OU Name: Chalfont Epilepsy Unit Guideline for Therapeutic Drug Monitoring of Antiepileptic Drugs-Version: 1.0. Index: TDM-FRM-SER-010 . Printed: 27-Feb-2026 11:25 Authorised on: 27-Feb-2026 . Authorised by: Cerian Dance and Sharon Allen-Phillips. Document Unique Reference: 35-134747459. Due for review on: 27-Feb-2028 Author(s): Sharon Allen-Phillips Guideline for Therapeutic Drug Monitoring of Antiepileptic Drugs-Version: 1.0. Index: TD M-FR M-SER-010. Printed: 27-Feb-2026 11:25 | Antiepileptic Drug | Ideal Sampling Time | Sample Volume (mL) | Time To Steadystate (Days) | Reference Range (mg/L) | | --- | --- | --- | --- | --- | | Brivaracetam | Immediately before oral dose | 0.2 | 1 to 2 | 0.2 to 2.0 | | Carbamazepine | Immediately before oral dose | 0.2 | 2 to 4A | 4 to 12 | | Carbamazepine epoxide (pharmacologically active metabolite of carbamazepine) | Immediately before oral dose | 0.2 | 7A | Up to 2.3 | | Clobazam | Immediately before oral dose | 0.2 | 7 to 10 | 30 to 300 (µg/L) | | Desmethyl Clobazam (Pharmacologically active metabolite of clobazam) | Immediately before oral dose | 0.2 | 7 to 10 | 300 to 3000
(µg/L) | | Clonazepam | Immediately before oral dose | 0.2 | 3 to 10 | 20 to 70 (µg/L) | | Eslicarbazepine | Immediately before oral dose | 0.2 | 3 to 4 | 3 to 5 | | Ethosuximide | Immediately before oral dose | 0.2 | 8 to 12 | 40 to 100 | | Felbamate | Immediately before oral dose | 0.2 | 3 to 5 | 30 to 60 | | Free Carbamazepine | Immediately before oral dose | 0.5 | 2 to 4A | 1 to 3 | | Free Lamotrigine | Immediately before oral dose | 0.5 | 3 to 8 | 1 to 7 | | Free Phenobarbital | Immediately before oral dose | 0.5 | 15 to 30 | 5 to 18 | | Free Phenytoin | Immediately before oral dose | 0.5 | 6 to 21 | 1 to 2 | | Free Valproic Acid | Immediately before oral dose | 0.5 | 2 to 4 | 5 to 10 | | Gabapentin | Immediately before oral dose | 0.2 | 1 to 2 | 2 to 20 | | Lacosamide | Immediately before oral dose | 0.2 | 2 to 3 | 10 to 20 | | Lamotrigine | Immediately before oral dose | 0.2 | 3 to 8 | 2.5 to 15 | | Levetiracetam | Immediately before oral dose | 0.2 | 1 to 2 | 12 to 46 | | 10-Hydroxycarbazepine (pharmacologically active oxcarbazepine) | Immediately before oral dose | 0.2 | 2 to 3 | 3 to 35 | | Phenobarbital | Immediately before oral dose | 0.2 | 15 to 30 | 10 to 40 | | Perampanel | Immediately before oral dose | 0.2 | 10 to 19 | 200 to 1000
(µg/L) | | Phenytoin | Immediately before oral dose | 0.2 | 6 to 21 | 10 to 20 | | Pregabalin | Immediately before oral dose | 0.2 | 1 to 2 | 2 to 8 | | Primidone | Immediately before oral dose | 0.2 | 2 to 5 | 5 to 10 | | Rufinamide | Immediately before oral dose | 0.2 | 1 to 2 | 30 to 40 | Guideline for Therapeutic Drug Monitoring of Antiepileptic Drugs-Version: 1.0. Index: TDM-FRM-SER-010 . Printed: 27-Feb-2026 11:25 Authorised on: 27-Feb-2026 . Authorised by: Cerian Dance and Sharon Allen-Phillips. Document Unique Reference: 35-134747459. Due for review on: 27-Feb-2028 Author(s): Sharon Allen-Phillips Guideline for Therapeutic Drug Monitoring of Antiepileptic Drugs-Version: 1.0. Index: TD M-FR M-SER-010. Printed: 27-Feb-2026 11:25 | Antiepileptic Drug | Ideal Sampling Time | Sample Volume (mL) | Time To Steadystate (Days) | Reference Range (mg/L) | | --- | --- | --- | --- | --- | | Stiripentol | Immediately before oral dose | 0.2 | 1 to 3 | 4 to 22 | | Tiagabine | Immediately before oral dose | 0.2 | 1 to 2 | 20 to 200 (µg/L) | | Topiramate | Immediately before oral do

FAQs

What is the ideal sampling time for antiepileptic drug therapeutic drug monitoring?

For most drugs, the guideline specifies sampling immediately before the oral dose. Sampling time relative to dosing is critical for drugs with significant diurnal variation.

How long does it take to reach steady state for different drugs?

The guideline provides a time-to-steady-state range (in days) for each antiepileptic drug. Carbamazepine introduced for the first time requires about 20 days due to autoinduction.

Which specimens can be used and how are results reported?

Testing is available from blood (plasma/serum) and saliva. Assays can be reported as total plasma or serum concentrations or as free, non-protein-bound concentrations.

Sep 4, 2026

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