Antiseizure Medicine Blood Levels

Blood levels of antiseizure medicines can be useful, but they are not a universal test of whether treatment is “working”.

The result must be interpreted alongside seizure control, side effects, dose, timing of the sample, pregnancy, other medicines and the person’s own previous levels.

What is therapeutic drug monitoring?

Therapeutic drug monitoring (TDM) means measuring the concentration of a medicine in blood and using that result to help guide clinical decisions.

A reference range is not a target for everyone

Laboratories often provide a population reference range.

Some people are seizure-free below that range, while others need higher concentrations without toxicity.

The most useful concentration can therefore be the level at which that particular person has good seizure control and acceptable side effects.

When levels can help

Common reasons include:

suspected poor adherence;
unexpected breakthrough seizures;
possible toxicity;
pregnancy;
major drug interactions;
changes in kidney or liver function;
checking an individual’s previous effective concentration;
complex pharmacokinetics.

Timing matters

A level drawn shortly after a dose can be very different from one drawn just before the next dose.

For many medicines, a trough sample—taken immediately before the next scheduled dose—is easiest to interpret consistently.

Phenytoin

Phenytoin has nonlinear pharmacokinetics: a small dose increase can sometimes produce a disproportionately large rise in blood concentration.

Blood-level monitoring is therefore particularly useful.

Protein binding matters

Some medicines, including phenytoin and valproate, are highly protein bound.

Low albumin, kidney disease or displacement by other medicines can change the relationship between the total measured concentration and the biologically active free concentration.

Carbamazepine

Carbamazepine can induce its own metabolism during the first weeks of treatment.

A level measured very early after starting treatment may therefore not represent the eventual steady-state concentration.

Newer medicines

Routine level testing is not equally useful for every newer antiseizure medicine.

For drugs such as levetiracetam or lacosamide, levels are often used selectively rather than as routine dose targets.

Lamotrigine

Lamotrigine levels can be particularly useful during pregnancy because clearance can increase substantially, causing concentrations to fall even when the dose has not changed.

Does every breakthrough seizure need a blood test?

No.

Clinicians first consider missed doses, illness, sleep deprivation, interactions, progression of the epilepsy, incorrect seizure classification and other causes.

Does a “normal” level prove adherence?

Not necessarily.

A single sample reflects only a particular point in time and can be influenced by when the last dose was taken.

Does a high level mean toxicity?

It increases concern, but symptoms matter.

Some people tolerate concentrations above population reference ranges, while others develop adverse effects within them.

Access is unequal

Therapeutic drug monitoring is widely available in many high-income health systems but limited in some low- and middle-income countries.

A systematic review found major differences in laboratory capacity and access.

The central point

The best antiseizure medicine level is not simply the number inside a laboratory range. It is the concentration interpreted in the context of that person’s seizures, side effects and circumstances.

Sources

Evidence checked: September 2026.