Using Free Antiepileptic Drug Levels for Therapeutic Drug Monitoring

When is therapeutic drug monitoring (TDM) of antiepileptic drugs indicated for clinical management
of epilepsy?

A: TDM of antiepileptic drugs (AEDs) may prove useful when initializing or adjusting treatment, establishing a personal therapeutic range after desired clinical response is reached, or when assessing or managing toxicity. TDM also might be needed when there is persistent seizure activity despite typical dosing, an AED formulation change, unexpected clinical response, question of AED compliance, and for patients with variable pharmacokinetics.

When is the free AED level clinically useful?

Labs can measure either the total or non-protein-bound (free) AED level. In most clinical situations, total AED measurement is adequate for TDM due to a relatively constant relationship between the free and protein-bound fractions. Analytically, the total AED level is less cumbersome, less costly, and less time-consuming to measure. Nevertheless, the free fraction most accurately reflects the active component of AEDs, and can be of clinical utility in certain scenarios.

Measurement of the free fraction is most useful with extensively bound AEDs. Examples of highly protein-bound AEDs include carbamazepine (~75% bound), phenytoin (~90%), and valproic acid (~90%). When the protein binding of these AEDs is disturbed, the free concentration may reflect the pharmacologically active concentration with greater accuracy than the total. This can occur in a variety of common situations, such as when patients are experiencing hepatic disease, renal disease, pregnancy, hypoalbuminemia, and uremia.

Another common reason to measure free AED levels is to assess potential toxicity and guide AED reintroduction. The free fraction is particularly useful in this scenario due to the fact that toxicity is often caused by factors that disrupt protein binding, such as drug-drug competition for available plasma protein binding sites or protein site displacement. For example, the percent of protein-bound valproic acid is dependent on saturation of available protein-binding sites. However, saturation can be reached unexpectedly if a patient is taking multiple AEDs or other medications—and once available plasma protein sites are saturated, the free fraction of valproic acid rises rapidly. This leads to a scenario where the total valproic acid concentration may be therapeutic, but the free (or pharmacologically active) fraction is starkly elevated and indicates possible toxicity to the clinician.

How are AED levels measured in the laboratory?

Labs use a variety of techniques to measure AED levels, including immunoassays, gas chromatography, liquid chromatography, and chromatography interfaced with a mass spectrometer. Immunoassays are typically performed in core hospital laboratories on automated chemistry analyzers or immunoanalyzers. Generally, they do not offer the analytical specificity or analyte multiplexing ability afforded by the other techniques listed. However, an advantage of immunoassays is their decreased turnaround time due to the 24/7 operations of a core laboratory with random access instrumentation. Short turnaround times are especially valuable in cases of suspected toxicity.

In order to measure the free fraction specifically, labs must separate out the non-protein-bound AED prior to analysis. Two techniques to accomplish this task are equilibrium dialysis or ultrafiltration. Both utilize a membrane with pores that are small enough to keep the protein-bound AED from passing through. Ultrafiltration also has the advantage of speed (it typically takes 20–40 minutes) due to the process being aided by centrifugation. After the non-protein-bound fraction is separated, it is collected for subsequent analysis. If a free AED assay is not offered by a chemistry analyzer vendor, labs can use a total AED assay to measure the free fraction. However, this may require optimization (e.g., increased sample volume and lower concentration calibrators) due to the lower concentrations of free versus total AED.

Adam J. McShane, PhD, DABCC (CC, TC), FADLM, is the medical director of the automated chemistry laboratory at Cleveland Clinic. +EMAIL: [email protected]