Undetectable phenytoin serum levels by an automated particle-enhanced turbidimetric inhibition immunoassay in a patient with monoclonal IgM lambda

Phenytoin is a drug used for the treatment of different types of seizures. Its variable pharmacokinetics, mainly a consequence of variable bioavailability, saturable protein binding and saturable hepatic metabolism, predisposes the drug to therapeutic drug monitoring. Several methods to analyze the drug in serum exist with immunoassays being the method of choice for routine measurements. Immunoassays are specific and sensitive, but cross-reactivity, possibly leading to erroneous serum levels, is a concern. We report a patient with falsely undetectable phenytoin serum levels.