Falsely elevated enzyme-multiplied immunoassay serum valproic acid results in 12 patients

A split sample study of 69 patients' serum for valproic acid analysis by enzyme-multiplied immunoassay technique (EMIT) and by gas-liquid chromatography revealed falsely elevated EMIT values in 12 patients. Of the 69 serum samples, 28 were also assayed by fluorescence polarization immunoassay and all results agreed with those of gas-liquid chromatography. Five of the 12 patients with elevated EMIT values were included in this subgroup. The unidentified interfering substance appears to be a heat-labile, high-molecular-weight compound, possibly a protein complex that can persist in blood for several months following discontinuation of the drug.