Analytical performance evaluation of a new turbidimetric immunoassay for valproic acid on the ADVIA® 1650 analyzer: effect of gross hemolysis and high bilirubin

Valproic acid is an anticonvulsant that requires careful therapeutic drug monitoring. Valproic acid is also used in psychiatric patients. Bayer Diagnostics (Tarrytown, NY) recently marketed a turbidimetric immunoassay for monitoring valproic acid concentrations in serum or plasma using the ADVIA® 1650 analyzer. We evaluated the performance of this new assay by comparing it with a widely used fluorescence polarization immunoassay (FPIA) on the AxSYM® analyzer (Abbott Laboratories, Abbott Park, IL). The total coefficient of variation (CV) for the low control of this new assay was 6.8% (mean=30.7, SD=2.1 µg/mL, n=44) while the corresponding CVs for the medium and high controls were 3.3% (mean=81.0, SD=2.7 µg/mL, n=44) and 5.9% (mean=142.9, SD=8.4 µg/mL, n=44), respectively. The assay is linear up to a serum valproic acid concentration of 170 µg/mL, and the detection limit is 4.4 µg/mL. We observed an excellent correlation between the FPIA of valproic acid and the turbidimetric assay using specimens from 52 different patients who were receiving valproic acid. Using the valproic acid concentrations obtained by the FPIA as the x‐axis, and the corresponding valproic acid concentrations obtained by the turbidimetric assay as the y‐axis, we developed the following regression equation: y=1.03 x+1.55 (r=0.98). With this new assay, high concentrations of bilirubin (unconjugated 30 mg/dL and conjugated 30 mg/dL) and gross hemolysis (4+, hemoglobin: 1,500 mg/dL) have no effect on measurements of valproic acid concentration. We conclude that the new turbidimetric assay for valproic acid can be used for routine therapeutic drug monitoring of valproic acid in clinical laboratories. J. Clin. Lab. Anal. 19:31–35, 2005. © 2005 Wiley‐Liss, Inc.