False negative ELISA screen for benzoylecgonine in blood using the Neogen® assay.

Enzyme-linked immunosorbent assay (ELISA) is a common analytical tool for screening samples in forensic toxicology laboratories, but it can be susceptible to interferences. The analysis of paired blood and oral fluid samples from the same subject led to the identification of a false negative ELISA screen for benzoylecgonine (BE) in blood. Investigation demonstrated that the issue was only with the Neogen® BE assay, and sample dilution did not remedy the problem. Based on subsequent monitoring of ELISA screens in blood, the false negative anomaly has only been identified in one specimen, and the cause remains undetermined.