Immunoassay drug testing methods, that have been modified from the manufacturers' recommended procedure to be used for the anaJysls of federally regulated specimens or other forensic samples require evaluation to ensure their scientific validity. These validation studies must demonstrate the accuracy, precision, and linearity of the modified immunoassay around the cutoff concentration, substantiate adequate rate separation, and verify the ability of the assay to differentiate positive and negative samples. Modification of the EMIT d.a.u, phencyclldlne assay In order to achieve the federally mandated cutoff concentration of 25 ng/mL is common. This study describes the validation of a modified EMIT phencyclldine assay and a protocol that allows for the evaluation of similarly modified immunoassays. © 1992, Oxford University Press. All rights reserved.