Table I. Coat-A-Count Methamphetamine RIA, Sodium Periodate Solution Stability Assay Without With 0.0075M Nal04 in eachtube

To the Editor: The paper by J. D'Nicuola et al., "Evaluation of Six Commercial Amphetamine and Methamphetamine Immunoassays for Cross-Reactivity to Phenylpropanolamine a dEphedrine in Urine " (1), reports that the DPC Coat-A-Count Methamphetamine can give false positive r sults at concentrations of 1000 ng/mL L-ephedrine and D-pseudoephedrine but not 100 ng/mL. Because L-ephedrine and D-pseudoephedrine are sold in some over-the-counter preparations and are excreted into urine largely as the parent drug, it is possible that concentrations i the range of 1000 mg/L might be encountered in urine specimens. The periodate treatment reported by E1Sohly (2) can be used directly in the Coat-A-Count assay to destroy the ephedrine and pseudoephedrine without effecting the performance of the assay (3). Because ephedrine and related compounds such as pseudoephedrine ar-hydroxyamines, they can be degraded by periodate treatment into smaller fragments which do not cross react with the antibody. Amphetamine and methamphetamine are not affected by periodate treatment (2). Adding NalO4 (50 L of 0.15 M NalO4 solution [4]) to produce afinal concentration of 0.0075M in the coated tube is sufficient to prevent the interference from ephedrine and related compounds in the DPC Coat-A-Count Methamphetamine radioimmunoassay (3). This concentration has no deleterious effects on the performance of the assay and reduces false positive readings due to ephedrine and pseudoephedrine to less than 10 % of the apparent concentration before treatment (Table I). Table II shows the cross-reactivity of ephedrine and pseudoephedrine somers i