Limits of Linearity and Detection for Some Drugs of Abuse*

The limits of linearity (LOL) and detection (LOD) are important factors in establishing the reliability of an analytical procedure for accurately assaying drug concentrations in urine specimens. Multiple analyses of analyte over an extended range of concentrations provide a measure of the ability of the analytical procedure to correctly identify known quantities of drug in a biofluid matrix. Each of the seven drugs of abuse gives linear analytical responses from concentrations at or near their LOD to concentrations several-fold higher than those generally encountered in the drug screening laboratory. The upper LOL exceeds the Department of Navy (DON) cutoff values by factors of approximately 2 to 160. The LOD varies from 0.4 to 5.0% of the DON cutoff value for each drug. The limit of quantitation (LOQ) is calculated as the LOD + 7 SD. The range for LOL is greater for drugs analyzed with deuterated internal standards compared with those using conventional internal standards. For THC acid, cocaine, PCP, and morphine, LOLs are 8 to 160-fold greater than the defined cutoff concentrations. For the other drugs, the LOL's are only 2 to 4-fold greater than the defined cutoff concentrations.