Abstract
A novel flow chromatographic immunoassay method using end-point dilution was developed in our laboratory as an alternative to the homogeneous enzyme immunoassay, for low-cost, simple, and fast semi-quantitative analyses of drugs of abuse in human urine. A serial dilution of the positive test sample using rapid test cassettes is continued until reaching the lowest dilution, with a visual detectable colored band at the definite cutoff concentration. The calibration curve of the equivalent dilution volume (end-point titers) vs concentration of the spiked urine sample exhibited excellent linearity with a coefficient of determination R2 of 0.9995 for cannabinoids and 0.9997 for opiates; this indicates that end-point dilution flow chromatographic immunoassay can be used in a semi-quantitative mode for drugs of abuse urine analysis. The developed dilution method exhibits a visual limit of detection (LOD) at the cutoff, and a linear relationship ≥ 50 ng/mL for cannabinoids and ≥ 300 ng/mL for opiates, without an upper limit of detection. The recovery of spiked urine samples is within 98–111% of theoretical values for cannabinoids and 101–110% for opiates. Good correlation was obtained between end-point dilution flow chromatographic immunoassays and homogenous immunoassays with a Thermo Scientific Indiko autoanalyzer for cannabis abuse specimens.
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