Immunoassay screen for qualitative detection of cocaine in urine.

Cocaine (benzoylmethylecgonine) is derived from the plant species Erythroxylon coca, which is widely grown in South America. Cocaine is a very common illicit drug and is popularly abused in the US. Cocaine abuse can produce euphoria, arousal, garrulousness, alertness, anxiety, insomnia, hyperactivity, paranoia, severe psychosis, and even suicide. Cocaine is rapidly metabolized, with less than 5% excreted unchanged in the urine. The two major metabolites, which result from enzymatic and nonenzymatic hydrolysis, are benzoylecgonine and ecgonine methyl ester. The metabolites may be detectable in urine for up to 3 weeks after long term, heavy use of cocaine. 

The cocaine screening assay uses a specific antibody, which can detect benzoylecgonine in urine. The assay is based on the competition of an enzyme glucose-6-phosphate dehydrogenase (G6PDH) labeled drug and the drug from the urine sample for a fixed amount of specific antibody binding sites. In the presence of free drug from the sample, the free drug occupies the antibody binding sites, allowing the drug-labeled G6PDH to interact with the substrate, resulting in enzyme activity. 

In the absence of drug from the sample, the specific antibody binds to the drug labeled with G6PDH and the enzyme activity is inhibited. This phenomenon creates a direct relationship between the drug concentration in the urine and the enzyme activity. The enzyme G6PDH activity is determined spectrophotometrically at 340 nm by measuring the ability to convert nicotinamide adenine dinucleotide (NAD) to NADH.