What Is Initial Screen And Confirmation Test?

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Urine drug test usually consists of two steps: first is the initial screen and second is the confirmation test. After urine sample is collected, it is split into 2 parts. First part is used for initial screening, which searches the signs of few (usually from 5 to 10) certain drugs using immunoassay test method. Mostly it is EMIT (Enzyme Multiplied Immunoassay Test), but in rare cases it could be ELISA (enzyme-linked immunosorbent assay) or CEDIA (cloned enzyme donor immunoassay). Immunoassay tests are simple, cheap and fast, but not reliable, so confirmation test on second part of sample is required in case of positive initial screen. The confirmation test uses GC/MS (Gas Chromatography/Mass Spectrometry) technique, which is expensive and complicated, but reliable. It is performed to confirm the presence of only one specific drug, which was detected by initial screen.


Enzyme Multiplied Immunoassay Technique, which had been developed in 1973 for quantitative and qualitative analysis of drugs and proteins in urine, was the first commercially used homogeneous immunoassay. As well as every immunoassay, EMIT utilizes specific antibodies that are designed to bind the required molecules in the sample without affecting other substances. Its main advantage over other methods is use of enzymes that bind antibodies, which did not bind to tested drug molecules, and are deactivated after this, what lets easily separate required substance from others. It is widely used not only for initial screen for drugs of abuse in urine, but for therapeutic drug monitoring in serum also. Nowadays EMIT can detect almost every illicit drug, including cannabinoids (marijuana), opiates (morphine, heroin), cocaine, amphetamines, phencyclidine, barbiturates, methadone, benzodiazepine and others. The concentration of substance must be above the specified cutoff level to be detected by EMIT assay. The cutoffs are set by authorities and can be different for every country. For example, in United States they are set by Substance Abuse and Mental Health Services Administration (SAMHSA) at the levels of 50 ng/ml for THC (marijuana metabolite), 150 ng/ml for cocaine, 2000 ng/ml for opiates etc.


Gas Chromatography - Mass Spectrometry is more complicated technology created in 1952 to detect particular substance in a given test sample. This method has a lot of applications except drug testing. Mainly it is used in forensic science: for fire investigations, identification of explosives and unknown samples from crime scene or human body, to search for poisons or drugs in biological samples of victims or suspects etc. GC/MS is also applicable for environmental monitoring, anti-doping tests, medical diagnosis, perfume, food and beverage analysis and even in astrochemistry for analysis of air and soil samples from other planets. The effect of this technology is based on the combination of two sophisticated techniques - gas chromatography, which separates and analyzes compounds without decomposition, and mass spectrometry, which identifies the type and amount of chemicals in the sample. The combination of these two methods provides a synergistic effect, enabling to identify unknown substances much more effective than in the case of their separate use.

gas chromatography - mass spectrometry equipment


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