Random workplace drug testing is conducted under the guidelines established by the Substance Abuse and Mental Health Services Administration. One of the most common testing procedures is urinalysis--testing urine samples of employees or prospective employees to determine if there are any illegal drug metabolites present. Drug screening is a two-step process: an initial immunoassay followed by gas chromatography/mass spectrometry or GC/MS confirmation. Potassium nitrate is an adulterant, a chemical added to the urine sample, designed to interfere with the detection of marijuana metabolites in the urine.
Marijuana Metabolites
It is important to note that urine is screened for drug metabolites, not the actual illegal drug itself. In the case of marijuana, the primarily metabolite 11-nor-9-Carboxy-THC, also known as THC-COOH, is formed in the body after cannabis is consumed. THC-COOH is not psychoactive, but it is the primary marijuana metabolite screened for in workplace drug testing.
Most drug metabolites are water-soluble and are rapidly excreted from the body. The marijuana metabolite THC-COOH, however, is fat-soluble. Its removal from the body is a much slower process. A single use of marijuana can result in positive urine tests up to 1 week later, and long-term marijuana use can produce positive results for up to 46 days after last use.
Immunoassays
Immunoassays are first performed on urine tests to detect specific drug metabolites. Immunoassays work on the principle of antigen-antibody interaction. Antibodies directed against specific drug metabolites bind selectively to those metabolites, and this binding is then detected using a variety of methods including radioisotopes, fluorescent compounds or enzymatic assays.
Potassium nitrate can be added to a urine sample to destroy the metabolites that would otherwise bind in the immunoassay. Potassium nitrate can directly destroy marijuana metabolites such as THC-COOH.
Gas Chromotography/Mass Spectrometry
Gas chromatography/mass spectrometry, or GC/MS, is a technique that uses the differences in the chemical properties between different molecules to separate them from a mixture. The gas chromatography separates the molecules, while the mass spectrometry downstream identifies the molecules.
Potassium nitrate added to a urine sample not only interferes with the immunoassay detection, but also interferes with the GC/MS confirmation stage due to the destruction of the TCH-COOH metabolite as well as the internal standard for the assay.
Overcoming the Effects of Potassium Nitrate
Potassium nitrate can destroy drug metabolites by reducing the compound. The use of reducing agents can overcome the effects of potassium nitrate. Compounds such as sodium hydrosulfite or sulfamic acid can be added before the sample is extracted from the immunoassay. However, this only allows detection of the remaining THC-COOH metabolite. It cannot recover the lost concentration of the marijuana metabolite when the potassium nitrate was originally added to the sample.
Potassium Nitrate Products
There are several products on the market that are specifically designed to interfere with drug testing, creating false negatives. One company originally marketed a product that upon analysis by the drug testing industry turned out to be a solution of potassium nitrate.
This led many labs to begin screening all urine samples for the presence of nitrate additives. That company now markets another drug that appears to be a combination of potassium nitrate, as well as other chemicals that prevent confirmatory testing.
