The commercial adulterant product Klear comes in two micro-tubes containing 500 mg of white crystalline material. This product readily dissolves in urine with no change in color or temperature of the urine. Klear may cause false-negative GC-MS confirmation of marijuana. ElSohly et al. (14) first reported this product as potassium nitrite and provided evidence that nitrite leads to decomposition of ions of A-9-THC and its internal standard. The authors reported that using a bisulfite step at the beginning of sample preparation could eliminate this interference. The group also investigated the effect of nitrite on immunoassay screening of other drugs. These drugs include cocaine metabolites, morphine, THC metabolites, amphetamine, and phencyclidine. Nitrite at a concentration of 1.0 M had no effect on the Abuscreen assay. At a higher nitrite concentration, the amphetamine assay becomes more sensitive and the THC assay becomes less sensitive. The GC-MS analyses of BE, morphine, amphetamine, and phencyclidine were not affected, whereas recovery of the THC metabolites was significantly reduced. Again, this interference could be eliminated by bisulfite treatment.
Nitrites also significantly affected the pH of urine samples, creating false-negative test results in immunoassays. Another important factor was the original drug concentration. However, for carboxyl-THC, regardless of the original drug concentration, all specimens with acidic pH showed negative immunoassay results using a SYVA EMIT dau or Roche Abuscreen OnLine® system or an onsite THC immunoassay (Roche OnTrak TesTstik™). Significant decreases in immunoassay results could be observed within 4 h of nitrite adulteration (15).
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