OPEN MINDS The Behavioral Health & Social Service Industry Analyst Industry News Urine sample adulteration with common substances can produce false negative results. According to a review in the January, 1989 issue of Clinical Chemistry News, some drugs of abuse were not detectable by enzyme immunoassays when urine was adulterated with certain substances: Amphetamines, cocaine, and opiates: Salt (NaCl) and liquid drain opener Barbiturates: Salt (NaCl) and liquid drain opener Benzodiazepines: Visine, liquid hand soap, and liquid drain opener Marijuana: Salt (NaCl . . .