Urine drug screening false-positive results

Patients who believe that samples taken from them will show positive analytical results frequently attempt to interfere with the analysis by adding various substances to the urine in order to hinder the detection of the drugs. Immunological methods can sometimes even give false positive results after manipulation of the urine, which, in the absence of confirmatory analysis, can place the credibility of the chosen method in doubt. Drug screening by GC/MS... 

The requirement for drug screening is to have the ability to detect as wide a range of compounds as possible in as little sample (plasma/serum/whole blood, urine/vitreous humour, stomach contents/vomit or tissues) as possible at a high sensitivity but with no false positive(s). Ideally some sample should be left to permit confirmation of the results using another technique such as paper chromatography or TLC, and quantification of any poison(s) present to aid clinical interpretation of the results. 

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