The risk of labelling a person with a false positive is high without confirmatory analysis. GC physically separates the compound and MS fragments it so that a fingerprint of the chemical can be obtained. Although sample preparation is extensive, when the methods are used together the combination is regarded as the 'gold standard' by most authorities. This combination is sensitive, specific and can identify all types of drugs in any body fluid. False positives A false positive occurs when results show that the drug is present when, in fact, it is not.
Some false positives are attributable to substances such as asthma or allergy medications.
Switching 'dirty' urine for 'clean' urine is the most common way to deceive a drug-screening test. Hiding condoms containing clean urine on the body or inside the vagina is one common method. Tubes may also be inserted into the urethra. People will add bleach, liquid soap and many other household products in attempts to mask drug use. One technique is to hide a masking substance under the fingernails.