Legal Information, including Disclosures and Disclaimers regarding presumptive field tests
In medical and forensic science, a presumptive test is an analysis of a sample, which establishes either:
A) The sample is definitely not a certain substance, or
B) The sample probably is the substance.
The presence of a substance, even present at a trace level, can be detected by a presumptive test.
In this case, pertinent examples include the cobalt thiocyanate test (Scott Reagent) for cocaine, the Duquenois reagent for marijuana, and the
Marquis reagent for narcotics. Other reagents for specific drugs are also available. Reagents used by Scott Company to determine the presence of illegal drugs or controlled substances are listed in the 2005 National Institute of Justice Color Test Reagents/Kits for Preliminary Identification of Drugs of Abuse: NIJ Standard–0604.01 (Dept of Justice)
These tests, and others like them, are presumptive tests, and can generally be used to establish the probable cause necessary to effect an arrest for the possession of an illegal drug or other controlled substance. An additional confirmatory test can be used after the presumptive test report is positive to confirm the identity of the substances or to measure the percentage purity or other quantitative analysis.
Presumptive field test kits are engineered to detect one (or more) specific substances and display a specific reaction appropriate to that particular substance. While presumptive testing is extremely reliable, faster, and less expensive than other methods of testing, it is possible (though unlikely) to receive a false positive result under certain conditions, when certain substances are introduced into the presumptive test.  We strongly advise the individual officer and appropriate agencies to use common sense and evaluate the totality of the circumstances before making an arrest.  
Because of this, Scott Company recommends that, in an effort both to aid the prosecution and to protect the legal and constitutional rights of the accused, in the absence of a written confession, signed by the suspect, the arresting agency should (where and when appropriate in accordance with local, county, state, or federal laws and ordinances) obtain a confirmation of the composition of any substance tested using a confirmatory testing method (such as a gas chromatography/mass spectrometry (GC/MS)) to obtain a confirmed analytical result before final charges are filed against an individual suspected of the sale, delivery, manufacture or possession of the suspected illegal drug or other controlled substance. 
If you have any questions, please don't hesitate to contact us, your local prosecutor or district attorney's office, or legal counsel. 
Thank You.