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There are many different types of drug tests. In a criminal case there is an important differerence between a "presumptive" test and a "conclusive" test. A presumptive test is a test usually performed by a police officer where the officer puts the alleged controlled substance into a plastic container purchased from a company, mixes it up, and looks at what color it changes. Because this is much cheaper, easier for the officer, and easy to explain, prosecutors like to rely on this test and prefer not to subject the alleged controlled substance to real testing at a lab.
Like most things in criminal cases, police and prosecutors what the public to believe their work, testimony, testing etc. is 100% all the time and that there is no chance of any error or problem. In a 1990 Wisconsin Court of Appeals Case, State v. Jackson 459 N.W.2d 260, (Wis. App 1990), the Court of Appeals disagreed with a trial judge who claimed a police officer was an expert and the fact that 3 of 14 "presumptive" tests indicated cocaine meant the defendant was guilty. Even though a jury found the defendant guilty of possession of cocaine, the Wisconsin Court of Appeals held that the case should have been dismissed because the officer's testimony was insufficient to show the alleged substance was cocaine. The Court made clear that color/presumptive drug tests have numerous problems including a 20-30% false positve rate:
"In its questions to Eigenfeld and the answers given to those questions, the trial court allowed the cobalt thiocyanate test evidence to stand. Also, the record notes the detective's "professional certainty" that the drug was cocaine. Thus, this court must resort to sources that are indisputably accurate via its inherent power of judicial notice to support ordinarily legislative facts that allow the appellate court to arrive at policy decisions that only incidentally relate to the case at bar. 31
The cobalt thiocyanate field test employed here is also known as the Scott test. 32 It is a field test that is a major improvement over past cocaine field testing, but it is not specific for cocaine. 33 It is also known as a nonspecific, 34 presumptive, 35 color screening test. 36
A few terms must be defined. Nonspecific, presumptive screening tests for drugs such as the test employed in this case, may result in what are called false positives, i.e. false identification of cocaine when the substance is some other alkaloid. "The Scott field test was designed to distinguish cocaine from other alkaloids, but is deficient in distinguishing cocaine from other drug mixtures such as lidocaine and PCP [phencyclidine]." 37 A test result that is specific for a particular drug is such if the result occurs only when that drug is present, 38 and are subject to few false positive results. 39 An example of a specific test for a drug such as cocaine is a Gas Chromatography/Mass Spectrometry. 40
Examples of the intended uses of color field tests such as the test used in this case, and their strengths and weaknesses are noted as follows:
Although color change tests are both easy and fast to conduct, they have major weaknesses. To begin with, color change tests are nonspecific:
If we assume that there are 20 distinguishable colors, then there are 8,000 possible responses in three-color tests and 160,000 possible responses in four color tests. Statistically, 250 different compounds (out of 2,000,000) would give the same three-color tests, and 12 would give the same four-color tests.
Color tests are often used as field screening procedures. When the police seize a suspected drug, they can conduct a color change test to determine whether there is probable cause to believe that the substance is a contraband drug. If the test is positive, the substance may be forwarded to a crime laboratory for further analysis. In some laboratories, upon further analysis twenty to thirty percent of the positive field color tests turn out to be false positives.
There is also an element of subjectivity in the interpretation of the test results. One analyst might identify a color as a light red while another analyst would record the test result as a pink. There are techniques for dealing with this problem. Many manufactures furnish color charts that can be placed next to the plate or tube for direct comparison. In addition, the Inter-Society Color Council-National Bureau of Standards (ISCC-NBS) has developed a set of color charts for the same purpose. Finally, the analyst can photograph the test result with high quality color film. The production of the charts and photograph at trial enables the trier of fact to make a direct comparison and doublecheck the analyst's evaluation of the test result. 41
In 1982, a United States Army forensic chemist had this to say of color field tests:
Field tests were designed to assist law enforcement agencies in drug investigations. They are simple and quick procedures for testing materials suspected of containing drugs which help the agent determine if a substance requires additional analysis by forensic laboratory personnel. Field tests were never intended to be used as a positive method of drug identification. 42
He later stated in the same report:
In summary, field tests are not confirmatory for drugs. They were never intended to be confirmatory nor should they be used as such in courts-martial or elimination boards."
Another problem with drug field tests are they encourage subjectivity by the officer. According to the Court of Appeals, "There is also an element of subjectivity in the interpretation of the test results. One analyst might identify a color as a light red while another analyst would record the test result as a pink."