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Field Drug Testing: How to Confront False Positives with Expert Testing

Anjelica Cappellino, J.D.

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— Updated on April 27, 2022

Field Drug Testing: How to Confront False Positives with Expert Testing

Field Drug Testing Expert WitnessThere are over two million Americans serving time in prison – more than any other country in the world. About half of all federal inmates, and 16% of all state prisoners, are serving time for drug convictions. These numbers have drastically increased over the past three decades. This is largely due to changes in prosecution and sentencing policies in the “War on Drugs.”

Since the inception of the War on Drugs, the number of Americans incarcerated for drug offenses has soared from 40,000 to nearly half a million. As acquittals become increasingly difficult to obtain – the federal courts enjoy an approximate 95% conviction rate – it is imperative for defendants and criminal defense attorneys alike to utilize every possible tool in their arsenal. With the advent of police field testing for drugs – test kits used to quickly identify, by color indicator, whether a substance contains narcotics – expert witnesses are a vital part of any drug charge defense. In fact, as these tests become more frequently exposed as faulty and unreliable, an expert’s testimony regarding drug testing may often be the one deciding factor in a defendant’s guilt or innocence.

Statistical Errors & Human Consequences

Drug Test Expert WitnessThe use of field testing to identify allegedly illegal substances has become an increasingly utilized police tactic over the years. At a relatively cheap cost – approximately $2 per test – police officers ostensibly can gauge whether a questionable substance is in fact contraband in a matter of minutes. However, there have been numerous cases where false positive field tests result in the wrongful arrest, prosecution, and detainment of an innocent individual.

So what can go wrong with field testing? Many things. Referred to as “presumptive testing,” field tests can only indicate that an illegal substance may be present. As a result, they often yield false positives and are far less precise than other forms of testing. Most field testing is colormetric; meaning the chemical contents of the testing kit mixed with the substance in question will change colors depending on the type of drug present. Being that the number of discernably different colors is relatively limited in comparison to the types of existing chemical compounds, there is a high likelihood that a variety of chemicals all ostensibly fall under the same color category during a field test.

Take for example the cobalt thiocyanate test, also known as the Scott test, which is used to identify cocaine. The test is considered nonspecific, in that the result is not specific to a particular drug. So while the Scott test may distinguish cocaine from other alkaloids, it is deficient in distinguishing cocaine from other drugs; which will result in false positives.

There are countless other ways a field test may yield a false positive. A study conducted by the Marijuana Policy Project demonstrated how over-the-counter Tylenol PM tested positive for cocaine, while Hershey’s chocolate tested positive for marijuana. A host of other common household items can yield a positive result; for example, Mucinex may test positive for heroin and morphine, and soap can produce a false positive for GHB. Most importantly, a false positive may be caused by the test’s exposure to air or extreme temperatures. The technical limitations of these tests compounded with the subjectivity and human error of police officers that are expected to interpret the results within minutes is a recipe for disaster.

Because an overwhelming percentage of defendants plead guilty instead of proceeding to trial, many innocent people are serving prison time and are saddled with the life-long consequences of a drug conviction on their record. A 2011 study found that prosecutors in nine out of the ten jurisdictions surveyed accepted guilty pleas based solely on the results of field tests. While it is difficult to ascertain exactly how many convictions are sustained from false positive drug tests, the data that is available is alarming. For example, the Florida Department of Law Enforcement lab system shows that 21 percent of evidence that the police tested as methamphetamine later proved to be a false positive.

Because the majority of crime labs do not test a substance after a defendant pleads guilty, proceeding to trial with the aid of an expert witness in the field of drug chemistry is a defendant’s strongest line of defense against field testing.

Expert Testing vs. Field Testing – How Experts Can Prevent Wrongful Convictions

Drug Test Expert WitnessFor the few defendants brave enough to face trial on a drug possession charge stemming from a field test; expert witnesses in the field of forensic toxicology and drug chemistry are key to vindication. Forensic toxicology experts are trained in the detection and interpretation of drugs within one’s system. While experts in the field of forensic drug chemistry perform laboratory tests to detect the presence, absence, or quantity of a controlled substance in the submitted evidence.

Unlike field testing, the analysis conducted by these expert witnesses provides a far more accurate evaluation of a substance. In contrast to the presumptive test of colormetrics, “confirmatory testing” uses instrumental analysis to positively identify the contents of the substance in question; which requires a multi-step process to separate the individual compounds to determine its chemical characteristics.

Referred to as qualitative analysis, this type of testing can determine the types of substances present in the sample. Confirmatory testing may also include quantitative analysis, which can determine the amount, or purity, present in the sample. Unlike police officers, forensic chemists are able to perform batteries of tests such as a microscopic analysis to view the structure of the substance, a microcrystalline test which crystallizes the substance in order to identify its components, an ultraviolet spectroscopy which measures the substance’s ability to absorb light, and gas chromatography, which is used to separate each component of the substance. Lastly, a toxicology expert may be used to confirm a defendant’s drug use, or lack thereof, if such information is available. This would corroborate the findings of the forensic chemist.

Typically, after testing the substance in question, an expert in forensic drug chemistry will be able to answer several important questions; What substances are present? Is any chemical within the substance illegal? And if so, how much of the sample contains an illegal substance? With these few questions, an expert witness can effectively vindicate an individual subject to a false positive drug test. Furthermore, expert witnesses in the fields of drug chemistry can testify as to the laboratory procedures, quality control, maintenance, and the details of the analysis report. While field testing is rife with variables, the laboratory testing and subsequent testimony of an expert witness can quell all reasonable doubt as to the identity of an illegal substance.

That is not to say that the expert testimony of a forensic chemist is without limitations. In order for the expert to properly conduct the requisite tests, the size and condition of the sample must contain enough material to be accurately measured, otherwise the analysis may be inconclusive. Also, the improper packaging of a substance – usually plant material such as marijuana – may destroy the sample to the point that it is unable to be accurately analyzed. This is even more of a reason for a defendant who believes he was a victim of a false positive field test to act swiftly and retain the services of an expert.

Overall, field testing of drugs is a documented problem within the criminal justice system; resulting in many innocent individuals pleading guilty to crimes that they did not commit for fear of lengthy prison sentences. But with the aid of expert witnesses, the playing field becomes a bit more leveled and the scales of justice a little less imbalanced.

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