Review: False Positives and False Negatives With a Cocaine-Specific Field Test and Modification of Test Protocol to Reduce False Decision

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Emily C. Lennert

Category: Chemistry

Keywords: presumptive test, field test, cocaine, crack, Scott test, false positive, false negative, error

Article to be reviewed:

  1. Tsumura, Y.; Mitome, T.; Kimoto, S. “False positives and false negatives with a cocaine-specific field test and modification of test protocol to reduce false decision.” Forensic Science International. 2005, 155 (2-3), 158–164.

Disclaimer: The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.

Summary: The Scott test is a popular field test for field identification of cocaine and crack. The Scott test involves three steps. Step 1: addition of cobalt thiocyanate, in the presence of cocaine, will form blue crystals (i.e. precipitates). Step 2: addition of hydrochloric acid, in the presence of cocaine, the blue crystals will disappear. Step 3: addition of chloroform, in the presence of cocaine, will form two layers where the lower layer is blue. The expected color changes at each step are characteristic of cocaine. If one of the expected color changes is not observed, then the unknown compound cannot be identified as cocaine. The study examined the specificity of the Scott test to correctly identify the presence of cocaine in an unknown sample. This directly refers to the occurrence of false positives and false negatives. The study states its goal: “to clarify the conditions that cause false negatives and false positives in the Scott test and to improve the test’s specificity.”1

Two Scott reagents were tested. The original Scott reagent which is comprised of: solution 1 was 2% cobalt (II) thiocyanate in water, diluted 1:1 in glycerine; solution 2 was concentrated hydrochloric acid; and solution 3 was chloroform. The second was an “improved” Scott reagent that is applicable to crack: solution 1 was 2% cobalt (II) thiocyanate in 10% acetic acid, diluted 1:1 in glycerine; solution 2 was 10% hydrochloric acid; and solution 3 was chloroform. Testing was performed on cocaine HCl samples, which were legally obtained from the Japanese police. Crack cocaine was made in lab from the legally obtained cocaine. Heroin HCl samples were also tested, to determine if heroin gave false positive results when tested with the Scott reagents, and were also obtained from Japanese police. The purity of the cocaine and laboratory made crack cocaine were checked by high-performance liquid chromatography (HPLC) prior to Scott reagent testing.

Several medications and drugs, such as lidocaine, heroin, ketamine, and promethazine, were tested for false positive indications for cocaine when tested with the Scott test. Of the drugs and medications tested, none showed the same color sequence result as pure cocaine. According to the study’s authors, this matches previous studies. Crack cocaine, of various purities and listed in the study’s result table as freebase cocaine A, B, and C, did not follow the expected color sequence for cocaine either. With crack cocaine, at step 2, the blue precipitates remained, which may lead to false negative determinations in the presence of crack cocaine. However, at the third step, the lower blue layer formed as expected. Four drugs showed the same color sequence as crack cocaine and could be perceived as giving a false positive result: chlorpromazine, an antipsychotic; diphenhydramine, commonly known as Benadryl; 5-methoxy-n-n-diisopropyltryptamine, a psychedelic drug known as “foxy”; and promethazine, an antihistamine.

To prevent the false identification of “foxy” and other drugs as crack, the authors of this study worked to improve the Scott test. When the precipitate remains after step 2, the supernatant, or top layer of liquid, without the blue precipitate, was removed to a new tube. Then, in step 3, the chloroform was added into the new tube, which contained the supernatant layer. The study states that the blue layer in step 3 will only form if the sample contains cocaine or crack cocaine. This allows the Scott test to successfully identify crack cocaine and eliminate the false positives or negatives that could occur at step 2.

The study also found that sample mass has an effect on the occurrence of error. False negatives occurred in cocaine samples when the sample was greater than 3 mg. Additionally, 2 mg of dibucaine, 2 mg of heroin, and 4 mg of ketamine each gave false positive indications for cocaine. The study states that a sample must be no more than 1 mg for the Scott test to be reliable.

Scientific Highlights:

  • The Scott test is a 3 step process.
  • The Scott test reliably indicates pure cocaine.
  • Crack cocaine detection is possible using a modified Scott reagent. Solution 1: 2% cobalt (II) thiocyanate in 10% acetic acid, diluted 1:1 in glycerine. Solution 2: 10% hydrochloric acid. Solution 3: chloroform.
  • Positive identification of crack cocaine with the modified Scott test, to avoid false positives, requires separation of the supernatant between steps 2 and 3.
  • A sample must be no more than 1 mg for the Scott test to be reliable.

Relevance: The research presented in the above article supports a currently used field test, the Scott test, and provides information and a method that will help to reduce the occurrence of error in the use of the Scott test.

Potential conclusions:

  • The Scott test can reliably indicate the presence of cocaine, but only if it is performed correctly. The same is true for the detected of crack cocaine.
  • False positives and false negatives are possible with the Scott test, but may be avoidable with proper procedure.