Presumptive and confirmatory tests
In medical and forensic science, a presumptive test is an analysis of a sample which establishes either:
- The sample is definitely not a certain substance
- The sample probably is the substance.
For example, the Kastle-Meyer test will show that a sample is not blood, or that the sample is probably blood (but it may be one of a range of less common substances). Further chemical testing is required to prove that the substance is blood.
Confirmatory tests are the tests required to confirm the analysis. Confirmatory tests cost more than simpler presumptive tests, which is why presumptive tests are often made to see if confirmatory tests are necessary.
In a like manner, in medicine, a presumptive diagnosis identifies the likely condition a patient has, and a confirmatory diagnosis confirms the presence of the condition.
Examples
- Acid phosphatase test for semen
- Cobalt thiocyanate test for cocaine
- Duquenois-Levine reagent for marijuana
- Kastle-Meyer test for blood
- Malachite green test for blood
- Marquis reagent for narcotics
- Benzidine test for blood which changes from clear to blue in color with the presence of blood
Presumptive testing of contraband samples
The tests for alkaloids and the Marquis test (performed on TLC plate as per the analytical manual, United States Department of Justice) are presumptive tests. The report obtained after performing these qualitative tests is final when the report is negative in all these tests. There is no need to perform confirmatory tests on negative reports obtained after performing presumptive tests on samples.
A confirmatory test is only required when the presumptive test report is positive for the substance; to confirm the substance's identity, or to measure the percentage purity or other quantitative analysis.
The presence of a substance, even presence at a trace level, can be detected by a presumptive test.
Presumptive test mistake
In March 2010, Australian Customs and Border Protection Service arrested and held for 5 days, a Filipino national at Melbourne Airport as the result of a presumptive test. A presumptive test swab of a 2.4 kg package of iced tea powder located in the passenger's baggage returned a positive result for methamphetamine and a second test showed amphetamine. Subsequent testing showed the powder was indeed iced tea.
The Australian Customs and Border Protection Service was ordered to pay $5,000 as damages to the Filipino.[1]
Recommendations by the U.S. Food and Drug Administration
The U.S. Food and Drug Administration issued a Guidance for Industry and FDA Staff entitled Premarket Submission and Labeling Recommendations for Drugs of Abuse Screening Tests. Its availability was announced in the Federal Register / Vol. 68, No. 231 on Tuesday, December 2, 2003 and is listed under Notices. It is important to understand presumptive testing because of its widespread use by employers and public entities. Most people who take a drug test will take a presumptive test because it is cheaper and faster than other methods of testing. However, it is not as accurate and can render false results. The FDA recommends confirmatory testing to always be conducted and further recommends placing a warning label on the presumptive drug test that reads, "This assay provides only a preliminary result. Clinical consideration and professional judgment should be applied to any drug of abuse test result, in evaluating a preliminary positive result. To obtain a confirmed analytical result, a more specific alternate chemical method is needed. Gas chromatography/mass spectrometry (GC/MS) is the recommended confirmatory method.