Presumptive and confirmatory tests
- The sample is definitely not a certain substance.
- The sample probably is the substance.
For example, the Kastle–Meyer test will show either that a sample is not blood or that the sample is probably blood but may be a less common substance. Further chemical tests are needed 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 so presumptive tests are often done to see if confirmatory tests are necessary.
Similarly, in medicine, a presumptive diagnosis identifies the likely condition of a patient, and a confirmatory diagnosis confirms is needed to confirm the condition.
Tests for alkaloids and the Marquis test, performed a thin layer chromatography plate as per the analytical manual, US Department of Justice, are presumptive tests. The report that is obtained after performing them is final when the report is negative in all the tests. There is no need to perform confirmatory tests on negative reports that are obtained after performing presumptive tests on samples.
A confirmatory test is required only when the presumptive test report is positive for the substance. It then confirm the substance's identityor measures the percentage purity or other quantitative analysis.
The presence of a substance, even at a trace level, can be detected by a presumptive test.
Presumptive test mistakeEdit
In March 2010, a Filipino national was arrested by Australian Customs and Border Protection Service at Melbourne Airport after multiple presumptive tests on 2.4 kg of iced tea in her baggage returned positive results for methamphetamine and amphetamine. Afterwards, she was turned over to Australian Federal Police and detained for 5 days pending trial on charge of importing a commercial quantity of a border-controlled drug.
Subsequent analysis confirmed that the substance was indeed lemon flavored iced tea, and the drug charge was dropped. The presiding magistrate made the exceptional order that the defendant be released from custody in the courtroom immediately, and also ordered that the Director of Public Prosecutions pay $5,000 to the Filipino. 
The US Food and Drug Administration issued a Premarket Submission and Labeling Recommendations for Drugs of Abuse Screening Tests. Its availability was announced in the Federal Register, Vol. 68, No. 231 on 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 take a presumptive test, cheaper and faster than other methods of testing. However, it is less accurate and can render false results. The FDA recommends for confirmatory testing to be conducted and the placing of a warning label on the presumptive drug test: "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."