Question

describe the context of how a false positive and a false negative fit into the five steps of systemic toxicological analysis?

Answer 1

Ans . Systematic toxicological analysis (STA) is a major part of the examination in forensic and clinical toxicology. STA is aimed at detecting and identifying all substances of toxicological relevance( i.e. drugs, drugs of abuse, poison and/or their metabolites) in biological material.

The usual practice in toxicological examination begins with the preliminary identification of alcohol and screening of a wide spectrum of acidic, neutral and basic organic drugs or poisons. If a toxin is detected , confirmatory and, if necessary, quantitative testing has to be performed. Quantitative testing determines how much of the substance is involved.

In general, a positive identification is achieved using atleast two independent analyses and preferably using different testing methods. For example, using an immunoassay for initial testing and then GC-MS or LC-MS to confirm results would suffice. A chromatographic confirmation test could also determine quantification, simplifying the testing process.

Immunoassays :- Most commonly used drug screening tests involve immunoassay techniques.Immunoassays are laboratory tests that use antibodies to detect a reaction with specific substances. Immunoassay screening tests are designed to detect whether a sample is positive or negative for the targeted drug. Four interpretations of a screening test are possible :-

1) a true-positive result occurs when the test correctly detects the presence of a drug.

2) a false-positive result is one where the test incorrectly detects the presence of a drug , where no drug is present.

3) a true-negative result occurs when the test correctly confirms the absence of a drug.

4) a false-negative result is one where the test fails to detect the presence of a drug when it is present.

For those samples that give positive screening results , confirmation test should be performed, preferably using mass spectrometry (MS) detection.