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Frequently Asked Questions – I. Drugs of Abuse Analysis


Why is the Immunoassay analysis method used for drugs of abuse?Immunoassays; is a diagnostic test consisting of a combination of the principles of chemistry and immunology which enable the precise detection of groups of substances used. The basic function of the homogeneous immunoassay method is to bind the labeled or unlabeled antigen to the binding sites of a specific antibody. For labeling, it may be an enzyme (EIA), radioisotope (RIA) or fluorophore (FPIA). Immunoassay is a quick, reliable and cost-effective method which provides qualitative or semi-quantitative DOA and quantitative TDM test results.


What is a qualitative test method?The qualitative test method is a method in which the final reaction absorbance generated in the immunoassay analysis technique is obtained “positive” or “negative” based on certain cut-off calibration absorbance level.


What is a semi-quantitative test method?
The semi-quantitative test method is a quantitative determination of the analyte concentration based on the comparison of absorbance obtained from the calibration curve with more than one calibrator.


What is a cutoff level?
Threshold concentration value is a concentration that distinguishes accurate and reproducible positive results from the negative results with given test methods. These values can be determined according to the limit of detection (LoD) of the test, verification method or other criteria. Threshold concentration values are determined by some institutions or by common opinion.


What is a LoD?
Limit of detection is defined as the lowest analyte concentration that a test method can detect.


What is a cross reaction?
This occurs when a test is affected by metabolites, drugs or structurally similar compounds rather than the expected analyte detection. There is positive and negative cross-reaction. The interaction with the compounds of the same class and the same structural characteristic as the substance to be detected is called a positive cross-reaction. Interaction with unintended or non-structured compounds is called a negative cross-reactivity.


Why a confirmation method is needed?
Verification of the analyte with more specific chromatographic analysis methods (GC/MS, LC/MS/MS etc.), which positively affects the positive test results obtained by screening tests, is required because of the cross reaction properties of immunoassays.


Can you obtain a positive result for THC if there is passive respiration of cannabis?
Passive inhalation is often used as an excuse for THC positive testing results. Many studies on this have shown that it is not possible to obtain a THC positive result above the threshold value due to passive inhalation.


What are the advantages and disadvantages of the immunoassay method?

– Applicable in the laboratory and at POCT
– Urine and oral fluid samples can be applied directly without pre-treatment
– Fast result
– Cost effective


– Negative cross reaction
– Restricted test parameters for some biological samples
– Failure to develop immediate testing for latest psychoactive substances


Which biological samples are used in drugs of abuse analysis?
Biological samples that can be used in forensic and clinical toxicology analyzes can be listed as urine, blood, hair, oral fluid, tissue, nail, intraocular fluid. The choice of biological sample depends on the purpose of the analysis to be performed. All above samples are used because the result obtained from forensic toxicology is used for legal purposes. In clinical toxicology, urine and hair are used to detect retrospective use disorders, and blood and oral fluid are the most commonly used samples to detect new usage.


Why are immunoassays used?

By combining the principles of chemistry and immunology, it is possible to use specimens to be analyzed in immunology for specific and sensitive measurements in diagnostic tests. The most basic principle for homogeneous immunoassay; unlabelled analyte in a sample competes with labelled analyte to bind an antibody.

The labeling may be enzyme, radioisotope, fluorophore or latex fragments. Drug of Abuse immunoassays gives qualitative or quantitative results. Same method results only quantitative for therapeutic drug and immunosuppressive drug testing. Monoclonal or polyclonal antibodies are used.