If we combine the principles of chemistry and immunology, it is possible to use samples to be analyzed in immunology for specific and sensitive measurements in diagnostic tests. The most basic principle for homogeneous immunoassay; It is based on the struggle of labeled or unlabeled antigens to bind to the antibody.
Labeling can be enzyme, radioisotope, fluorophore or latex fragments. In drugs used for abuse (Drug of Abuse), immunoassays can be studied qualitatively or quantitatively. This method is completely quantitative in therapeutic drug therapy and immunosuppressive drug tests. Immunoassays use monoclonal or polyclonal antibodies.
Immunoassays; They are diagnostic tests consisting of a combination of chemistry and immunology principles that enable the sensitive detection of the substance or substance groups used. The basic mechanism of the homogeneous immunoassay method is that labeled or unlabeled antigen competes to bind to the binding sites of a specific antibody. For labeling, it can be an enzyme (EIA), radio isotope (RIA) or fluorophore (FPIA). Immunoassay method provides qualitative or semi-quantitative measurement of addictive substance (DOA) screening tests and quantitative measurement of Drug Level Monitoring (TDM) tests quickly, reliably and economically.
Qualitative testing method; It is the method that produces a "positive" or "negative" result depending on whether the final reaction absorbance in the immunoassay analysis technique is obtained below or above a certain threshold value (cut-off) calibrator absorbance.
It is necessary to confirm the analyte that causes positivity in positive test results obtained with screening tests with more specific chromatographic analysis methods (GC/MS, LC/MS/MS, etc.) due to the cross-reactivity properties of immunoassays.
Semi-quantitative testing method; It is the determination of the analyte concentration quantitatively by comparing the absorbance obtained as a result of the analysis according to the calibration curve obtained by using more than one calibrator.
Passive inhalation is often the excuse used to explain THC positive test results. Many studies on this subject have shown that it is not possible to obtain positive results above the threshold value as a result of passive breathing and THC analysis.
The threshold concentration value is the concentration that accurately and consistently (reproducibly) distinguishes a positive result from a negative result with existing test methods. These values may be determined based on the test's lowest level of detection (LoD), verification method, or other criteria. Threshold concentration values are determined by some institutions or by common opinion.
Biological samples that can be used in forensic and clinical toxicology analyzes can be listed as urine, blood, hair, oral fluid, tissue, nail, and intraocular fluid. The selection of biological samples depends on the purpose of use of the analysis results. Since the results obtained in forensic toxicology are used for legal purposes, all the samples listed above are used. In clinical toxicology, urine and hair are the most commonly used samples to detect retrospective use disorders, and blood and oral fluid are the most commonly used samples to detect new use.
Lowest detection level; It is defined as the lowest analyte concentration that a test method can measure.
Advantages:
- Ability to be applied in a laboratory environment or at the bedside.
- Urine and oral fluid samples can be studied directly without pre-treatment.
- Fast result time.
- Cheap testing cost.
Disadvantages:
- Undesirable cross reactions.
- Limited test parameters for some biological samples.
- Failure to immediately develop tests for new psychoactive substances.
The test is affected by metabolites, drugs or structurally similar compounds other than the analyte to be detected. There are desired and undesirable cross reactions. Interaction with compounds in the same class and with similar structural features as the substance to be detected is the desired cross reaction. Interaction with undesirable or structurally dissimilar compounds is an undesirable cross-reaction.
