By Kevin Shanks, D-ABFT-FT
In toxicology testing, qualitative and quantitative testing are the norm, but every so often something prevents a result from being acquired. This “something” is called interference and will be displayed on the Axis Forensic Toxicology report as unsuitable due to interference. Interferences can originate from both endogenous and exogenous sources.
Blood samples in test tubes
Image taken by Charlie-Helen Robinson (2021)
Endogenous sources of interference such as drug metabolites and components in the biological matrix may affect the ability of the analytical test to measure the analyte of interest with accuracy and precision. Forensic and postmortem specimens are more susceptible to endogenous interferences. These interfering substances present in the specimen may lead to competition for charge from the mass spectrometer and cause fluctuation or differences in the signal of the drug of interest. During analytical method validation, we assess potential sources of endogenous interferences by testing multiple sources of matrix (i.e. ten sources of authentic postmortem blood), but this testing may not encapsulate all of the variations that could be present.
Exogenous sources of interference include agents used by the person such as prescription and over the counter medications, various ingested substances, environmental contaminants, and sample additives. During the validation of the analytical method prior to implementation in the laboratory, we assess any common known exogenous interferences by spiking blood, urine, or tissues with the most commonly encountered substances in the forensic toxicology laboratory. While we test the most common compounds, the testing does not cover all known prescription and over the counter medications that are available on the market.
During routine testing, if we encounter a sample that displays interference for a specific test, we attempt to achieve a valid result by rerunning the specimen at a dilution. This theory behind diluting the sample is to remove some of the biological matrix which may be causing the interference. If a valid result is attained using the dilution, then it will be reported as such on the final toxicology report. But, if we still cannot achieve a valid quantitative or qualitative result, then the result will be reported as unsuitable due to interference.
In the end, the quality of the analytical toxicology results acquired through testing is dependent on the quality of the sample being analyzed. Due to the inherent nature of forensic toxicology and the effects of postmortem biology, interferences can and do happen.
If you have a question about potential endogenous or exogenous interferences or a specific result on an Axis toxicology report, please reach out to our toxicologists at [email protected].
References
Guidelines for the Interpretation of Analytical Toxicology Results. Disposition of Toxic Drugs and Chemicals in Man. Twelfth Edition. Randall C. Baselt. Biomedical Publications. Pages xxx-xlii. (2020).
Introduction to Forensic Toxicology. Clarke’s Analytical Forensic Toxicology. Sue Jickells and Adam Negrusz. Pharmaceutical Press. Pages 1-12. (2008).
Postmortem Toxicology. Clarke’s Analytical Forensic Toxicology. Sue Jickells and Adam Negrusz. Pharmaceutical Press. Pages 191-218. (2008).
Postmortem Forensic Toxicology. Principles of Forensic Toxicology. Fourth Edition. Barry Levine. AACC, Inc. Pages 3-14. (2017).
Forensic Drug Testing. Principles of Forensic Toxicology. Fourth Edition. Barry Levine. AACC, Inc. Pages 31-48. (2017).