Forensic blood testing after long-term exposure to chemical interferents
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Abstract
The field of forensic science encounters many types of evidence. To narrow down what is probative, there are presumptive and confirmatory tests that may be performed on scene and in the laboratory. Most of these tests are performed with one goal in mind: to provide linkages. Biological linkages are made via deoxyribonucleic acid (DNA) left behind on evidence. Blood is the most common source of DNA for investigators. Blood analysis is performed in a specific order, starting with a visual inspection then presumptive testing, and lastly confirmatory testing. Typically, if an item yields a negative presumptive test then confirmatory testing isn’t necessary. However, some tests that yield a negative presumptive result aren’t necessarily indicative of the absence of blood. Interferents contained in common household cleaning products and other items can potentially cause false negative results.This research was a continuation of an earlier study from approximately three years ago that explored the relationship between interferents and false negative results during blood analysis. That study concluded that stronger chemical interferents resulted in more false negatives. It also found that less concentrated blood samples produced more frequent false negatives. A review of the literature reveals that several chemical interferents present in everyday household products have the ability to mask the presence of blood. A variety of these common products were tested, including concentrated chemical interferents contained in these products.
Previously analyzed samples from 2021 were retested approximately three years later in this study for a results comparison. Specifically, this current study focused on the distribution of false negatives for both presumptive (Kastle-Meyer and Orthotolidine) and confirmatory (RSID™ and HemaTrace®) tests for blood. Detailed preliminary visual inspections were also performed. Upon inspection, many samples contained less than a 1:10 concentration of blood which was difficult to discriminate by eye. Typically, in forensic case work, a negative visual inspection for a stain would preclude further testing. In this study, false negatives results were obtained for most interferents during presumptive testing, while confirmatory tests yielded mostly positive results. Certain household cleaners yielded false negatives for both presumptive and confirmatory testing. In these instances, the apparent lack of blood evidence would likely preclude DNA testing.
Research that identifies factors that contribute to false negative blood test results is important for the forensic community. More sensitive and specific tests can help limit the discarding of potential forensic evidence. This experiment seeks to provide results that can be useful in casework, especially for potential blood evidence that has been exposed to similar chemical interferents over a long period of time.
Description
2024