The Validation of Forensic DNA Analysis From Bloodstains Treated by Presumptive Test Reagents

DNA analysis requires untainted samples.

Physical evidence in crime investigations often include bloodstains. However, some dilute bloodstains appear invisible to the naked eye. Presumptive tests provide a means to identify such evidence but pose a risk of destroying the sample, rendering it useless for further testing. Several reagents exist which identify blood without destroying the sample.

Presumptive Test Reagents

Presumptive test reagents change upon contact with blood. This helps identify very dilute, nearly invisible bloodstains at crime scenes. Examples include phenolphthalein or o-tolidine, colorless solutions that turn an intense pink when added to hydrogen peroxide in the presence of blood. Luminol, Fluorescin and Bluestar emit light when exposed to blood. Luminol must be used in total darkness, while Bluestar and Fluorescin can be used in partial light.

DNA Analysis

Some presumptive test reagents render the sample useless for further processing, including DNA analysis. Benzidine, an older presumptive test reagent, interferes with serological tests and degrades DNA over time. Luminol and Bluestar do not interfere with immunologic tests nor do they have any effect on DNA degradation. This allows bloodstain samples previously treated with Luminol and Bluestar to be used for further testing, including DNA analysis.

Validation

In order to test whether presumptive reagents affect samples, investigators check known blood samples. Using blood drawn from a known donor, researchers test both treated and nontreated samples of that blood. They mimic bloodstain samples found at crime scenes by placing the blood on various objects, letting them dry over several hours or days, and then treating the stains with presumptive reagents. Further analysis, including DNA analysis, and comparison with the untreated blood sample from the same known donor verifies whether or not the presumptive reagent has an effect on the sample.

References

• Bioforensics.com: The Presumptive Reagent Fluorescein for Detection of Dilute Bloodstains and Subsequent STR Typing of Recovered DNA
• NFSTC.org: The Effect of Luminol on Presumptive Tests and DNA Analysis Using the Polymerase Chain Reaction
• Forensic Science International: Effect of Presumptive Tests Reagents on Human Blood Confirmatory Tests and DNA Analysis Using Real Time PCR
• enotes.com: Blood, Presumptive Test
• Journal of Forensic Identification: Technical Note