No Blood, No Proof: Study Challenges Post-Cleanup DNA Evidence
by Jo Ellen Nott
A Flinders University study published in the Australian Journal of Forensic Sciences provides defense attorneys with powerful tools to challenge forensic evidence in cases involving attempted crime scene “clean-ups.” The study reveals how variable and unreliable forensic interpretations of cleaned evidence can be, findings that should prompt defense counsel to scrutinize prosecution claims about biological evidence far more aggressively.
The research underscores a well-established defense principle: the “innocent explanation.” In many violent crimes, particularly domestic disputes, the mere presence of a person’s DNA on a surface is insufficient for a conviction if that presence can be explained by cohabitation or casual contact. This study arms defense counsel with concrete scientific data showing that even when DNA persists after cleaning, its biological source may be impossible to determine, undermining the prosecution’s ability to connect that DNA to a violent act rather than ordinary household contact.
Critically for the defense, the study demonstrates that many common cleaning agents successfully remove hemoglobin, a blood-derived protein used as a marker in forensic testing, while leaving behind trace DNA. When this occurs, the prosecution cannot establish that the remaining DNA came from blood rather than from innocent sources such as skin cells deposited through everyday living. If the biological source of the DNA cannot be identified as blood, the evidentiary link between the DNA and an alleged violent event is fundamentally weakened.
The researchers tested seven cleaning methods on cotton and metal surfaces using both wet and dried blood. Of the seven methods, bleach produced the greatest removal of hemoglobin from cotton, while hot running water produced the greatest removal of hemoglobin from knife blades. For DNA, cold running water with a sponge was most effective on cotton, while bleach was most effective on knife blades. The variation in these results is itself a key defense point. Removal rates for hemoglobin and DNA depended on multiple variables, including whether the blood was wet or dried, the type of surface involved, and the specific cleaning method used. That variability undercuts any prosecution claim that a cleaned item can be interpreted through a one-size-fits-all forensic assumption. Defense counsel should demand that the prosecution account for each of these variables rather than relying on generalized assertions about what cleaning does or does not eliminate.
The study also exposes two distinct risks in forensic laboratory workflow that defense attorneys should be prepared to challenge. First, in some jurisdictions, a negative blood-detection test may lead analysts not to proceed to DNA analysis at all, meaning potentially exculpatory DNA evidence may never be developed. Second, when a DNA profile is generated after a negative blood test, the biological source of that DNA may be misinterpreted or assumed without adequate scientific basis. For defendants, both scenarios are significant. A negative blood result does not end the scientific inquiry, and a post-cleanup DNA result does not, by itself, establish what biological material was actually present. Defense counsel should challenge any testimony that leaps from “DNA was found” to “blood was present” without independent confirmation of the biological source.
A related study by members of the same research group, published in Forensic Science International, used imaging flow cytometry to attempt to differentiate between cell types (e.g., skin versus vaginal cells) months after a crime. While the technique showed some promise, the researchers themselves recommended further testing before it is used in casework or presented as evidence in court. Defense attorneys should take note that any prosecution attempt to introduce imaging flow cytometry results to identify cell types from aged samples should be challenged on the ground that the technique’s developers have expressly cautioned it is not yet validated for forensic casework.
Source: Anderson, R., Mitchell, N., Van Oorschot, R. A. H., & Goray, M. (2025). Persistence and detection of blood derived haemoglobin and DNA after attempted cleaning of crime-related items. Australian Journal of Forensic Sciences, 1–22.
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