The research, published as a proof-of-concept pilot study in the International Journal of Legal Medicine, compared two walking scenarios. In the first, a volunteer stepped into a standardized 20-milliliter pool of fresh human blood and then walked ten consecutive steps at a sustained pace, mimicking someone moving steadily away from a blood source. In the second, the walker paused for two to three minutes after the first step, allowing the blood film on the outsole to partially dry before resuming at a slower pace, simulating a person who lingers near a blood pool before leaving. Each condition was run twice, producing four complete depletion sequences, with every single footprint photographed under fixed, calibrated conditions.

The numbers tell a dramatic story of rapid loss. In the continuous walking sequence, the initial blood pool covered roughly 160 square centimeters, but the very first footprint already carried only 60 to 70 square centimeters of blood. By the second step, that figure had collapsed to 20 to 30 square centimeters, and by the third it fell to 10 to 20 square centimeters. More than 80 percent of the transferable blood vanished within the first three contacts, a pattern the authors describe as consistent with a negative exponential decay model, in which the bulk of available blood is shed almost immediately and subsequent steps leave progressively smaller residues.

Yet quantity is not the whole story. Despite this steep quantitative collapse, the overall geometry of the shoe, its tread pattern and outline, remained recognizable for a surprisingly large number of consecutive steps in the continuous sequence. The researchers analyzed each image using custom Python and OpenCV algorithms, converting photographs into HSV color space to isolate blood pixels, then computing a battery of indices: a Fragmentation Density Index tracking how broken up the pattern became, a Spatial Dispersion Index measuring how far residual deposits spread from the footprint’s core, and a Transfer Ratio comparing each step’s blood area to the first. As blood area shrank, fragmentation relentlessly increased, with the highest values appearing in the terminal stages when only isolated microdeposits remained.

The interrupted partial-drying sequence produced an unexpected twist. Counterintuitively, letting the blood partially dry did not preserve the footprint longer. Instead, interpretability declined sharply around the fourth step, the point at which the overall tread geometry could no longer be reliably reconstructed. What partial drying changed was not longevity but geography: residual deposits became more localized and concentrated rather than diffusely scattered. The authors caution that this Step 4 transition is specific to their experiment, not a universal forensic threshold, and that their design cannot separate the individual effects of the pause, the drying itself, and the slower walking pace that followed.

Perhaps the most striking discovery concerns the mismatch between what can be seen and what can be detected chemically. Using Combur Test strips, a tetramethylbenzidine-based reagent that responds to the pseudoperoxidase activity of hemoglobin, the team tested traces at each stage. Fresh deposits reacted strongly, but after roughly 24 hours of drying, direct testing of the traces was predominantly negative, even where blood was still visible to the eye. The crucial move came next: rehydrating the dried traces with physiological saline restored positive reactions, even in advanced stages of depletion and even on the shoe’s outsole itself after a full day.

This dissociation between morphological persistence and biochemical persistence carries real weight for criminal investigations. A footprint that has degraded beyond recognition, reduced to scattered punctiform flecks with no reconstructable tread pattern, may still harbor biologically reactive blood material. The study suggests that apparently worthless, fragmented impressions could retain forensically relevant evidence, including material potentially suitable for confirmatory testing, long after their value for pattern comparison has evaporated. Investigators who dismiss heavily depleted traces as uninformative may be discarding evidence that a combined morphological and biochemical approach could recover.

The authors are careful to frame their conclusions as hypothesis-generating rather than definitive. The study involved a single 59-kilogram subject, one worn athletic shoe in EU size 43, one blood source collected in EDTA tubes, and one smooth non-porous surface resembling linoleum. Blood viscosity, water loss, film thickness, and plantar pressure were not directly measured, and the image segmentation retained an analyst-dependent component without formal inter-rater validation. Combur itself is only a presumptive test, indicating heme reactivity but not confirming that a stain is human blood. Real crime scenes introduce countless variables, from carpet and concrete to rain and scrubbing, that this controlled pilot could not address.

Even so, the work opens a productive line of inquiry at the intersection of fluid physics, biomechanics, and forensic science. Blood is a complex non-Newtonian fluid whose wetting, spreading, and drying behavior on surfaces has only recently attracted systematic study, and previous research has shown that DNA can survive on shoe soles for hundreds of steps. The Italian team’s integrated approach, fusing digital morphometry with presumptive chemistry, offers a template for future studies incorporating multiple subjects, diverse footwear, varied substrates, and gait analysis. The broader message is already clear: the footprint you can see and the blood you can detect follow two different clocks, and forensic science needs to read both.

Subject of Research: Depletion dynamics of blood-bearing footwear impressions in forensic bloodstain pattern analysis

Article Title: Blood footwear impression depletion: comparative morphological and biochemical analysis of continuous and interrupted partial-drying transfer sequences

Article References: Aquila, I., Gualtieri, S., Grimaldi, G., & Sacco, M. A. (2026). Blood footwear impression depletion: comparative morphological and biochemical analysis of continuous and interrupted partial-drying transfer sequences. International Journal of Legal Medicine. https://doi.org/10.1007/s00414-026-04035-1

Image Credits: AI Generated

DOI: 10.1007/s00414-026-04035-1

Keywords: forensic science, bloodstain pattern analysis, footwear impressions, blood transfer, hemoglobin detection, Combur Test, morphometric analysis, crime scene reconstruction, partial drying, exponential decay, legal medicine, trace evidence

The work is spread across three publications: one paper already published in The Alberta Law Review and two forthcoming articles in The Canadian Bar Review and The Dalhousie Law Journal. Together, they trace a persistent lag between what scientists have learned about the limits of certain forensic methods and what courts continue to allow as evidence. Gordon’s central argument is straightforward: when someone’s liberty is at stake in a criminal trial, a technique that has been used for decades should not be taken for granted, especially where the underlying science has never been rigorously validated. Her stated hope is that the research will help judges and lawyers understand what these techniques can and cannot actually tell them, so that forensic evidence is presented in a way that accurately reflects its scientific support.

The most familiar of the three techniques is forensic ballistics, in which bullets and cartridge cases recovered from crime scenes are compared with test firings from suspect firearms. Examiners look for matching striations and impressions transferred from the weapon’s barrel and breech mechanism, and they routinely testify that a bullet came from a specific gun to the practical exclusion of all others. Ballistics evidence has been accepted in Canadian courts for more than a century, and it has played an important role in securing convictions across North America. Yet Gordon found that the scientific research does not support the level of confidence with which ballistics experts often testify. In practical testing, examiners misidentified bullets 37 per cent of the time — an error rate that, if replicated in casework, would represent a serious threat to the integrity of convictions built on such testimony.

The legal landscape around firearms identification may be beginning to shift. In 2023, a Chicago court became the first in the United States to bar the use of ballistics-matching evidence in a criminal case, a decision that signaled growing judicial willingness to question methods once considered untouchable. In Canada, however, Gordon’s forensic firearms article documents that the evidence continues to be routinely admitted, largely on the strength of precedent rather than fresh validation studies. Her papers call on judges to exercise their gatekeeping role more actively, ensuring that experts do not stray beyond what the underlying science can support and that claims of certainty are tempered by the documented error rates now emerging from the research literature.

The second technique under scrutiny is the use of drug-detection dogs, commonly known as sniffer dogs. In Canada, police may deploy such dogs to conduct a search where they have reasonable suspicion that the search will reveal evidence of a drug offence — a deliberately lower threshold than what is ordinarily required for a warrantless search. The Supreme Court of Canada has justified this exception on the grounds that sniffer-dog searches are minimally intrusive, narrowly targeted and highly accurate. Gordon’s review of the scientific evidence that has accumulated since the Court’s most recent rulings in 2013 indicates that these assumptions no longer hold up. In her assessment, the dogs are not nearly as reliable as many courts appear to believe, and the searches that follow their alerts can in fact be quite invasive.

The empirical record on detection dogs is sobering. One Australian study found that drugs were not discovered following nearly 75 per cent of police dog alerts, meaning that three out of four alerts led to searches that turned up nothing. Dogs may alert in the absence of drugs for a variety of reasons, including poor training, residual odours left behind from earlier contact with narcotics, and even odours associated with some common baking ingredients. Handler influence adds another layer of unreliability: a dog can be unintentionally cued by its handler, for example when the handler slows down or behaves differently near a person they already suspect. In such cases, the alert may reflect the officer’s expectations rather than the dog’s independent detection of contraband, undermining the very premise of using the animal as an objective investigative tool.

The consequences of unreliable alerts extend beyond wasted police time. The searches that follow a dog alert can cause fear, anxiety, anger and humiliation for the people subjected to them, and Gordon notes that they may have a particularly negative impact on racialized people, who are disproportionately stopped and searched. Among the reforms she proposes are clearer standards for the training, certification and performance monitoring of detection dogs, so that their real-world accuracy can be measured and audited rather than assumed. She also argues that the same legal standard that generally applies to other warrantless searches should apply to sniffer-dog searches, removing the special exemption that currently allows them to proceed on mere reasonable suspicion.

The third discipline examined is bloodstain pattern analysis, which involves examining the shapes, sizes and locations of bloodstains at a crime scene in order to reconstruct the physical events that produced them — the direction of a blow, the position of a victim, the type of force involved. Canadian courts have long accepted the technique as a physical science, but Gordon’s historical digging reveals an origin story that bears little resemblance to laboratory science. The discipline grew out of the work of a chemist experimenting in his basement, who later helped establish the field’s scientific journal and its early textbooks. Those institutional trappings gave the emerging field many of the outward markers of a scientific discipline, even as its foundational claims went largely untested. Practitioners today often invoke concepts from fluid dynamics when explaining their conclusions, despite frequently having little or no formal training in physics.

The rigorous studies that have begun to emerge paint a troubling picture of the discipline’s accuracy. One study found that analysts correctly classified bloodstain patterns only about 70 per cent of the time on rigid surfaces, and just 62 per cent of the time on fabrics — performance levels that fall well short of what one would expect from a settled physical science. Another study found that different analysts frequently reached different conclusions when examining the very same evidence, raising questions about the consistency and reproducibility of the method. Additional research showed that contextual information could bias examiners: what analysts were told about the case or the crime scene could influence the conclusions they drew, a form of cognitive contamination that is well documented in other forensic disciplines but particularly damaging in a field that claims to read physical evidence objectively.

Taken together, the three papers deliver a coordinated call for reform rather than a blanket rejection of forensic practice. Gordon’s argument is not that these techniques are worthless, but that their reliability has been overstated and their limitations under-communicated to the juries and judges who depend on them. The research urges courts to more actively exercise their gatekeeping role by ensuring that forensic evidence is genuinely reliable before it reaches a jury, and it calls for greater scientific literacy among lawyers and judges so that cross-examinations and admissibility hearings can probe expert claims with the depth they deserve. The broader lesson resonates well beyond the three disciplines studied: courtroom longevity is not a measure of scientific validity, and techniques that have gone unchallenged for a century deserve the same empirical scrutiny as any new method entering the witness box. As the gap between forensic practice and forensic science continues to close, the cases that hinge on a dog’s alert, a bloodstain’s shape or a bullet’s striations may look very different in the years ahead.

Subject of Research: Scientific reliability of sniffer dogs, bloodstain pattern analysis and ballistics evidence in criminal courts

Article Title: Sniffer dogs, bloodstain patterns and ballistics evidence under the microscope in new studies

Article References: Sniffer dogs, bloodstain patterns and ballistics evidence under the microscope in new studies. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: forensic science, sniffer dogs, ballistics, bloodstain pattern analysis, criminal law, evidence reliability, courtroom admissibility, police searches, error rates, expert testimony, University of British Columbia, judicial gatekeeping