Written by Jarrett Ambeau, trial attorney, court-qualified expert in forensic DNA interpretation, and holder of a Master of Science in forensic DNA and serology, at The Ambeau Law Firm.
A pattern of blood is not a video of what happened.
Bloodstain pattern analysis claims to reconstruct a violent event from the stains left behind. An analyst looks at drops and spatter and tells a story: where people stood, how the blows fell, who is lying. It sounds scientific. Much of it is interpretation.
Let’s say Joe says he found his roommate already hurt and knelt to help. The state’s analyst looks at the blood on Joe’s clothes and calls it “impact spatter” from an attack. The same stains could come from Joe leaning over a wounded friend. The stains do not speak. The analyst does.
This is one of the most persuasive and most dangerous forms of forensic testimony. It turns a chaotic scene into a confident narrative. The defense job is to show the jury how much of that narrative is assumption.
What is bloodstain pattern analysis?
Bloodstain pattern analysis is the study of the size, shape, and distribution of blood at a scene. The idea is that different actions leave different patterns. A hard blow, a cast-off swing, a passive drip, each is said to look distinct.
In practice, the analyst classifies stains and then infers the events that made them. That second step is where the trouble starts. Two analysts can look at the same scene and tell two different stories, because the method leans heavily on training, experience, and assumption.
The discipline borrows the language of physics, angles, trajectories, and points of origin. But applying that math to a real, messy scene requires assumptions about drop size, surface, and motion that often cannot be verified. Precise math on unreliable inputs produces confident nonsense.
The science: where the interpretation breaks down
Blood behaves differently on different surfaces. A drop on smooth tile spreads one way. The same drop on carpet, wood, or fabric behaves another way. If the analyst misjudges the surface, the whole reconstruction tilts.
Scenes are rarely clean. Stains overlap. Earlier events get covered by later ones. A wounded person moves, and so do bystanders trying to help. Every one of those facts adds a variable the analyst cannot fully account for, yet the final opinion often hides all of them behind a single confident conclusion.
Then there is the examiner’s mindset. When the analyst already knows the police theory, the interpretation tends to bend toward it. This is called contextual bias, and it is not a character flaw. It is how human perception works. Good science controls for it. Much of bloodstain practice does not.
Why juries should be cautious
The National Academy of Sciences studied this field and did not mince words. Its 2009 report found that many bloodstain conclusions are subjective and that the opinions of analysts are more subjective than most jurors realize.
The report warned that the uncertainties in this discipline are enormous, and that some analysts overstate what the patterns can prove. That warning, from the scientific community itself, is something a jury is entitled to hear.
The law: reliability and the limits of opinion
Bloodstain testimony is expert opinion, and expert opinion must clear a reliability bar. Louisiana applies Code of Evidence Article 702 under the standard set in State v. Foret, 628 So.2d 1116 (La. 1993), which adopted the Daubert v. Merrell Dow, 509 U.S. 579 (1993) reliability factors.
Those factors ask hard questions of this field. Can the method be tested? What is its error rate? Is the specific conclusion generally accepted, or is it one analyst’s guess? When the reconstruction rests on unverifiable assumptions, we argue it fails the reliability test and should be excluded or narrowed.
Even when some testimony comes in, its scope can be limited. There is a difference between describing a stain and declaring exactly how a person was struck. We push the court to keep the analyst inside what the science can actually support.
How we challenge bloodstain testimony: law and science together
We separate observation from interpretation. An analyst can describe a stain. Whether that stain proves a specific act is a different, weaker claim, and we make the analyst own the difference on the stand.
On the law, we file a Daubert-Foret challenge under Article 702 and press the court to exclude reconstructions built on untested assumptions. We use the NAS findings to show that the discipline’s own scientists question its reliability.
On the science, we retain independent experts to offer alternative, innocent explanations for the same patterns. We probe the analyst’s training, error rate, and whether they knew the state’s theory before they “analyzed” the scene. Knowing the answer in advance shapes what people see.
At trial, we make the assumptions visible. What surface did you assume? What drop size? Did you account for the victim moving? Each honest answer chips away at the confident story and opens room for reasonable doubt.
Which cases turn on bloodstain evidence?
This evidence appears in the gravest cases. Homicide, manslaughter, and serious violent-crime prosecutions where the fight is over what actually happened and who did what. When the state uses stains to cast a defendant as the attacker, the reliability of that reading is everything.
These are also the cases where an innocent explanation is often the truth. People help the wounded. People walk through scenes. The blood tells you something happened, not who is guilty, and we keep that distinction front and center.
Frequently asked questions
Isn’t bloodstain analysis settled science?
No. Classifying a stain can be reliable. Reconstructing exactly how it got there is far more subjective, and national scientific reviews have said so.
The blood was on my client. Doesn’t that prove an attack?
Not by itself. Blood transfers in many innocent ways, including helping a victim. The pattern alone rarely proves who caused the injury.
Can this testimony be limited or excluded?
Often, yes. Under the Daubert and Foret reliability standard, we can move to exclude unsupported reconstructions or to limit the analyst to what the science supports.
Do you use outside experts?
Yes. We work with independent analysts who can offer competing, credible explanations and expose the assumptions behind the state’s story.
Authoritative resources
The concerns are documented in the National Academies’ report, Strengthening Forensic Science in the United States, which addresses the subjectivity of bloodstain pattern interpretation.
Related forensic evidence challenges
Forensic evidence rarely stands alone. We challenge the full range of scientific proof the state uses, including firearms and toolmark analysis, fingerprint and latent print analysis, and forensic toxicology and DUI testing. For the full picture, see our overview of what a forensic science lawyer does.
How The Ambeau Law Firm can help
If the state built a story out of blood, that story can be tested and taken apart. We challenge the interpretation, offer the innocent explanation, and hold the analyst to the science. Contact The Ambeau Law Firm to discuss your case.