Forensic biology evidence is often spoken about in court as though it is straightforward: blood was found, it was tested, and it matches the accused.
The language can sound objective and definitive.
However, the scientific reality is frequently more nuanced.
In many cases, a sample described as blood has never been scientifically confirmed to be blood at all. That distinction is often overlooked — yet it can have significant implications for how forensic evidence should be interpreted, challenged, and ultimately weighed by the court.
“The Blood Matches My Client”: A Common Assumption
A recent discussion with a criminal lawyer illustrates how easily assumptions can harden into accepted facts.
The lawyer stated confidently: “The blood matches my client.”
When asked what supported the conclusion that the sample was blood, the answer was simple: “It’s common knowledge. It’s labelled as a blood swab.”
That label alone had framed the lawyer’s entire understanding of the forensic DNA evidence.
What the Case Materials Actually Showed
A preliminary review of the forensic material revealed several critical issues that had gone unnoticed.
1. Suspected blood at the crime scene
The sample was collected by a Crime Scene Officer from a stain they suspected to be blood. Suspicion at the scene is not confirmation. It is merely a starting point for further testing.
2. Presumptive Blood Testing Only
Only presumptive blood testing had been conducted. Presumptive tests are screening tools designed to indicate the possible presence of blood.
Importantly, these reagents are known to produce positive reactions with many other substances, including:
- – metals
- – cleaning agents
- – plant material
- – other biological fluids
A positive presumptive test does not confirm that blood is present.
3. No Confirmatory Blood Testing
No confirmatory blood testing had been performed. Confirmatory tests are specifically designed to establish whether blood is present. Without them, forensic science cannot reliably conclude that a stain is blood.
4. No Visual Indication of Blood
The swab itself did not have the appearance of being bloodstained. While appearance alone is not determinative, it provides important context when scientific confirmation is absent.
5. Low-Level, Mixed DNA Profile
The DNA result obtained was low-level and mixed. Such profiles introduce additional interpretive complexity, including the possibility of:
- – secondary transfer
- – environmental DNA
- – indirect contact
They do not establish the biological source of the DNA or the mechanism of its deposition.
What Can – and Cannot – Be Scientifically Concluded
Based on the available forensic evidence, it was not scientifically possible to conclude that:
- – blood was present on the swab, or
- – any blood present belonged to the defendant.
At most, the evidence demonstrated:
- – a suspected bloodstain
- – a positive presumptive test with known limitations
- – a low-level mixed DNA profile including the defendant
This is a materially different proposition from “blood matching the defendant.”
How Forensic Assumptions Become Legal “Facts”
This type of misunderstanding follows a familiar pattern in criminal cases involving forensic evidence:
- A stain is suspected to be blood
- It is labelled a “blood swab”
- The label is repeated in reports
- Repetition creates shorthand
- Shorthand becomes assumption
- Assumption becomes accepted fact
Once this occurs, the defence discussion often shifts away from whether the evidence is valid and towards how damaging it is — a shift that may be entirely unjustified.
Why This Matters for Criminal Defence Lawyers
Forensic DNA evidence does not identify the biological source of DNA unless appropriate testing has been performed. DNA results do not explain how, when, or from what substance DNA was deposited.
Accepting forensic labels at face value risks conceding crucial ground.
Criminal lawyers should routinely ask:
- Was the substance scientifically confirmed to be blood?
- What specific tests were performed?
- What are the limitations of those tests?
- Does the DNA profile quality support strong conclusions?
- Are alternative explanations consistent with the findings?
These questions go to the heart of reliability, weight, and fairness in forensic evidence.
A Perspective Shift in 15 Minutes
In this case, a short discussion was enough to completely change the lawyer’s perspective on the forensic DNA evidence. What initially appeared to be powerful prosecution material was revealed to be ambiguous, limited, and open to challenge.
No new testing was required – only careful attention to what the science could actually support.
Conclusion: Always Question the Label
A blood swab is not necessarily blood.
In forensic science, labels, assumptions, and repetition do not equal proof. For criminal lawyers, recognising the difference can fundamentally alter case strategy and outcomes.
The most important questions remain:
- What was actually tested?
- What was proven — and what was assumed?
In forensic DNA evidence, those distinctions matter more than most people realise.