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Minnesota Criminal Defense & Forensic Law Firm
Injustice Anywhere is a Threat to Justice Everywhere
Serving Southwest Minnesota
We Accept the Following Forms of Payment:
Injustice Anywhere is a Threat to Justice Everywhere
Serving Southwest Minnesota
We Accept the Following Forms of Payment:
Injustice Anywhere is a Threat to Justice Everywhere
When people hear the words DNA evidence, they often imagine a laboratory pulling a clean genetic profile from a single drop of blood and instantly identifying the right person. Television and movies have reinforced the idea that DNA evidence is always precise, conclusive, and virtually impossible to challenge.
The reality is far more complicated.
Before any DNA profile is ever generated, interpreted, or compared, biological evidence can go through a process involving serology testing of the DNA. Serology is meant to help determine what biological material is present, where it came from, and whether it is even suitable for DNA analysis at all.
DNA serology testing is the forensic testing of biological material to determine:
Serology is not DNA profiling. It does not identify a person. Instead, it answers a critical preliminary question: What biological evidence do we actually have here?
It is typically after serology is completed that a laboratory decide whether to proceed with DNA extraction, quantification, and profiling. A laboratory can however go back after testing (such as differential extraction) to do confirmatory testing for sperm.
Forensic serology focuses on identifying biological fluids commonly encountered in criminal investigations. These include:
Blood may be visible or invisible to the naked eye. Serology testing helps determine whether a stain is likely blood and whether it may be human.
Semen testing is common in sexual assault cases. Serology seeks to identify components such as sperm cells or seminal fluid markers.
Saliva may be deposited through licking, kissing, biting, or touching objects with the mouth.
This can include:
Not every biological material yields usable DNA, and not every positive serology test leads to a DNA profile.
Serology testing typically occurs in two stages: presumptive tests and confirmatory tests. Presumptive tests are screening tests. They suggest that a particular biological material may be present, but they are not definitive. For example:
Presumptive tests are intentionally sensitive, meaning they can produce false positives. Confirmatory tests are more specific and are designed to confirm the presence of a particular biological material. However, even confirmatory tests have limitations, and not all materials can be conclusively confirmed.
A critical issue in many cases is whether a lab:
One of the most consequential aspects of serology is evidence selection.
A laboratory analyst must decide:
These decisions are often subjective.
For example:
These choices can unintentionally (or sometimes improperly) shape the narrative of the case.
DNA mixtures — samples containing DNA from multiple people — are one of the most complex and contested areas of forensic science today.
What many jurors never hear is this:
Mixtures are often created at the serology stage.
Examples include:
Once a mixture is created, interpretation becomes far more subjective, and the risk of error increases substantially.
Transfer, Persistence, and Secondary Transfer
Serology cannot tell you how biological material got somewhere. Biological material can be transferred through:
Serology also cannot determine:
A positive serology result does not automatically equal guilt.
Serology is typically performed early in the forensic process, often shortly after evidence is received.
This makes it a critical point for:
Questions that matter include:
Contamination at the serology stage can permanently taint evidence.
Serology documentation is often:
Important details may be missing, such as:
Without thorough documentation, it becomes nearly impossible to independently evaluate the reliability of the testing.
Although many laboratories strive to be neutral, serology involves human judgment.
Analysts must make decisions about:
Bias — whether conscious or unconscious — can influence these decisions, especially when analysts are aware of the allegations or expected outcomes.
Most courtroom discussions of DNA focus on:
Serology is often treated as a technical footnote. That is a mistake. If the biological material was misidentified, improperly selected, contaminated, or combined, the DNA results are only as reliable as the serology that preceded them.
A defense attorney with DNA experience does not simply accept serology results at face value.
Key questions include:
Serology is often where a defense can:
If DNA serology is part of your case, it does not automatically mean:
It means biological material was detected — nothing more, nothing less.
Understanding that distinction is essential.
(Screening tests — sensitive but NOT specific)
Presumptive tests are designed to err on the side of detection, not certainty. They indicate that a biological material may be present, but they do not conclusively identify a substance.
A positive presumptive test does not prove that a particular biological fluid is present.
What it detects:
How it works:
What it actually proves:
Common False Positives:
Defense relevance:
What it detects:
How it works:
Common False Positives:
Defense relevance:
What it detects:
How it works:
Common False Positives:
Defense relevance:
What it detects:
How it works:
Common False Positives:
Additional limitations:
Defense relevance:
What it detects:
How it works:
Common False Positives:
Defense relevance:
What it detects:
How it works:
Common False Positives:
Defense relevance:
(More specific — but still not infallible)
Confirmatory tests aim to identify a biological material with greater specificity, but they still have limitations and may not conclusively prove source, timing, or relevance.
What it detects:
How it works:
Limitations / False Results:
Defense relevance:
What it detects:
Limitations:
Defense relevance:
(e.g., ABAcard® HemaTrace®)
What it detects:
How it works:
Potential False Positives:
Defense relevance:
What it detects:
How it works:
Limitations:
Defense relevance:
What it detects:
How it works:
Potential False Positives:
Defense relevance:
(e.g., RSID™-Saliva)
What it detects:
Limitations / False Positives:
Defense relevance:
CRITICAL LIMITATIONS COMMON TO ALL SEROLOGY TESTS
Regardless of test type, serology cannot determine:
Decisions made at the serology stage can permanently shape a case.
Once evidence is cut, swabbed, combined, or consumed:
Having defense counsel who understands DNA and serology can make a significant difference in preserving issues and protecting your rights.
From a defense perspective, serology issues often include:
Serology results are foundational, not conclusive.
A positive serology test means “something reactive was detected” — not “this proves guilt.”
Every DNA profile depends on:
That is why experienced DNA-focused defense counsel scrutinizes serology as well as the DNA statistic.
DNA serology testing is not glamorous. It is not definitive. But it is foundational.
As a defense attorney who focuses on DNA cases, I evaluate serology with the same seriousness as DNA profiling itself — because without reliable serology, there is no reliable DNA evidence. If your case involves biological evidence, understanding serology is not optional. It is essential.
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