A Plain-English Guide for Criminal Defense Clients
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:
- Whether biological material is present at all
- What type of biological fluid it may be (blood, saliva, semen, etc.)
- Where that material is located on an item
- Which areas should be tested for DNA
- Which areas should not be tested for DNA
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.
Common Biological Materials Tested in Serology
Forensic serology focuses on identifying biological fluids commonly encountered in criminal investigations. These include:
- Blood
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
Semen testing is common in sexual assault cases. Serology seeks to identify components such as sperm cells or seminal fluid markers.
- Saliva
Saliva may be deposited through licking, kissing, biting, or touching objects with the mouth.
- Other Biological Material
This can include:
- Vaginal fluid
- Menstrual blood
- Urine (less commonly used for DNA)
- Skin cells (often referred to as “touch DNA,” though serology for skin cells is limited)
Not every biological material yields usable DNA, and not every positive serology test leads to a DNA profile.
Presumptive vs. Confirmatory Testing
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:
- A test that reacts to the presence of hemoglobin may indicate possible blood
- A test that reacts to enzymes found in saliva may suggest possible saliva
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:
- Relied too heavily on presumptive testing
- Failed to perform confirmatory testing
- Overstated what a test result actually proves
How Evidence Is Selected for DNA Testing
One of the most consequential aspects of serology is evidence selection.
A laboratory analyst must decide:
- Which stains or areas to test
- How many samples to take
- Whether to test a whole item or cut portions
- Whether to prioritize certain locations over others
These decisions are often subjective.
For example:
- Why was one stain tested but not another?
- Why was a particular area assumed to be “probative”?
- Why was evidence from one person’s clothing prioritized over another’s?
These choices can unintentionally (or sometimes improperly) shape the narrative of the case.
The Problem of Mixtures Begins in Serology
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:
- Cutting too large an area of fabric
- Swabbing areas with overlapping biological material
- Combining multiple stains into one sample
- Testing areas likely to contain DNA from innocent contact
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:
- Direct contact
- Secondary transfer (touching an object after someone else)
- Innocent, everyday activities
- Prior unrelated events
Serology also cannot determine:
- When the material was deposited
- Whether it was deposited during a crime
- Whether it was deposited consensually or innocently
A positive serology result does not automatically equal guilt.
Chain of Custody and Contamination Risks
Serology is typically performed early in the forensic process, often shortly after evidence is received.
This makes it a critical point for:
- Cross-contamination
- Environmental contamination
- Analyst handling errors
- Improper packaging or storage
Questions that matter include:
- Were items packaged together?
- Were gloves changed between items?
- Were tools cleaned properly?
- Were controls used and documented?
Contamination at the serology stage can permanently taint evidence.
Documentation and Transparency Problems
Serology documentation is often:
- Sparse
- Vague
- Heavily summarized
- Difficult for non-scientists to interpret
Important details may be missing, such as:
- Exact locations of stains
- Photographs before cutting or swabbing
- Justification for evidence selection
- Notes (or photos) about weak or ambiguous results
Without thorough documentation, it becomes nearly impossible to independently evaluate the reliability of the testing.
Serology Is Not Objective Truth
Although many laboratories strive to be neutral, serology involves human judgment.
Analysts must make decisions about:
- What is “likely” biological material
- What is worth testing
- What results are meaningful
- How results are reported
Bias — whether conscious or unconscious — can influence these decisions, especially when analysts are aware of the allegations or expected outcomes.
Why Serology Is Often Overlooked in Court
Most courtroom discussions of DNA focus on:
- DNA profiles
- Statistical probabilities
- Match numbers
- Likelihood ratios
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.
How a DNA-Focused Defense Attorney Evaluates Serology
A defense attorney with DNA experience does not simply accept serology results at face value.
Key questions include:
- Were proper tests used?
- Were results overstated?
- Was confirmatory testing appropriate?
- Were alternative explanations considered?
- Were laboratory protocols followed?
- Was evidence selection justified?
- Were mixtures unnecessarily created?
Serology is often where a defense can:
- Expose weaknesses in the evidence
- Challenge assumptions
- Limit overstatements
- Educate the court and jury
What Clients Should Understand About Serology Evidence
If DNA serology is part of your case, it does not automatically mean:
- The biological material came from you
- The material was deposited during a crime
- The testing was flawless
- The conclusions are unquestionable
It means biological material was detected — nothing more, nothing less.
Understanding that distinction is essential.
- PRESUMPTIVE SEROLOGY TESTS
(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.
- PRESUMPTIVE TESTS FOR BLOOD
- Kastle–Meyer Test (Phenolphthalein Test)
What it detects:
- The presence of hemoglobin’s peroxidase-like activity
How it works:
- Phenolphthalein reagent reacts with hydrogen peroxide
- If hemoglobin is present, the solution turns pink
What it actually proves:
- Possible presence of a substance with peroxidase activity
Common False Positives:
- Plant peroxidases (e.g., horseradish, potatoes)
- Certain fruits and vegetables
- Cleaning agents (bleach residues if not neutralized properly)
- Some metals and oxidizing agents
- Chemical disinfectants
Defense relevance:
- The test does not confirm blood
- The test does not confirm human blood
- Environmental contamination is common
- Leuco Crystal Violet (LCV)
What it detects:
- Peroxidase activity similar to hemoglobin
How it works:
- Color change (purple/blue) in the presence of oxidizing agents
Common False Positives:
- Paints and dyes
- Copper compounds
- Certain cleaning chemicals
- Plant materials
Defense relevance:
- Highly sensitive but notoriously non-specific
- Often used at scenes rather than labs
- Tetramethylbenzidine (TMB / Hemastix)
What it detects:
- Peroxidase activity associated with hemoglobin
How it works:
- Blue-green color change
Common False Positives:
- Bleach
- Rust
- Copper salts
- Some cleaning products
- Certain food substances
Defense relevance:
- Commonly used in field testing
- Especially prone to environmental false positives
- Luminol / Bluestar (Chemiluminescent Tests)
What it detects:
- Trace peroxidase activity
How it works:
- Emits blue light in the presence of oxidizers
Common False Positives:
- Bleach and cleaning agents
- Copper and brass
- Certain paints
- Some adhesives
- Plant materials
Additional limitations:
- Dilutes biological material
- Can degrade DNA
- Can spread contamination
Defense relevance:
- Positive reaction ≠ blood
- Reaction ≠ human
- Can complicate later DNA interpretation
- PRESUMPTIVE TESTS FOR SEMEN
- Acid Phosphatase (AP) Test
What it detects:
- Acid phosphatase enzyme, found in high concentrations in semen
How it works:
- Color change (purple) in the presence of AP enzyme
Common False Positives:
- Vaginal secretions
- Fecal material
- Some plant materials
- Certain cosmetics
- Bacterial contamination
Defense relevance:
- Acid phosphatase is not unique to semen
- Enzyme levels vary widely among individuals
- PRESUMPTIVE TESTS FOR SALIVA
- Amylase Tests (Phadebas, SALIgAE, etc.)
What it detects:
- Alpha-amylase enzyme
How it works:
- Detects starch digestion by amylase
Common False Positives:
- Feces
- Vaginal fluid
- Sweat
- Tears
- Some food residues
Defense relevance:
- Amylase is present in multiple bodily fluids
- High variability between individuals
- CONFIRMATORY SEROLOGY TESTS
(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.
- CONFIRMATORY TESTS FOR BLOOD
- Takayama Crystal Test (Hemochromogen Test)
What it detects:
- Hemoglobin crystals formed under specific conditions
How it works:
- Microscopic observation of characteristic crystals
Limitations / False Results:
- Requires sufficient intact hemoglobin
- Old, degraded, or chemically treated stains may fail
- Does not determine species (human vs. animal)
Defense relevance:
- Negative result does not rule out blood
- Positive result does not establish human origin
- Teichmann Crystal Test (Hemin Crystal Test)
What it detects:
- Hemin crystals derived from hemoglobin
Limitations:
- Less sensitive than Takayama
- Rarely used in modern labs
Defense relevance:
- Subjective interpretation under microscope
- Human Hemoglobin Immunological Tests
(e.g., ABAcard® HemaTrace®)
What it detects:
- Human hemoglobin proteins
How it works:
- Antibody-based reaction specific to human hemoglobin
Potential False Positives:
- Cross-reactivity with some primates
- Improper storage or degraded samples
- Rare environmental protein interference
Defense relevance:
- Does not indicate how or when blood was deposited
- CONFIRMATORY TESTS FOR SEMEN
- Microscopic Identification of Spermatozoa
What it detects:
- Physical presence of sperm cells
How it works:
- Microscopic examination after staining
Limitations:
- Azoospermia (no sperm production)
- Vasectomy
- Low sperm count
- Degraded samples
Defense relevance:
- Absence of sperm ≠ absence of semen
- Presence of sperm ≠ proof of a criminal act
- Prostate-Specific Antigen (PSA / p30)
What it detects:
- PSA protein found in seminal fluid
How it works:
- Immunological antibody test
Potential False Positives:
- Male urine
- Some female biological samples
- Cross-reactivity at low levels
Defense relevance:
- PSA does not indicate sperm presence
- PSA does not indicate time of deposition
- CONFIRMATORY TESTS FOR SALIVA
- Saliva-Specific Immunoassays
(e.g., RSID™-Saliva)
What it detects:
- Saliva-specific proteins (e.g., alpha-amylase isoforms)
Limitations / False Positives:
- Cross-reactivity with other body fluids
- Environmental degradation
- Variable expression among individuals
Defense relevance:
- Positive result ≠ intentional deposition
- Positive result ≠ time frame
CRITICAL LIMITATIONS COMMON TO ALL SEROLOGY TESTS
Regardless of test type, serology cannot determine:
- When biological material was deposited
- How it was transferred
- Whether it was deposited during a crime
- Whether contact was consensual or innocent
- Who deposited the material
- Whether the material is relevant to the alleged act
Why Early Legal Guidance Matters
Decisions made at the serology stage can permanently shape a case.
Once evidence is cut, swabbed, combined, or consumed:
- It may never be retested
- Errors may be irreversible
- Independent analysis may be limited
Having defense counsel who understands DNA and serology can make a significant difference in preserving issues and protecting your rights.
WHY THIS MATTERS FOR THE DEFENSE
From a defense perspective, serology issues often include:
- Overstating presumptive results as “confirmed”
- Failing to disclose false-positive risks
- Testing irrelevant or highly transferable areas
- Creating mixtures at the serology stage
- Inadequate documentation of testing locations
- Confirmation bias in evidence selection
Serology results are foundational, not conclusive.
- BOTTOM LINE FOR CLIENTS AND COURTS
A positive serology test means “something reactive was detected” — not “this proves guilt.”
Every DNA profile depends on:
- What was tested
- How it was tested
- Why it was tested
- And what assumptions were made beforehand
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.
