Forensic trace evidence

Forensic trace evidence provides the laboratory foundation for identifying biological fluids and reconstructing crime scene events through chemical and microscopic analysis. Suspected blood samples are first analysed using sensitive presumptive screening tests (e.g., UV light). Microchemical confirmatory tests (e.g., Teichmann test, Takayama test) are then conducted to confirm the presence of blood. Analysis of suspected semen samples involves identifying specific chemical and antigen markers and visualizing spermatozoa on microscopy. Specialized species identification, achieved through the precipitin test, further refines the biological profile of recovered evidence. Further studies for species identification include DNA-based methods (e.g., cytochrome b analysis) and microscopy. In addition to fluid identification, forensic biology uses durable markers such as hair, which are distinguished from animal sources using the medullary index and can be used for nuclear and mitochondrial DNA analysis. Adherence to precise collection protocols is essential to avoid contaminating evidence.

The forensic identification of blood stains follows a tiered analytical process, progressing from highly sensitive presumptive tests to specific confirmatory microchemical and instrumental methods.

Presumptive testing

Presumptive tests are used to rapidly screen suspected bloodstains. While these tests are highly sensitive, they are not specific to blood and may produce false-positive results if certain plant peroxidases or chemical oxidants are present.

Confirmatory testing

Confirmatory tests definitively identify the presence of blood by reacting with the hemoglobin or its derivatives to form characteristic crystals that are visible under a microscope.

• Teichmann test (hemin crystal test)
  • Procedure: Suspected blood is heated with a solution containing chloride and glacial acetic acid.
  • Result: formation of dark-brown, rhombic crystals of hemin (hematin hydrochloride)
• Takayama test (hemochromogen crystal test)
  • Procedure: Suspected blood is treated with a specialized Takayama reagent containing pyridine and glucose.
  • Result
    • Formation of pink, feathery, needle-like crystals of pyridine hemochromogen
    • Considered more reliable than the Teichmann test
• Absorption spectrometry
  • An analytical technique that measures the absorption of electromagnetic radiation to quantify specific spectral bands of hemoglobin and its derivatives (e.g., oxyhemoglobin, methemoglobin)
  • Can estimate the age of a blood stain, but its reliability decreases with exposure to environmental elements (e.g., temperature, humidity)

The identification of seminal fluid is a critical component of forensic investigations that involve sexual assault. The process involves identifying specific chemical and antigen markers and visualizing spermatozoa on microscopy.

UV light

• Seminal stains exhibit a characteristic blue-white fluorescence when exposed to UV light.
• This is the primary method for locating stains on large surfaces such as clothing or bedding.

Enzymatic tests

• Acid phosphatase test
  • Mechanism: detects the enzyme acid phosphatase, which is secreted in extremely high concentrations by the prostate gland
  • Result: An intense purple color develops within seconds.
  • Limitations: While this test is highly sensitive, it is only presumptive as low levels of acid phosphatase are also found in other biological fluids (e.g., vaginal secretions).
• Creatine kinase test: to detect creatine kinase, which is present in high concentrations in seminal fluid

Antigen tests

• p30 (prostate-specific antigen)
  • A reliable marker for identifying seminal fluid using immunochromatography
  • Prostate-specific antigen may also be present in sweat and fecal material; interpret positive results with caution.

• Seminal vesicle-specific antigen
  • Semenogelin is the predominant protein produced by the seminal vesicles and is a useful marker for identifying semen.
  • Detected using immunochromatography

Microchemical tests

Microchemical tests use reagents to assess for seminal fluid in a stain. The reagents react with constituents of seminal plasma to form characteristic crystals.

Microscopy

• The visualization of intact spermatozoa is the most reliable confirmatory test for semen.
• Interpretation
  • Motile sperm: indicates very recent acts of coitus (typically within 6–12 hours)
  • Nonmotile sperm: can be recovered from the vaginal canal for up to 3 days after the incident
• Aspermic or oligospermic samples: If the perpetrator has a low sperm count or has undergone a vasectomy, identification is only possible using confirmatory antigen testing.

Hair is one of the most durable forms of trace evidence recovered from crime scenes because it is highly resistant to chemical decomposition and putrefaction. Forensic hair analysis is used to determine the species of origin and identify an individual using DNA analysis.

Anatomy of the hair shaft

A forensic microscopic examination focuses on the three primary morphological layers of the hair shaft:

• Cuticle
  • The outermost layer
  • Composed of overlapping protein scales
  • The scale pattern is often specific to certain animal species but relatively uniform in humans.
• Cortex: the middle layer containing the bulk of the hair's mass and the pigment granules (melanin) that determine hair color
• Medulla: the innermost central core of the hair shaft

Determination of species

• The most critical forensic distinction in hair analysis is differentiating human hair from animal hair. This is primarily done by calculating the Medullary index.
• Medullary index = diameter of the medulla / diameter of the hair shaft

Forensic significance of the hair root

The condition of the hair root provides evidence regarding the mechanism by which the hair was removed.

• Forcibly removed hair: A follicular tag (tissue attached to the hair root) is often present. This tissue contains nuclear DNA, which can be used to identify an individual.
• Naturally shed hair: The root is typically dry and club-shaped without any adhering tissue.

DNA identification from hair

• Nuclear DNA: available only from the root or follicular tag of forcibly removed hair
• Mitochondrial DNA
  • Can be extracted from the hair shaft
  • While mitochondrial DNA is not unique to an individual, it can be used to establish or exclude maternal lineage and is particularly useful for analyzing very old or degraded hair samples.

Once a biological stain or tissue is confirmed as animal or human in origin, forensic investigators must determine the specific species from which the sample originated. This step is critical for differentiating between evidence relevant to a crime and nonforensic biological contaminants (e.g., differentiating human blood from animal blood at a crime scene).

Precipitin test

The precipitin test is a classic immunological assay used to determine the species of origin of biological samples, most commonly blood stains. It is based on the immunological reaction between an antigen (the forensic sample) and the corresponding antibody (antihuman or antianimal serum).

• Mechanism
  • Antigen-antibody reaction
  • If the forensic sample is human in origin, it will react with antihuman serum to form a visible precipitate.
• Procedure: A diluted extract of the biological stain is placed in contact with a layer of antihuman serum, typically in a small capillary tube.
• Result
  • If the sample is of human origin, a white precipitin ring (cloudy precipitate) forms at the interface between the two liquids.
  • The absence of a ring indicates that the sample is of a different species.
• Forensic significance
  • High sensitivity: The test can identify species origin even in dried or severely aged biological stains (e.g., blood stains that are several years old).
  • Specificity: By using specialized sera (e.g., antidog, anticat, or anticattle), forensic scientists can identify the exact species of a nonhuman biological sample.

DNA-based species identification

While the precipitin test remains a foundational serological tool, forensic laboratories increasingly use DNA analysis for species identification.

• Cytochrome b analysis: Sequencing of the mitochondrial cytochrome b gene enables the definitive identification of thousands of different species.
• Polymorphic markers: DNA analysis is preferred when samples are severely degraded or serological protein markers can no longer be reliably detected.

Morphological examination

In addition to serological and DNA tests, morphological examination of specialized tissues can assist in species identification.

• Hair: The medullary index is used to differentiate species.
• Skeletal remains: Identification is based on the size, density, and distinct anatomical features of bones (e.g., differentiating human cranial fragments from animal bone).

The successful reconstruction of a crime scene and the definitive identification of an individual depend on the systematic collection of biological evidence and an understanding of the factors that influence its viability and degradation.

Forensic evidence collection protocols

Adherence to strict collection and handling protocols is essential to prevent contamination and ensure that evidence is admissible in court.

• Avoidance of contamination: Investigators must use personal protective equipment (PPE), including gloved hands and face masks, to avoid introducing their own DNA or biological material into a crime scene.
• General biological materials
  • Blood: Samples should be collected from both victims and suspects for grouping and DNA characterization.
  • Saliva
    • Saliva stains or buccal swabs may be collected for DNA analysis.
    • If stain samples are required, they should be air-dried, stored without preservatives, and sent in properly labeled sterile containers.
  • Hair and fibers: The collection includes loose scalp and pubic hair, pulled reference hair samples as needed, and any foreign hair or fibers found on the body, clothing, and/or at the scene.

DNA analysis samples

Specific tissues are preferred based on their resilience and the quantity of viable genetic material they contain.

Sexual offense evidence

See "Forensic evaluation" in "Sexual violence."