Trace evidence

Forensic trace evidence provides the laboratory foundation for identifying biological fluids and reconstructing crime scene events through chemical and microscopic analysis. The definitive identification of blood and semen relies on a progression from sensitive presumptive screening, such as the acid phosphatase or UV fluorescence tests, to specific microchemical confirmatory tests like the Teichmann and Takayama crystal tests. Beyond fluid identification, forensic biology utilizes resilient markers such as hair, which can be distinguished from animal sources via the medullary index, and provides a source for both nuclear and mitochondrial DNA analysis. Specialized species identification, achieved through the precipitin test, further refines the biological profile of recovered evidence. Adherence to precise collection protocols is essential for ensuring that biological evidence remains legally robust for the administration of justice.

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 for the rapid screening of suspected stains. While highly sensitive, they are not specific to blood and may produce false-positive results with certain plant peroxidases or chemical oxidants.

Confirmatory testing

Confirmatory tests definitively identify the presence of blood by reacting with the hemoglobin or its derivatives to form characteristic crystals 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: Blood is treated with a specialized Takayama reagent (containing pyridine and glucose).
  • Result
    • Formation of salmon-pink, feathery (needle-like) crystals of pyridine hemochromogen
    • This is considered the most reliable microchemical test for blood.
• Absorption spectrometry
  • The most reliable method for identifying the specific spectral bands of hemoglobin and its derivatives (e.g., oxyhemoglobin, methemoglobin)
  • This method is effective even for very old or severely degraded stains.

The identification of seminal fluid is a critical component of forensic investigations involving sexual assault or established paternity. The analytical process relies on identifying specific chemical and antigen markers and the microscopic visualization of spermatozoa.

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: development of an intense purple color within seconds
  • Limitations: While highly sensitive, this test is only presumptive, as low levels of acid phosphatase are also found in other biological fluids (e.g., vaginal secretions).
• Creatine phosphokinase test: detects the enzyme creatine phosphokinase, which is present in high concentrations in seminal fluid

Antigen tests

• p30 (PSA)
  • An effective marker for seminal fluid identification using chromatographic immunoassay methods
  • PSA may also be present in sweat and fecal material; therefore, positive results should be interpreted cautiously.

• Seminal vesicle-specific antigen
  • Semenogelin is the predominant protein produced by the seminal vesicles and is a useful marker for semen identification.
  • Detected by immunochromatographic methods

Microchemical tests

Microchemical tests rely on the reaction of reagents with specific chemical constituents of seminal plasma to form characteristic crystals.

Microscopy

• The visualization of intact spermatozoa is the most reliable confirmatory test for the presence of semen.
• Interpretation
  • Motile sperm: indicates very recent acts of coitus (typically within 6–12 hours)
  • Non-motile sperm: can be recovered from the vaginal canal for up to 3 days post-incident
• Aspermic/oligospermic samples: In cases where the perpetrator has a low sperm count or has undergone a vasectomy, identification must rely entirely on confirmatory antigen testing.

Hair is one of the most resilient forms of trace evidence recovered from crime scenes, as it is highly resistant to chemical decomposition and putrefaction. Forensic analysis of hair is utilized to determine the species of origin and to provide sources for individual DNA identification.

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 achieved 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: often exhibits a follicular tag (tissue adhering to the root) containing nuclear DNA, which can be used for individual identification
• Naturally shed hair: typically has a club-shaped, dry root without adhering tissue

DNA identification from hair

• Nuclear DNA: available only from the root or follicular tag of forcibly removed hair
• Mitochondrial DNA (mtDNA)
  • Can be extracted from the hair shaft
  • While mtDNA 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.

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

The precipitin test

The precipitin test is a classic microserological method used to determine the species origin of biological stains, most commonly blood stains. It is based on the immunological reaction between an antigen (the forensic sample) and a specific antibody (anti-human or anti-animal serum).

• Mechanism
  • The test utilizes the principles of the antigen-antibody reaction.
  • If the forensic sample is human, it will react with anti-human serum to form a visible complex.
• Procedure: A diluted extract of the biological stain is placed in contact with a layer of anti-human 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., anti-dog, anti-cat, or anti-cattle), forensic scientists can identify the exact species of a non-human biological sample.

DNA-based species identification

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

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

Microscopic differentiation

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

• Hair: utilizing the medullary index to differentiate species.
• Skeletal remains: identification 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 individuals 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 prevent the introduction of their own DNA or biological material.
• 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, 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."