9.6 Serology, Bloodstain Pattern, and Tool Mark Evidence Collection

9.6 Serology, Bloodstain Pattern, and Tool Mark Evidence Collection

Every task in the Advanced Forensic Science Knowledge section of the Advanced Skills List is phrased as collect: collect data or material for serology, collect data or material on tool marks, collect trace evidence. That phrasing is the exam's guidance. The investigator is not the analyst, and items in this section test what you preserve and how, not what conclusion you draw.

---

Serology at the Scene

Presumptive testing

Presumptive tests screen for the peroxidase-like activity of hemoglobin. They are sensitive, fast, and cheap, and they are not specific.

False positives are the tested point. Plant peroxidases — horseradish, potato, turnip, many vegetables — react. Some metals, notably copper and iron salts, react. Oxidizing agents, especially bleach, react, which matters because bleach is exactly what someone cleaning a scene uses. Luminol in particular reacts with bleach, with some rust and metal surfaces, and with certain plant material, so a luminol-positive floor is a lead, not a finding.

A positive presumptive test supports only that the stain may be blood. It does not establish that it is blood, that it is human, or that it is the decedent's.

Confirmatory and species testing

• Crystal tests — Takayama (hemochromogen) and Teichmann (hemin) — confirm the presence of blood by producing characteristic crystals under microscopy
• Immunochromatographic hemoglobin tests (for example, cards that detect human hemoglobin) indicate human origin, with the known caveat of cross-reaction with some higher primates and ferrets
• DNA establishes source, and is the only method that individualizes

Other biological fluids

• Semen — acid phosphatase as a presumptive screen, p30 / prostate-specific antigen as a confirmatory human-semen indicator, and microscopic identification of spermatozoa
• Saliva — amylase-based screening
• Urine — creatinine and urea screening
• Alternate light examination causes many biological stains to fluoresce, which is a locating technique, not an identification

Collection

• Wet stains: absorb onto a sterile swab, then air dry completely before packaging
• Dry stains on a movable item: collect the whole item where possible
• Dry stains on a fixed surface: double-swab technique — a swab lightly moistened with sterile water followed immediately by a dry swab over the same area
• Always collect a substrate control from an unstained area of the same surface
• Package in breathable paper — never sealed plastic, which traps moisture and degrades DNA
• Change gloves between items, use single-use or cleaned instruments, and wear a mask to prevent the collector's own DNA from reaching the sample

---

Bloodstain Patterns: What the Investigator Documents

Bloodstain pattern analysis is a specialist discipline, and the investigator's contribution is documentation that preserves the pattern for someone else to interpret. A swab destroys the very thing the analyst needs.

Terminology

Current terminology organizes patterns by mechanism:

Passive / gravity-driven: drip stains, drip trails, flow patterns, pools, saturation stains, and transfer stains (contact stains, including wipe and swipe patterns).

Spatter: impact patterns from a force applied to a liquid blood source; cast-off patterns from blood flung from a moving object; expirated patterns from blood forced from the airway, often with bubble rings and dilution; arterial patterns with surge or spurt morphology; and drip and satellite spatter.

Altered stains: void patterns where an object blocked deposition and was subsequently removed, clotted stains, diluted stains, insect stains (which frequently mimic spatter), sequenced stains showing overlay order, and stains altered by wiping or by decomposition fluid.

Two of these carry disproportionate investigative weight. A void tells you an object or person was present and was removed — its shape may identify what stood there. Insect stains are the most common misinterpretation in an indoor scene with a decomposing decedent, and are distinguished by their irregular distribution, accompanying fly specks, and presence on surfaces such as ceilings and light fixtures.

Directionality and angle

A bloodstain's shape records its travel:

• The stain is elongated in the direction of travel, and the tapered, pointed, or scalloped end points in the direction the drop was moving
• The impact angle is calculated as θ = arcsin (width ÷ length) of the elliptical stain
• A drop striking perpendicular to a surface produces a circular stain (90 degrees); increasingly acute angles produce increasingly elongated ellipses
• Area of origin is estimated by the analyst from the convergence and the calculated angles of multiple well-formed stains

Surface texture strongly affects stain morphology, which is why the analyst needs to know the substrate and why a control photograph of the surface matters.

Documenting a bloodstain scene

1. Overall photographs establishing the room and the relationship of the patterns to the body and to the furniture
2. Roadmapping — adhesive scales or numbered labels placed adjacent to patterns, then mid-range photographs establishing each pattern's location
3. Close-up photographs perpendicular to the surface with a scale in the same plane as the stain, which is required for any subsequent angle measurement
4. Overlapping sequential photographs across large patterns so they can be reassembled
5. Sketch and measure the patterns' positions and dimensions
6. Photograph before applying any presumptive reagent, and document luminol results with long-exposure photography noting that reagent application alters the stain
7. Only then collect samples for DNA, from the periphery of a pattern where possible so the pattern is preserved

---

Tool Marks

Class, subclass, and individual characteristics

• Class characteristics are shared by all tools of a type — a screwdriver blade of a given width, a bolt cutter jaw profile
• Subclass characteristics arise from the manufacturing process and are shared by a limited group of tools made in the same run; they are a well-recognized source of erroneous association
• Individual characteristics are the random imperfections a specific tool acquires from manufacture, use, and damage, and are the basis for any identification claim

Mark types

• Impressed (static) marks — the tool is pressed into a softer surface, as in a pry mark on a door jamb or a hammer strike
• Striated (dynamic) marks — the tool moves across the surface under pressure, producing parallel striae, as when a screwdriver slips or a bolt cutter shears

The rule that gets tested

Never fit a suspected tool into a tool mark. Doing so can alter both the mark and the tool, creates the appearance of a fabricated association, and transfers material between them. Comparison is performed in the laboratory with test marks made in appropriate media on a comparison microscope.

Documentation and collection

• Photograph with a scale, perpendicular to the mark, and then again with oblique lighting from several directions, since low-angle light is what makes impressed detail visible at all
• Collect the whole item bearing the mark where it can be removed — a door jamb section, a padlock, a window frame member
• Where the item cannot be removed, cast the mark with a silicone casting material such as a polyvinylsiloxane product, after photographing; label the cast with the case number, date, initials, and orientation before it is lifted
• Package the tool separately from the marked item, and never in the same container
• Protect the working surfaces of a recovered tool by wrapping or suspending it so the tip is not abraded in transport, and remember that the tool may also bear trace evidence and DNA from the surface it acted on — collect those before the tool is handled for comparison

Tool marks on bone and cartilage

Skeletal tool marks are a forensic anthropology subject with its own vocabulary. Saw marks in dismemberment cases are described by the kerf — the cut channel — with its kerf floor shape, kerf width, striae, and any breakaway spur or false start, and these features can indicate saw type and blade characteristics. Knife and chop marks on bone and cartilage may retain class-level blade features. Costal cartilage, hyoid, and sternal cartilage retain tool marks well, which is why the pathologist and anthropologist should be alerted before those structures are processed, and why the investigator's scene documentation of any recovered implement matters to that examination.

A caution on comparison claims

The scientific limits of pattern comparison disciplines have been examined closely in the last two decades, and overstated identification claims have been a recurring source of error. The Board-level investigator's defensible position is to document thoroughly, preserve completely, and leave the comparison conclusion to the qualified examiner — and, when testifying, to describe what was collected and how, rather than what it proved.