Chapter 10 — Key Takeaways

One-page field card. Two questions, two completely different levels of validity. Never let one borrow the other's credibility.

The core split

Blood asks two questions that must stay in separate boxes: (1) What is it, and whose is it? → serology into DNA → near the strong end of the spectrum. (2) What happened? → bloodstain pattern analysis → a real physical core wrapped in contested interpretation.

The two-stage funnel (§10.1–10.2)

• Presumptive test = fast, cheap, sensitive screen. Positive = "could be — keep going." Negative = a reliable exclusion (the powerful result).
• Confirmatory test = specific; establishes the stain is the target (human blood).
• Order: screen → confirm → DNA (whose?). Never let a presumptive flag pose as an identification.
• All blood screens detect heme's peroxidase activity, not "blood" — which is why they false-positive.

Serology → DNA handoff (§10.3)

• Serology = what / which fluid. DNA = whose (Chapters 7–9). Keep them separate.
• Collection ritual: document first → test a small edge → wet-then-dry swab → air-dry, package breathable paper, never sealed plastic → controls + chain of custody.
• The ceiling on a DNA result is set at the scene. (Case Study 10.1: O. J. Simpson — strong chemistry, lost on collection/handling.)
• Inherited traps: the transfer problem (presence ≠ action ≠ timing) and mixtures (preserve, don't assume single-source).

Bloodstain pattern analysis (§10.4–10.6)

• Defensible core: a free-falling drop is a sphere; stain directionality from shape; angle of impact from $\sin(\text{angle}) = \text{width}/\text{length}$; area of origin as a bounded region with real uncertainty.
• Categories: passive (gravity), transfer/contact, spatter (force-projected droplets — a description, not a cause).
• The stringing method places origin too HIGH (it ignores the droplet's parabolic, gravity-curved path).
• Distrust: blow-by-blow reconstruction, single confident mechanism, the narrative that matches the detective's theory.
• Bias host: ambiguous patterns + analyst told the case theory = contextual bias. Fix = blind analysis (Chapter 31).

Method-validity verdict

Blood DETECTION (serology → confirmatory → DNA): STRONG-to-MODERATE. Chemistry understood, error modes (false positives) named, rigorous confirmatory/DNA backstop. The risk is narrative, not chemistry.

BLOODSTAIN PATTERN ANALYSIS: SPLIT. Directionality/angle/area-of-origin = defensible with stated uncertainty. Event-level reconstruction = weak/contested — the 2009 NAS report found it "more subjective than scientific" with enormous uncertainties and inadequately established error rates. (Case Study 10.2: David Camm — same eight stains, opposite expert conclusions, three trials.)

What you can honestly say on the stand

For blood ID: "The stains tested presumptively positive for blood and were confirmed as human blood; source attribution was determined by DNA analysis" — never "this is the victim's blood" from a presumptive test alone.

For BPA: "The stains' directionality is consistent with a source low and to the left; the area-of-origin estimate is roughly 30–50 cm above the floor, with substantial uncertainty; this pattern is difficult to reconcile with [account A] and consistent with [account B]." Never narrate the crime, name a precise origin point, declare a single mechanism without basis, or say "to a reasonable degree of scientific certainty" about an event-level conclusion.

Themes advanced

• Theme 1 (exclusion > proof): a presumptive negative and an area-of-origin "this account is hard to reconcile" are both clean exclusions; inclusions come with caveats.
• Theme 2 (validity spectrum): blood ID is strong; event-level BPA is NAS-flagged and contested — same chapter, opposite ends.
• Theme 3 (bias): BPA is a prime host for contextual bias; blind analysis is the fix.
• Theme 4 (CSI effect): jurors expect blood to "tell the story"; an analyst who obliges is feeding a fiction.

Cold case (this chapter)

Doorframe stain: human blood, deposited before the fire (lower edge below char line). Low spatter: inconsistent with collapse-in-fire, consistent with a pre-fire impact. Status: force likely applied before the fire — a defensible sequence and exclusion, not a suspect. Whose blood (DNA) and what the body says (autopsy, Chapter 11) come next.