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Presumptive-Test Decision Tool

Run the Presumptive-Test Decision Tool Fullscreen

About This MicroSim

Forensic testing runs in two stages, and this MicroSim lets investigators live inside the first one. A presumptive test is fast, cheap, and sensitive: it tells you a stain might be blood so a lab does not waste a slow, expensive confirmatory test on a ketchup smear. The Kastle-Meyer test uses phenolphthalein reagent and hydrogen peroxide. Blood contains hemoglobin, which acts like a peroxidase and flashes the reagent bright pink within seconds.

Here is the catch this tool is built to teach: other things act like a peroxidase too. Horseradish is the classic false positive — it screens strongly positive but is not blood at all. So a pink flash means "consistent with blood, confirm it," never "this is blood." A presumptive test screens; only a confirmatory test identifies.

How to Use It

  1. Pick a sample from the Sample dropdown: Stain A (reddish-brown), Rust water, Ketchup, Horseradish extract, or Blood simulant.
  2. Press Run Presumptive Test and watch the reagent drop fall. Peroxidase-active samples (blood simulant, horseradish, and Stain A) flash bright pink — a presumptive positive. Rust reacts weakly and ketchup shows no change.
  3. Read the Verdict panel. A pink result means the stain could be blood, but it is a presumptive call only.
  4. Press Run Confirmatory Test to reveal the truth. Only Stain A and the Blood simulant are truly blood — horseradish is exposed as a false positive.
  5. Press Reset and try another sample to compare how the screen and the confirmation agree or disagree.

Learning Objective

Judge which stains warrant confirmatory testing and reason about false positives, distinguishing a presumptive screen from a confirmatory identification (Bloom Level 5 — Evaluate).

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/presumptive-test-decision-tool/main.html"
        width="100%" height="530" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 15–20 minutes Bloom level: Evaluate (L5) — judge which stains warrant confirmatory testing.

Warm-up. Ask investigators: "If a reddish-brown smear flashes bright pink with the Kastle-Meyer reagent, do we now know it is blood? What could a defense attorney say?"

Guided questions:

  • Run the test on Horseradish extract, then confirm it. Why does a non-blood sample screen positive, and what does that tell you about the meaning of a pink flash?
  • Which samples does the presumptive test screen out, sparing the lab a slow confirmatory test? Why is that screening still valuable even though it cannot identify blood?
  • A presumptive test "screens" while a confirmatory test "identifies." Using two samples from the tool, explain the difference in what each result lets you claim in court.

Extension. Discuss why labs prefer a highly sensitive presumptive test (few false negatives) even at the cost of occasional false positives, and how chain-of-custody plus confirmatory testing together protect an innocent suspect.

References

Specification

This MicroSim was generated from a specification in Chapter 6: Forensic Serology and Biological Fluid Analysis.

Design note: the reagent chemistry is stylized for teaching — the pink flash, weak rust reaction, and hidden "truth" are qualitative outcomes chosen to illustrate false positives, not measured reaction kinetics.