Page 10 - Stained With Suspicion Blood Detection

Post-Lab and

Calculations

Solutions

1. What did you observe when you tested the 5. What did you learn about correctly
positive control card? processing evidence? Why is this procedure
The pink reaction occurred between one minute important?
and three minutes. Answers will vary. Proper evidence handling is
vital for courtroom use, as poor handling can be
grounds to have evidence thrown out.
2. What did you observe when you tested the
negative control card?
If a pink reaction was observed, it occurred later 6. What did your group conclude about the
than three minutes. stain on the carpet? Did your test detect the
presence of blood?
Yes, there was blood on the carpet.
3. What common food items provided a false
positive in the presumptive blood test?
Answers will depend on the food items that 7. Based on your knowledge of the crime(s),
were tested. Ketchup, horseradish, and others what is your hypothesis about the events
can cause false positives. surrounding the substance on the carpet of
the truck?
Answers will vary.
4. Why do police officers perform a presumptive
test in the field? Based on your experiments, why
is it important to do a confirmatory test later?
Police test for the presence of blood for early
clues about what happened at a scene, but
forensic technicians must confirm the presence
of blood in case the stain was ketchup or
horseradish, which can give a false positive.

10 THE MYSTERY OF LYLE AND LOUISE