Chapter 20 — Exercises

Work these without looking back at the chapter first; then check yourself. Items marked † have full worked solutions in the answers appendix. There are no answers in this file. The set mixes recall, applied reasoning and calculation, evidence interpretation, "spot the overstatement," ethics, and a cold-case extension.

A. Recall and definitions

1. Define forensic toxicology in one sentence, and name its three component activities (the three questions of §20.1).

2. † State the three questions of forensic toxicology in order, label each as identification, quantitation, or interpretation, and explain why certainty "drains away" from the first to the third.

3. Define a metabolite. Why does finding a metabolite in urine tell you the parent drug was in the body and processed, even when the parent drug itself is no longer detectable?

4. Distinguish screening from confirmation. Name the typical method for each, and state the one-sentence rule about which result may go in a report as an identification.

5. Define immunoassay and explain cross-reactivity. Why does cross-reactivity make an immunoassay presumptive only?

6. Define BAC and give the unit it is conventionally expressed in (U.S.). Is the measurement of BAC the hard part of alcohol toxicology, or the interpretation? Explain in one sentence.

7. State Paracelsus's principle ("the dose makes the poison") in your own words, and define the therapeutic, toxic, and lethal ranges.

8. Define postmortem toxicology and postmortem redistribution (PMR). In one sentence, why does PMR make a corpse's blood concentration an unreliable mirror of the antemortem level?

9. Why is vitreous humor described as the body's "protected archive"? Name its two forensic virtues.

10. What is retrograde extrapolation of BAC, and what phase of the alcohol curve does it assume the person was in?

B. Applied reasoning and calculation

11. † A decedent's drug concentration is reported from heart (central) blood. Explain, mechanistically, why this value may overstate the concentration that was circulating at the moment of death, and name the single best alternative specimen and why it is preferable.

12. A driver's blood, drawn 2 hours after a crash, measures a BAC of 0.12. Assuming the driver was past peak and eliminating at an illustrative rate of about 0.015 per hour, estimate the BAC at the time of the crash, showing your arithmetic. Then state the single most important assumption that, if wrong, would make this estimate an overstatement.

13. † Re-read the absorption–elimination curve (§20.4, Figure 20.1). Explain how the same measured BAC could correspond to a higher crash-time value in one scenario and a lower crash-time value in another. What information would you need to tell the two apart?

14. A urine screen is positive for a cannabis metabolite. A prosecutor wants to argue the driver was impaired at the time of a 9 p.m. crash. Explain, using the difference between exposure and effect, why urine is the wrong specimen to support that specific claim.

15. A blood opioid concentration falls squarely in the textbook "lethal range." Give two distinct reasons this finding does not, by itself, establish that the opioid was the cause of death.

16. † A body shows a combined presence of an opioid, a benzodiazepine, and alcohol, none individually in its own lethal range. Explain why reading each drug in isolation could miss the cause of death, and name the interpretive concept involved.

17. A chronic, long-term user of a drug is found dead with a blood concentration that would be "lethal" for a naive person. Explain how tolerance complicates the interpretation, and what additional, non-toxicological information would help.

18. Rank these specimens by the detection window they typically offer, shortest to longest, and state the question each is best suited to answer: hair; blood; urine.

19. A workplace immunoassay flags "amphetamines" for an employee taking an over-the-counter decongestant. Walk through what should happen next before any adverse action, and name the chemical reason the screen alone is unsafe to act on.

20. Explain why a confirmatory test that rests on the same chemical principle as the screen (e.g., a second immunoassay) is not a true confirmation. What property must the confirmatory method have instead?

C. Evidence interpretation

21. † Re-read Figure 20.1 ("One measured number, two different stories"). Write (a) the single sentence an honest toxicologist could say on the stand about the crash-time BAC, and (b) the single sentence that would be an overstatement. State what makes the difference.

22. A toxicology report states: "Diazepam was confirmed in peripheral (femoral) blood at [concentration]. This concentration is consistent with use in the toxic/incapacitating range; interpretation of effect in this individual is limited by unknown tolerance." Identify three separate things this report does well (where it is appropriately honest).

23. An expert testifies, in a single breath: "The drug was present at this level, so the deceased was impaired, so he could not have walked to the next room." Separate this into the three kinds of statement from the §20.1 "At the Bench" callout, and identify which is the weakest and why.

24. A BAC of 0.09 is measured in a body recovered from a fire several days after death. Before treating this as proof of antemortem drinking, list two postmortem processes that could affect the number, and name the specimen comparison that helps distinguish genuine drinking from postmortem alcohol production.

25. A report gives a postmortem drug concentration with no sampling site stated. Explain why this omission alone should make a careful reader withhold an interpretation, and what single piece of information would most change how the number is read.

D. Spot the overstatement / junk-science alert

26. † A prosecutor's slide reads: "Toxicology proves the defendant was high at the moment of the offense." Identify two distinct problems with this statement (think: which of the three questions it conflates, and the present-vs-impairing gap) and rewrite it honestly.

27. A police press release states a roadside field test "tested positive for cocaine," and a suspect was arrested. Using the screen-vs-confirmation architecture, explain what is missing before this is an identification, and why presumptive field tests have produced wrongful arrests.

28. An expert offers a single back-calculated BAC ("0.14 at the time of the crash") with no stated assumptions and no range. List the elements whose absence should make a reader distrust the number.

29. A toxicologist testifies that a urine-positive result "shows the driver was under the influence." Name the specific overstatement (what urine can and cannot establish) and give the defensible version.

30. A report converts a minimum claim ("the drug was at least present") into a maximal one ("the drug caused the death") without intervening reasoning about concentration, tolerance, or combination. Name which range concept (therapeutic/toxic/lethal) the leap skips, and why the leap is unearned.

E. Ethics and reasoning

31. † A toxicologist is told, before analysis, "this is an obvious overdose — the family found him with pills." Using the Cognitive-Bias Watch in §20.4 (and previewing Chapter 31), explain how that framing could distort which tests are ordered and how ambiguous results are read, and describe the safeguard.

32. Two qualified toxicologists, given the same postmortem blood concentration, offer different interpretations of whether it was "incapacitating" for the decedent. Is this necessarily a scandal? Explain by reference to tolerance, sampling site, and the population basis of the ranges.

33. You are asked to testify that a confirmed drug identification also proves the person was impaired at a specific past moment. Explain why you should decline to let the certainty of the identification vouch for the impairment claim, and what you can honestly say instead.

34. Why is the relative scarcity of stated assumptions in an interpretive toxicology opinion itself a fact a careful court should weigh — even when the underlying identification is unimpeachable?

35. A defense expert and a prosecution expert agree completely on the identification and quantitation but disagree sharply on the interpretation. Explain why this pattern is exactly what the §20.1 / §20.6 framing predicts, and which half of the testimony a competent cross-examination should target.

F. Synthesis and validity spectrum

36. † Place these on the NAS 2009 / PCAST 2016 validity spectrum, justifying each: a confirmed GC-MS drug identification (this chapter / Chapter 23); an interpretive claim that a postmortem concentration was "incapacitating for this person" (this chapter); single-source nuclear DNA (Chapter 7); bite-mark comparison (Chapter 16, previewed).

37. Explain, in one paragraph, how forensic toxicology occupies two positions on the validity spectrum at once, and why the discipline of this book is to keep the high-spectrum statement from vouching for the low-spectrum one. Give an example statement of each.

38. In one paragraph, explain how this chapter advances at least two of the book's four themes (exclusion over proof; the validity spectrum; cognitive bias; the CSI effect cutting both ways). Name which themes and how.

G. Cold-case extension

39. † Cold Case. Using only what the toxicology of Marcus Diallo's specimens establishes, write the entry you would add to the Mill Creek evidence log (Appendix I). State (a) the defensible inference at its true strength, (b) the honest verb, (c) at least three things this evidence specifically does not establish (including whether it names a perpetrator and whether the sedative is the cause of death), and (d) why "drugged before death" corroborates the homicide finding (Chapter 11) without independently proving it.

40. Cold Case, integrative. The toxicology establishes that the victim was chemically incapacitated before death. List two other evidence types from earlier or later chapters that, combined with the incapacitation finding, would begin to connect a person to the crime — and state plainly why the toxicology alone cannot make that leap. (Think: who had access to a sedative; who can be placed at the scene.) Note explicitly that you must not name a suspect on the strength of the toxicology.

H. Short writing

41. In 150–200 words, explain to a juror why a confirmed drug identification is genuinely strong evidence and why "the drug was present" is not the same thing as "the person was impaired" — and why the law sometimes sets a per se alcohol limit precisely because of that gap.

42. † In 150–200 words, contrast the two halves of forensic toxicology — detection (identification and quantitation) and interpretation — explaining what each rests on, where each sits on the validity spectrum, and where an honest expert must concede uncertainty under cross-examination.