Lecture Notes: Time of Death Calculations

Key Topics

• Postmortem Interval
• Algor Mortis
• Rigor Mortis
• Vitreous Humor Potassium
• Time of Colonization and Insect Activity

Postmortem Interval

• Determined by forensic pathologists in the ME's office.

• Algor Mortis:

  • Body temperature decreases post-death to match ambient temperature.
  • Rate of decrease: ~1 to 1.5°C per hour, not strictly linear.
  • Calculation:
    • Formula: ( (37°C - \text{current body temperature}) / T )
    • T = 1.5 if ambient temperature < 0°C; T = 0.75 otherwise.

• Rigor Mortis:

  • Muscle stiffening post-death, beginning with jaw and neck (1-4 hours).
  • Full body rigor after 8 hours, dissipates in reverse order by 24-36 hours.
  • Decomposition (after 36 hours) precludes use of rigor mortis.

• Vitreous Humor Potassium:

  • Potassium accumulates post-death due to lack of active transport.
  • Calculation uses vitreous humor sample:
    • Formula: ( 5.26 \times [\text{potassium concentration}] - 30.9 )
    • Convert to days by dividing by 24.

Time of Colonization

• Difference between Time of Colonization and Postmortem Interval:

  • Postmortem Interval: Physiological decomposition processes.
  • Time of Colonization: Interval from death to insect activity.

• Precolonization Interval:

  • Time between death and insect colonization.
  • Varies (minutes to weeks).

• Postcolonization Interval:

  • Period from when insects colonize to when eggs are laid.
  • Used to backtrack to time of colonization.

Calculating Time of Colonization

1. Identify Insect Species and Age:

   • Focus on immatures (eggs, larvae, pupae).
   • Development data needed (species-specific).

2. Understand Base Temperature:

   • Temperature where insect development stops.
   • Species and population-specific.

3. Calculate Accumulated Degree Days:

   • Accumulate time in relation to temperature.
   • Formula: (Temperature - Base temperature) * Days

4. Environmental Data Conversion:

   • Obtain temperature data (often from NOAA).
   • Convert Fahrenheit to Celsius if necessary.
   • Calculate average daily temperature.

5. Reverse Time Calculation:

   • Start from date of discovery and move backward.
   • Calculate degree days for each day backward.

6. Match Development Data with Environmental Data:

   • Look for matching accumulated degree days.
   • Determine time of colonization range.*

Key Takeaways

• Keep calculations step-by-step; avoid overwhelming complexity.
• Separate tasks into manageable parts.
• Use tools like Excel for efficiency but understand each step manually first.
• Practice with assignments to reinforce learning.
• Reach out with questions for clarification.