## Investigation of Choice for Postmortem Interval Estimation ### Why Vitreous Humor Potassium (K⁺) is Best **Key Point:** Vitreous humor K⁺ concentration increases linearly after death at a rate of approximately 1.4 mEq/L per hour, making it the most reliable biochemical marker for estimating postmortem interval (PMI) in the 12–72-hour window. **High-Yield:** The vitreous humor is protected from bacterial contamination and environmental changes because it is enclosed within the globe, making K⁺ rise predictable and less affected by external factors compared to blood or other body fluids. ### Comparison of Postmortem Biochemical Markers | Investigation | Time Window | Reliability | Limitations | |---|---|---|---| | Vitreous K⁺ | 12–72 hours | Excellent (linear rise ~1.4 mEq/L/hr) | Affected by antemortem renal disease, dehydration | | Gastric content analysis | 0–4 hours | Moderate | Highly variable digestion rates; depends on meal type | | Blood alcohol | Variable | Poor for PMI | Reflects antemortem consumption, not death timing | | Liver glycogen | 0–12 hours | Moderate | Rapid depletion; unreliable in prolonged illness | **Clinical Pearl:** In the given case, early rigor mortis and core temperature of 32°C suggest death occurred 8–12 hours prior. Vitreous K⁺ would confirm this estimate with greater precision than other methods. ### Mechanism of K⁺ Rise **Key Point:** After death, cellular autolysis and bacterial decomposition cause breakdown of cell membranes, releasing intracellular K⁺ into the extracellular space. The vitreous humor, being avascular and isolated, accumulates K⁺ at a predictable rate. **Formula for PMI estimation:** $$\text{PMI (hours)} = \frac{[K^+] - 2.5}{1.4}$$ (where [K⁺] is vitreous potassium in mEq/L; baseline ≈ 2.5 mEq/L) **Warning:** This formula is an approximation; individual variation can be ±5–10 hours. It is most reliable between 12–72 hours postmortem.
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.