Describe the estimation of the time since death with the methods available and their limitations.

General Principles

• The time since death (postmortem interval) can never be stated precisely, and the honest answer is always a range with the reasoning stated. Every method rests on a rate that varies with temperature, body build, clothing and the cause of death
• No single method is reliable; the estimate is built by combining several, and where they conflict the discrepancy itself is informative — often indicating that the body was moved, refrigerated or exposed to an unusual environment
• The accuracy falls sharply with time: within the first day an estimate to within a few hours may be possible; within a week, to within a day or two; and beyond that, only to within weeks or months
• Record the ambient conditions — temperature, humidity, air movement, whether the body was covered, indoors or outdoors, in water or buried — without which none of the observations can be interpreted

The Individual Methods

Special Circumstances

• Bodies recovered from water — cooling is roughly twice as fast; putrefaction is delayed while submerged but proceeds rapidly once the body is recovered and exposed to air; washerwoman'S hands (bleached, wrinkled, sodden skin of the palms and soles) appear within a few hours and progress to skin slippage and degloving over days; and adipocere may form
• Buried bodies — decomposition is about eight times slower than in air; depth, soil type, moisture, coffin and clothing alter it substantially
• Burnt bodies — heat stiffening supervenes and rigor mortis will never occur; the ordinary methods are inapplicable
• Refrigerated or air-conditioned bodies — all changes are greatly retarded, and this is a very common source of error in Indian hospital mortuaries where the interval before autopsy may be long
• Infants and children — cool far faster because of the high surface area to mass ratio; rigor is feeble or absent in the premature

Building the Estimate in Practice

• At the scene — record the ambient temperature, humidity, air movement, whether the body is covered, clothed, indoors, outdoors, in water or buried, and the surface on which it lies.
• None of this can be reconstructed later → Record the rectal temperature with the time, and the time of every subsequent observation → Assess lividity — its extent, and whether it shifts on turning the body (fully, partially or not at all) → Assess rigor in each group — eyelids, jaw, neck, upper limbs, lower limbs — and whether it is developing or resolving → Note decomposition — green discolouration, marbling, bloating, blisters, skin slippage → Collect entomological specimens if any insect activity is present, and the soil beneath → At autopsy, take vitreous humour for potassium, and record the stomach contents with their state of digestion → Obtain the circumstantial evidence — last meal, last contact, newspapers, milk, mobile phone and camera records → Combine the methods
• give a range
• report any disagreement between them rather than averaging it away

Applied Aspects

• Give a range, never a point, and state the assumptions on which it rests; a doctor who says "death occurred at about 9 pm" will be asked how they excluded every variable, and will not be able to answer
• Record the ambient temperature and conditions at the scene, at the time of the examination; nothing can be reconstructed afterwards and its omission renders the temperature reading useless
• Use several methods and report the disagreements; a discrepancy usually means the body was moved or refrigerated, and that is often the most useful thing the doctor can say
• Ask about circumstantial evidence — the last meal, the last contact, newspapers and milk — which frequently narrows the interval far better than any medical observation and costs nothing
• Do not use stomach contents to time death without knowing when the last meal was eaten and its size; and remember that shock, head injury and opioids arrest gastric emptying entirely
• Note whether the body was refrigerated and for how long before the autopsy; in Indian practice this is often the single largest confounder and is regularly left out of the report
• Take vitreous humour routinely; potassium gives an estimate up to about 100 hours and the fluid resists putrefaction when blood has become useless
• Entomology takes over after the third day and remains informative for months, which is exactly where every classical method has already failed
• A partly developing and a partly resolving rigor look identical at one examination; the other findings, not the rigor alone, decide which it is
• Accuracy falls sharply with time — hours in the first day, a day or two within a week, and only weeks or months thereafter; say so rather than implying uniform precision
• Bodies in water cool about twice as fast, so temperature-based estimates must be corrected or abandoned
• Stomach contents time the last meal, not death, and gastric emptying stops altogether in shock, head injury, fear and opioid use
• Circumstantial evidence is often far more precise than any medical method — a stopped watch, uncollected milk or a last message costs nothing to ask about
• Record the assumptions with the range; an estimate stated without its conditions cannot be defended when one of those conditions turns out to have been different
• Rigor and lividity together are more informative than either, because a contradiction between them points to movement or refrigeration rather than to a particular interval
• Do not average away a discrepancy; report both observations and the inconsistency, which may matter more to the investigation than the interval itself
• Insect specimens must be halved — preserved and reared — because species identification usually requires the adult and the species sets the developmental rate
• Entomology dates colonisation, so wrapping, burial, a closed room or refrigeration must be considered before the figure is read as the time since death
• The plateau makes temperature useless for the first two hours, and a formula applied then will return a figure close to zero however long the person has been dead