A Molecular Approach to Estimating Postmortem Interval

A technical report recently made available by NIJ through the National Criminal Justice Reference Service, "Estimating Postmortem Interval: A Molecular Approach", written by Clifton P. Bishop, presents evidence that postmortem interval (PMI) may be estimated using a molecular approach that relies upon RNA segments.

From the abstract:

Accurate estimates of postmortem interval (PMI) can be crucial to the successful investigation of a suspicious death. Knowing the PMI can help investigators eliminate suspects if they have a valid alibi for when the death occurred, allowing investigators to focus on other leads. Early PMIs can be determined by various physical and biochemical changes occurring shortly after death, such as algor mortis. However, these changes are only reliable for estimating relatively short PMIs.

Forensic entomologists can obtain longer PMIs by knowing the lifecycle of insects that colonize the corpse and the sequence of colonizing insects (which arrive first, which are secondary, etc.). Forensic entomology is subject to many environmental factors (temperature being one of the principle variables), and it requires both access to the bodies and extensive information on the local insect population. The author set out to use a molecular approach similar to one previously used to estimate the age of a bloodstain in an approach that is analogous to Carbon-14 dating, relying upon RNA segments.

Previous studies indicated that humidity plays an insignificant role in the rate of RNA decay in tooth pulp, but results presented in this study indicate that temperature plays a significant role. The author determined that a better estimate of PMI can be obtained when data are plotted according to Accumulated Degree Days (ADD) rather than calendar days. ADD is a measure of how much accumulated temperature a system has been exposed to and is often used in forensic entomology studies.

The author was able show that an equation can be derived for estimating PMI within a 95 percent confidence interval, though the equations might need to be seasonal-specific (specific over a given temperature range). The results strongly support the feasibility of developing predictive PMI equations for humans given sufficient samples. When fully developed, this means of estimating PMI could be used for samples collected anywhere in the world without specialized knowledge of insect fauna. This approach is more cost-effective than current options and could allow for estimations of extended PMIs over what is now possible with forensic entomology.

Click here to read the publication (PDF, 27 pages).

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