A post-mortem interval is the time that has elapsed between the time a person dies and the time the body is examined. Commonly used in homicide investigations, the knowledge of the post-mortem interval is useful to pinpoint the time the person died. Ways in which pathologists can calculate post-mortem interval include identification of insect species living in the body, rate at which the person's body has decomposed, and other indicators such as the presence of rigor mortis.
Through study and research over the years, pathologists have been able to develop reference charts for the rate of body decomposition after death. An example is rigor mortis, which refers to the temporary stiffening of the body that occurs for some time after death. If a deceased person is not in rigor mortis, he or she is either in the very early stages of decomposition, or is past the rigor mortis stage. Generally, determining a post-mortem interval involves taking into account several signs.
Rigor mortis, for example, can be assessed in conjunction with an internal temperature of the person's body, commonly of the liver. As cooling occurs after death, a residual temperature, along with rigor mortis, can give a medical examiner a relatively accurate idea of post-mortem interval, and therefore time of death. Another major indicator of the time since death is insect activity on the body, which is useful after rigor mortis has passed. The stages of lifecycle of the insects, from eggs to larvae to adults, can help pinpoint the time the insects first colonized the body.
These reference standards are not always precise, though, as there are many confounding variables in the field of pathology. The environment that the person's body is part of can vary widely in factors like temperature, humidity and presence of water. As well as the naturally occurring characteristics of the location of the body, manmade factors like the type of clothing the person is wearing can also affect the rate of decomposition, and interfere with the interpretation of the post-mortem interval.
Determining post-mortem interval is very useful for investigation of suspicious deaths. It can help investigators figure out the last time the person was seen, figure out what happened in the time immediately leading up to the death, and vindicate or implicate other people in the death. Generally, identification of the post-mortem interval is not useful information for post-mortem examination for people who have died in non-suspicious circumstances.