Postmortem interval

The postmortem interval, or PMI, is the time between death and the examination of a body. Establishing it helps investigators test witness statements and alibis, work out when a death happened and judge whether the circumstances were natural or suspicious. Because death is rarely observed, the interval is almost always an estimate given as a range rather than an exact time.

In the first day or two, examiners rely on early body changes. Algor mortis is the cooling of the body toward the surrounding temperature; tools such as the Henssge nomogram correct the cooling rate for body weight, room temperature and factors like clothing. Livor mortis is the purplish discoloration where blood settles under gravity, which becomes fixed within about a day. Rigor mortis is the stiffening and later relaxation of the muscles. Chemistry helps too, such as the steady rise of potassium in the vitreous fluid of the eye.

After two or three days these signs lose precision, and estimates turn to the stage of decomposition and to forensic entomology, which reads the age of fly larvae and other insects that colonize a body in a predictable order. Newer approaches study RNA breakdown and shifts in the body's microbes. Combining methods gives the most reliable estimate.