Time of death dependent criteria in vitreous humor: accuracy of estimating the time since death.

Since more than 40 years reports on the rise of potassium concentration [K(+)] in vitreous humor have been published with different statements concerning the accuracy of death time estimation. In the last years several statistical approaches for a more accurate estimation of the time since death have been reported. While in most investigations the PMI has been used as the independent and [K(+)] as the dependent variable in linear regression analysis between PMI and [K(+)], recently it has been suggested to use [K(+)] as the independent variable for regression analysis. Changing the variables would lead to a higher accuracy of death time estimation. This has also been recommended for regression analysis between hypoxanthine concentration [Hx] and time since death. This hypothesis has been checked on independent cases with potassium and hypoxanthine in vitreous humor. Linear regression with [K(+)] or [Hx] as independent variable has revealed a slightly more accurate death time estimation compared to a linear regression with PMI as independent variable. Thus, e.g. the accuracy could have been improved from +/-25.96 to +/- 23.27h by using [K(+)] as independent variable. Another statistical approach has re-evaluated six large studies on the rise of vitreous [K(+)] using a local regression analysis (Loess procedure). Based on this re-evaluation an accuracy of death time estimation has been recommended (95% limits of confidence of +/-1h in the early PMI and +/-10h, 110h postmortem) which has surpassed even optimistic results of earlier investigations. This recommended accuracy of death time estimation has been checked on a random sample of 492 cases. Only 153 cases have been within the predicted postmortem interval, 339 lay outside with a systematic overestimation of the time since death.