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 ± 23.27 h 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 ±1 h in the early PMI and ±10 h, 110 h 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.