Application of Mixed-effect Model in PMI Estimation by Vitreous Humor

Abstract

To test the changes of the potassium （K⁺） and magnesium （Mg²⁺） concentrations in vitreous humor of rabbits along with postmortem interval （PMI） under different temperatures, and explore the feasibility of PMI estimation using mixed-effect model. After sacrifice, rabbit carcasses were preserved at 5 ℃, 15 ℃, 25 ℃ and 35 ℃, and 80-100 μL of vitreous humor was collected by the double-eye alternating micro-sampling method at every 12 h. The concentrations of K⁺ and Mg²⁺ in vitreous humor were measured by a biochemical-immune analyser. The mixed-effect model was used to perform analysis and fitting, and established the equations for PMI estimation. The data detected from the samples that were stoned at 10 ℃, 20 ℃ and 30 ℃ with 20, 40 and 65 h were used to validate the equations of PMI estimation. The concentrations of K⁺ and Mg²⁺ [f（x,y）] in vitreous humor of rabbits under different temperature increased along with PMI （x）. The relative equations of K⁺ and Mg²⁺ concentration with PMI and temperature under 5 ℃~35 ℃ were fK⁺（x,y）=3.413 0+0.309 2 x+0.337 6 y+0.010 83 xy-0.002 47 x² （P<0.000 1）, and fMg²⁺（x,y）=0.745 6+0.006 432 x+0.033 8 y （P<0.000 1）, respectively. It was proved that the time of deviation for PMI estimation by K⁺ and Mg²⁺ was in 10 h when PMI was between 0 to 40 h, and the time of deviation was in 21 h when PMI was between 40 to 65 h. the ambient temperature range of 5 ℃-35 ℃, the mixed-effect model based on temperature and vitreous humor substance concentrations can provide a new method for the practical application of vitreous humor chemicals for PMI estimation.