Urea nitrogen levels in postmortem serum, vitreous humor, and cerebrospinal fluid

Urea nitrogen (UN) levels are considered relatively stable postmortem; however, many uncertainties remain. This study aimed to clarify differences in UN levels among postmortem serum (Se), vitreous humor (VH), and cerebrospinal fluid (CSF), and to evaluate the influence of postmortem interval (PMI) on these levels. UN concentrations were analyzed in Se, VH, and CSF from 123 medicolegal autopsy cases (86 males and 37 females; age range 21–92 years), which were classified into five PMI groups (< 1 day, 1–2 days, 3–4 days, 5–6 days, ≥ 7 days). In all three matrices, UN levels differed significantly between cases with PMIs shorter than 1 day and those with PMIs of 7 days or longer, whereas in cerebrospinal fluid, a significant difference was also observed between PMIs shorter than 1 day and 5–6 days. In cases with PMIs shorter than 1 day, UN levels differed significantly among the three matrices, being highest in serum and lower in vitreous humor and cerebrospinal fluid. In contrast, no significant differences among matrices were observed in cases with PMIs of 1 day or longer. These findings suggest that postmortem UN levels are influenced by PMI.