They often sample vitreous fluid (the clear gel inside the eye) during an autopsy because it is relatively protected from early postmortem breakdown, bacterial contamination, and some of the changes that affect blood after death. That makes it useful for certain lab measurements.
The main reasons are:
Electrolytes and chemistry — things like potassium, sodium, chloride, glucose, urea, and creatinine can help evaluate dehydration, severe electrolyte disturbance, diabetic ketoacidosis, kidney failure, or marked hyperglycemia before death. Time-since-death estimation — vitreous potassium tends to rise after death in a somewhat predictable way, so it can contribute to estimating the postmortem interval. It’s only one clue, not an exact clock. Toxicology — sometimes for alcohol, drugs, or when blood results may be harder to interpret because of postmortem changes.
A simple way to think of it: after death, blood chemistry can become unreliable fairly quickly, but vitreous fluid often preserves a cleaner biochemical snapshot.
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