Creatinine is a by-product of muscular metabolism. In the natural and normal scheme of things, this substance or waste product can be eliminated from the body. A high-serum creatinine level may cause kidney damage. In relation to the above question as to how high can creatinine levels go before death, it must be noted that kidneys have strong compensatory ability and by that as long as its still 50 percent functional, creatinine level won't be that high. Which leads us to a conclusion that, the lesser the kidney function level is, the higher the creatinine level.
On the other hand for the blood urea nitrogen (BUN) is the most stable blood constituent following death as it reaches antemortem (before death) levels and even after moderate decomposition.
Therefore, as long as the kidneys are functional, regardless of other factors such as the patient's condition/ diagnosis, these blood constituents are nearly normal.
