Protein is broken down into amino acids which are then used by the body. A byproduct of protein use is ammonia, which the liver converts into urea. Urea is then excreted in urine by the kidneys. Any illness or disease affecting the kidneys will cause an increase in BUN levels in the blood. Liver disease, bleeding within the stomach or intestines, or even a very high protein diet can also cause BUN levels to increase. There are numerous medications that can affect BUN levels.
Creatinine is produced during muscle activity, and levels do not change much day-to-day. Creatinine is also excreted by the kidneys. Decreased kidney function leads to increased levels of creatinine, but these levels are not affected by liver disease. Increased creatinine blood levels indicate a decrease in kidney function. The BUN-to-Creatinine Ratio is used to diagnose illness due to kidney versus liver function
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