Similar to how our digestive tract produces waste, cells also form waste as a result of normal cellular function. Two examples of the most common waste products are creatinine and urea. Creatinine is a waste product formed as a result of normal muscle activity. Urea is the by-product of protein and amino acid breakdown. Our body uses amino acids and proteins to build new cells and tissues which are both essential to sustaining life. Other examples of waste, or toxins, are acids, simple sugars, alcohol, and even certain medications. Healthy individuals with normal kidney function do accumulate creatinine and urea in their body. As the kidney function decreases, there is an increase in these two waste products found in the blood. Creatinine is therefore a useful indicator of the overall kidney function. Regular blood tests are given to patients with kidney problems to monitor the levels of these two important waste products.