At all stages of renal insufficiency, the sCr is a much more reliable indicator of renal function than the BUN because the BUN is far more likely to be affected by dietary and physiologic conditions not related to renal function (Table 193.1). For example patients with congestive heart failure and intact kidneys commonly present with a BUN of 50 to 70 mg/dl and an sCr below 1.2 mg/dl. Of course, sCr may rise under some of these extrarenal factors, but seldom will it exceed 3 to 4 mg/dl. The stages of renal failure have been defined according to the sCr as follows: