Prerenal and renal azotemia is differentiated on the basis of-

the different types of azotemia, which may arise from or be a pa of renal failure, have different causes: when fluid flowing through the kidneys isn&;t enough to remove the nitrogen (prerenal azotemia) when the urinary tract is obstructed by something or by a rupture (postrenal azotemia) which may be differentiated by level of creatinine Prerenal azotemia refers to elevations in BUN and creatinine levels resulting from problems in the systemic circulation that decrease flow to the kidneys. In prerenal azotemia, decreased renal flow stimulates salt and water retention to restore volume and pressure Azotemia is an elevation of blood urea nitrogen (BUN) and serum creatinine levels. The reference range for BUN is 8-20 mg/dL, and the normal range for serum creatinine is 0.7-1.4 mg/dL. Each human kidney contains approximately 1 million functional units, nephrons, which are primarily involved in urine formation. Ref Harrison20th edition pg 278