Hepatorenal syndrome (HRS) is a common cause of acute kidney injury (AKI) in cirrhotic patients and has a one month survival rate of 50% and a 3 month survival rate of 20%. The leading theory behind HRS is selective vasoconstriction of renal vasculature in the setting of decreased systemic vascular resistance. Patients with liver cirrhosis suffer from a large degree of third spacing in the form of peripheral edema and ascites. In addition treatment with multiple drugs, including diuretics puts these patients at higher risks of prerenal AKI and ischemic acute tubular necrosis (ATN). AKI occurring due to HRS, prerenal AKI and ischemic or nephrotoxic ATN have different pathophysiologic mechanisms and are treated differently with significantly different outcomes. While renal perfusion is expected to be reduced in HRS and prerenal AKI, it is normal or increased in ATN. Prerenal AKI has the most favorable prognosis among these pathologies and treatment simply consists of volume expansion with blood, albumin, crystalloids or colloids. In clinical practice vasoactive agents such as midodrine and octreotide are used to increase the tone of splanchnic vessels and to improve renal perfusion. These interventions would not affect renal function in cases with ATN. Unfortunately, the diagnostic criteria proposed by the International Club for Ascites (ICA) for HRS are not specific and do not always exclude patients with other forms of acute kidney injury. Therefore, availability of a simple diagnostic tool for measurement of renal blood flow (RBF) at the bedside would be of great value in management of cases with cirrhosis of the liver presenting with acute reduction in kidney function. However, currently, there are no practical and simple tools available for this purpose.