The cardiorenal syndrome (CRS) indicates how close the relationship is between 
heart and kidney during failure of these organs. At present, the classification 
of the syndrome includes five types of CRS: types I and II which are strictly 
related to initial heart failure (both acute and chronic), types III and IV 
which include initial kidney failure, and type V which includes several systemic 
diseases. Many pathophysiological pathways have been described illustrating how 
heart and kidney disease are involved in clinical conditions. The diagnosis of 
CRS is based on both blood tests and ultrasound imaging. Several biomarkers 
indicating levels of heart and kidney function have emerged over the last few 
decades which can be used to predict kidney failure in patients with acute or 
chronic heart disease. Kidney injury biomarkers have also to be tested, 
especially those indicating glomerular and tubular damage. Renal ultrasound and 
trans-thoracic echocardiography can provide further information on heart and 
kidney failure in patients with cardio-renal syndrome at any stage.