Oliguria in the setting of critically ill patients is usually treated by admini-stering fluids and furosemide [1]. Invasive therapies, namely renal replacement therapies (RRT), are reserved for patients in whom less invasive measures have failed [2], especially if acute pulmonary oedema complicates the clinical picture [1]. Intravascular volume depletion elicits a kidney response consisting of augmented sodium retention at Henle's loop and water at the collecting tubules. In such conditions, loop diuretics such as furosemide would be less effective to improve diuresis and water loss than osmotic diuretics such as mannitol [3, 4]. This case report aims to highlight the utility of the assessment of the glomerular and tubular functions to identify an ineffective diuretic strategy and to select a successful one, which prevented the use of invasive RRT. A 33-year-old female patient suffering from preeclampsia (gestational age was 35 + 6 weeks) was admitted to our Post-surgical Intensive Care Unit (PICU) after an urgent caesarean section performed under spinal anaesthesia, without further incidents.