The prognosis of acute kidney injury (AKI) depends on early diagnosis and therapy. A multitude of causes are classified according to their origin as prerenal, intrinsic (intrarenal), and post-renal.Prerenal AKI means a loss of renal function despite intact nephrons, for example, because of volume depletion and/or hypotension.There is a broad spectrum of intrinsic causes of AKI including acute tubular necrosis (ATN), interstitial nephritis, glomerulonephritis, and vasculitis. Evaluation includes careful review of the patient’s history, physical examination, urinalysis, selected urine chemistries, imaging of the urinary tree, and eventual kidney biopsy. The history should focus on the tempo of loss of function (if known), associated systemic diseases, and symptoms related to the urinary tract (especially those that suggest obstruction). In addition, a review of the medications looking for potentially nephrotoxic drugs is essential. The physical examination is directed towards the identification of findings of a systemic disease and a detailed assessment of the patient’s haemodynamic status. This latter goal may require invasive monitoring, especially in the oliguric patient with conflicting clinical findings, where the physical examination has limited accuracy.Excluding urinary tract obstruction is necessary in all cases and may be established easily by renal ultrasound.Distinction between the two most common causes of AKI (prerenal AKI and ATN) is sometimes difficult, especially because the clinical examination is often misleading in the setting of mild volume depletion or overload. Urinary chemistries, like calculation of the fractional excretion of sodium (FENa), may be used to help in this distinction. In contrast to FENa, the fractional excretion of urea has the advantage of being rather independent of diuretic therapy. Response to fluid repletion is still regarded as the gold standard in the differentiation between prerenal and intrinsic AKI. Return of renal function to baseline or resuming of diuresis within 24 to 72 hours is considered to indicate ‘transient, mostly prerenal AKI’, whereas persistent renal failure usually indicates intrinsic disease. Transient AKI may, however, also occur in short-lived ATN. Furthermore, rapid fluid application is contraindicated in a substantial number of patients, such as those with congestive heart failure.‘Muddy brown’ casts and/or tubular epithelial cell casts in the urine sediment are typically seen in patients with ATN. Their presence is an important tool in the distinction between ATN and prerenal AKI, which is characterized by a normal sediment, or by occasional hyaline casts. There is a possible role for new serum and/or urinary biomarkers in the diagnosis and prognosis of the patient with AKI, including the differential diagnosis between pre-renal AKI and ATN. Further studies are needed before their routine determination can be recommended.When a diagnosis cannot be made with reasonable certainty through this evaluation, renal biopsy should be considered; when intrarenal causes such as crescentic glomerulonephritis or vasculitis are suspected, immediate biopsy to avoid delay in the initiation of therapy is mandatory.
