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A 72-year-old-man, recently treated for resistant Escherichia coli urosepsis, is sent to the emergency department from his nursing home because of new left-sided hemiparesis. He has a past history of chronic kidney disease (CKD), hypertension, and osteoarthritis. Medications include lisinopril, 20 mg/d and naproxen, 400 mg twice a day. On physical examination, he has a low-grade fever (100.4°F), his initial blood pressure is 130/66 mm Hg, and his heart rate is 88 beats per minute (bpm). His respiratory rate is 28 breaths per minute, and his hemoglobin oxygen saturation by pulse oximetry is 89% on supplemental oxygen by nasal cannula. He has a flaccid left hemiparesis and is stuporous.

He undergoes computed tomography (CT) of the head, which shows no hemorrhage. He has a CT angiogram of the chest, which shows no evidence of pulmonary embolism. His temperature rises quickly to 102°F, the blood pressure falls to 78/50 mm Hg, and the heart rate rises to 120 bpm. His initial white blood cell count is 29.9 with 44% bands, and a urine analysis shows pyuria and bacteriuria. Treatment is initiated with vancomycin, gentamicin, and cefepime. Intravenous normal saline is given by rapid infusion, according to the sepsis protocol. The patient is intubated and placed on mechanical ventilation. The plasma creatinine, 1.6 mg/dL on arrival, increases to 2.0 mg/dL by the following day, and the blood urea nitrogen (BUN) rises from 22 mg/dL to 36 mg/dL. Urine output has been about 20 mL/h for the past 8 hours.

The patient’s daughter, a registered nurse, is in the room when you make rounds. She is aware of her father’s history of CKD and wants to know if he is in “kidney failure.” What do you tell her?

Although acute renal failure (ARF) is conceptually simple to characterize as “an abrupt decrease in renal function,” a commonly accepted definition of ARF emerged only very recently. Absence of a uniform definition of ARF substantially hampered research and limited comparisons between different populations. Therefore, in 2002, an international group of experts developed a consensus definition of ARF—using changes in plasma creatinine concentration and hourly urine output—and gave it the acronym RIFLE (Risk, Injury, Failure, Loss, and End-stage).1 Many investigators have since validated that classification scheme in various clinical settings, and a recent meta-analysis showed a graded impact on mortality by RIFLE stage.2

The RIFLE criteria represented real progress in the field of ARF research. Nonetheless, shortcomings of RIFLE were soon recognized: there was frequent discordance between the stage assignment by the plasma creatinine and the urine output criteria.3 In addition, smaller changes in plasma creatinine (≥ 0.3 mg/dL)4 over a short time (48 hours) were shown to be associated with important clinical outcomes.5

With the goal of refining the ARF definition, a second international group met in 2004. Their consensus produced the Acute Kidney Injury Network (AKIN) criteria.6 They recommended that ...

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