Glomerular filtration rate (GFR) is an accepted measure for assessment of kidney function. For the critically ill patient, creatinine clearance is the method of reference for the estimation of the GFR, although this is often not measured but estimated by equations (i.e., Cockroft-Gault or MDRD) not well suited for the critically ill patient. Functional evaluation of the kidney rests in serum creatinine (Crs) that is subjected to multiple external factors, especially relevant overhydration and loss of muscle mass. The laboratory method used introduces variations in Crs, an important fact considering that small increases in Crs have serious repercussion on the prognosis of patients. Efforts directed to stratify the risk of acute kidney injury (AKI) have crystallized in the RIFLE or AKIN systems, based in sequential changes in Crs or urine flow. These systems have provided a common definition of AKI and, due to their sensitivity, have meant a considerable advantage for the clinical practice but, on the other side, have introduced an uncertainty in clinical research because of potentially overestimating AKI incidence. Another significant drawback is the unavoidable period of time needed before a patient is classified, and this is perhaps the problem to be overcome in the near future. 1. Epidemiology of Acute Kidney Injury in the ICU Acute kidney injury (AKI) can be defined as a decrease of the glomerular filtration rate (GFR) that appears acutely, is maintained for some time, causes an accumulation of waste products from metabolism and uremic toxins, and conditions a mishandling of body fluids and a loss of the ability to maintain homeostasis of electrolytes and acid-base balance. In the intensive care setting, AKI presents with a high incidence and, once established, has an important impact in the patient and the resources [1, 2]. The reported incidence of AKI in the intensive care units (ICUs) shows a wide variability depending on the population analyzed and the criteria employed for its definition, but when this is based in the new systems for stratification, as RIFLE [3] or AKIN [4], more than 30% of ICU patients are found to present with some degree of AKI [5], and mortality rate increases according to this degree of renal dysfunction [6, 7]. These figures are a good measure of the magnitude of the problem, and even when functional recovery after AKI is good, it has been demonstrated that the development of severe AKI can lead to an increase in long-term mortality [8] with an estimation of the incidence of chronic kidney disease (CKD) after an episode
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