Acute kidney injury (AKI) is the abrupt loss of kidney function, resulting in the retention of urea and other nitrogenous waste products and in the dysregulation of extracellular volume and electrolytes. The term AKI has largely replaced acute renal failure (ARF), reflecting the recognition that smaller decrements in kidney function that do not result in overt organ failure are of substantial clinical relevance and are associated with increased morbidity and mortality. The term ARF is now reserved for severe AKI, usually implying the need for renal replacement therapy.
The loss of kidney function that defines AKI is most easily detected by measurement of the serum creatinine, which is used to estimate the glomerular filtration rate (GFR). Three problems are associated with the use of serum creatinine to quantitatively define AKI:
●Serum creatinine does not accurately reflect the GFR in a patient in whom it is not in steady state. In the early stages of AKI, the serum creatinine may be low, even though the actual (not estimated) GFR is markedly reduced, since there may not have been sufficient time for the creatinine to accumulate (see "Assessment of kidney function"). When the serum creatinine is rising, estimates of GFR based on creatinine values will overestimate the true GFR; conversely, estimates of GFR will underestimate the true GFR during recovery of kidney function, when the serum creatinine concentration is declining.
●Creatinine is removed by dialysis. As a result, it is usually not possible to assess kidney function by measuring the serum creatinine once dialysis is initiated. One exception is when the serum creatinine continues to fall on days when hemodialysis is not performed, indicating recovery of renal function.
●Numerous epidemiologic studies and clinical trials have used different cut-off values for serum creatinine to quantitatively define AKI .