AKI can be diagnosed if any one of the following is present:

Increase in SCr by ≥0.3 mg/dl (≥26.5 μmol/L) within 48 hours; or
Increase in SCr to ≥1.5 times baseline, which has occurred within the prior 7 days; or
Urine volume < 0.5 mL/kg/h for 6 hours. Staging
The RIFLE criteria, proposed by the Acute Dialysis Quality Initiative (ADQI) group, aid in assessment of the severity of a persons acute kidney injury. The acronym RIFLE is used to define the spectrum of progressive kidney injury seen in AKI:

Risk: 1.5-fold increase in the serum creatinine, or glomerular filtration rate (GFR) decrease by 25 percent, or urine output <0.5 mL/kg per hour for six hours. Injury: Two-fold increase in the serum creatinine, or GFR decrease by 50 percent, or urine output <0.5 mL/kg per hour for 12 hours. Failure: Three-fold increase in the serum creatinine, or GFR decrease by 75 percent, or urine output of <0.3 mL/kg per hour for 24 hours, or no urine output (anuria) for 12 hours. Loss: Complete loss of kidney function (e.g., need for renal replacement therapy) for more than four weeks. End-stage kidney disease: Complete loss of kidney function (e.g., need for renal replacement therapy) for more than three months. Evaluation
The deterioration of kidney function may be signaled by a measurable decrease in urine output. Often, it is diagnosed on the basis of blood tests for substances normally eliminated by the kidney: urea and creatinine. Additionally, the ratio of BUN to creatinine is used to evaluate kidney injury. Both tests have their disadvantages.
People who have experienced AKI are at increased risk of developing chronic kidney disease in the future. Management includes treatment of the underlying cause and supportive care, such as renal replacement therapy. Signs and symptoms
The clinical presentation is often dominated by the underlying cause. The various symptoms of acute kidney injury result from the various disturbances of kidney function that are associated with the disease. Accumulation of urea and other nitrogen-containing substances in the bloodstream lead to a number of symptoms, such as fatigue, loss of appetite, headache, nausea, and vomiting. Marked increases in the potassium level can lead to abnormal heart rhythms, which can be severe and life-threatening. Fluid balance is frequently affected, though blood pressure can be high, low, or normal.Pain in the flanks may be encountered in some conditions (such as clotting of the kidneys blood vessels or inflammation of the kidney). This is the result of stretching of the fibrous tissue capsule surrounding the kidney. If the kidney injury is the result of dehydration, there may be thirst as well as evidence of fluid depletion on physical examination. Physical examination may also provide other clues as to the underlying cause of the kidney problem, such as a rash in interstitial nephritis (or vasculitis) and a palpable bladder in obstructive nephropathy. Causes
Prerenal
Prerenal causes of AKI ("pre-renal azotemia") are those that decrease effective blood flow to the kidney and cause a decrease in the glomerular filtration rate (GFR).