Diagnosis of AKI

Acute kidney injury is diagnosed on the basis of clinical history and laboratory data. A diagnosis is made when there is rapid reduction in kidney function, as measured by serum creatinine, or based on a rapid reduction in urine output, termed oliguria.

Definition

Introduced by the Acute Kidney Injury Network (AKIN), specific criteria exist for the diagnosis of AKI:

1. Rapid time course (less than 48 hours) 
2. Reduction of kidney function 

• Absolute increase in serum creatinine of ≥0.3 mg/dl (≥26.4 μmol/l)  
• Percentage increase in serum creatinine of ≥50%  
• Reduction in urine output, defined as <0.5 ml/kg/hr for more than 6 hours 

Staging 

The RIFLE criteria, proposed by the Acute Dialysis Quality Initiative (ADQI) group, aid in the staging of patients with AKI: 

• Risk: serum creatinine increased 1.5 times or urine production of <0.5 ml/kg for 6 hours 

• Injury: doubling of creatinine or urine production <0.5 ml/kg for 12 hours 

• Failure: tripling of creatinine or creatinine >355 μmol/l (with a rise of >44) (>4 mg/dl) OR urine output below 0.3 ml/kg for 24 hours

• Loss: persistent AKI or complete loss of kidney function for more than 4 weeks

• End-stage renal disease: complete loss of kidney function for more than 3 months 

Further testing

Once the diagnosis of AKI is made, further testing is often required to determine the underlying cause. These may include renal ultrasound and kidney biopsy. Indications for renal biopsy in the setting of AKI include:

1. Unexplained AKI 
2. AKI in the presence of the nephritic syndrome 
3. Systemic disease associated with AKI