Chapter 9. Acute Kidney Injury

• Acute increase in blood urea nitrogen (BUN) and serum creatinine.
• May be associated with oliguria or normal urine output.
• Symptoms and signs depend on cause.

Acute kidney injury (AKI) is a life-threatening disease process occurring in approximately 5% of all hospitalized patients and accounting for up to 30% of the admissions to intensive care units. AKI is preferred to acute renal failure as both kidney and injury are more patient-appropriate terms. Patients with AKI, regardless of their associated comorbid conditions, have a greater than 5-fold increased mortality rate. AKI is characterized by a reduction in the glomerular filtration rate (GFR) resulting in retention of nitrogenous wastes (creatinine, BUN, and other molecules that are not routinely measured). Early in the course of AKI patients are often asymptomatic and the condition is diagnosed only by observed elevations of BUN and serum creatinine levels or oliguria. An initial rise in serum creatinine of 0.5 mg/dL or a 25% increase in serum creatinine is often used to define AKI, although there is no definitive definition.

Oliguria (urine output less than 400 mL per 24 hours or 15 mL per hour) occurs commonly in AKI and may be an important indicator of renal dysfunction. However, urine output cannot be the only measure of kidney function. Patients with nonoliguric AKI usually have a better prognosis primarily due to less severe injury and/or a higher incidence of nephrotoxic-induced AKI in the nonoliguric group. Unfortunately, there has been little improvement in survival from AKI since the advent of hemodialysis and the mortality remains greater than 50% in many studies.

The RIFLE criteria consists of various graded levels of kidney injury based upon percent rise in serum creatinine, urine output and outcome measures.

Risk: 1.5-fold increase in the serum creatinine or GFR decrease by 25 percent or urine output <0.5 mL/kg per hour for six hours

Injury: Twofold increase in the serum creatinine or GFR decrease by 50 percent or urine output <0.5 mL/kg per hour for 12 hours

Failure: Threefold increase in the serum creatinine or GFR decrease by 75 percent or urine output of <0.5 mL/kg per hour for 24 hours, or anuria for 12 hours

Loss: Complete loss of kidney function (eg, need for renal replacement therapy) for more than four weeks

ESRD: Complete loss of kidney function (eg, need for renal replacement therapy) for more than three months

The AKIN (Acute Kidney Injury Network) criteria are a modification of the RIFLE criteria and include both diagnostic and staging system.

Stage 1. Increase in serum creatinine ≥0.3 mg/dl or 1.5 to 2 fold increase from baseline or urine output less than 0.5 mL/kg per hour for more than 6 hours