3 Acute Kidney Injury Practice Questions

Recognizing acute kidney injury

Acute kidney injury (AKI) is an abrupt decline in kidney function identified by changes in serum creatinine, urine output, or both. Under the KDIGO definition, AKI is present if any one of these criteria is met:

• Serum creatinine rises by at least $0.3\ \text{mg/dL}$ within $48$ hours.

• Serum creatinine rises to at least $1.5$ times baseline within $7$ days.

• Urine output is below $0.5\ \text{mL/kg/hour}$ for at least $6$ hours.

A qualifying creatinine rise meets the definition even when urine output is unchanged. For example, a rise from $1.0\ \text{mg/dL}$ to $1.3\ \text{mg/dL}$ within $48$ hours meets the criterion.

Classifying causes

Causes are commonly grouped by where the problem occurs. More than one mechanism may contribute in the same patient.

• Prerenal: Inadequate kidney perfusion. Prolonged vomiting accompanied by low blood pressure and poor oral intake is an example.

• Intrinsic renal: Injury within the kidneys. Acute glomerulonephritis is an example.

• Postrenal: Obstruction to urine flow. An enlarged prostate causing urinary retention is an example.

Applying AKI stages

Stage AKI using both serum creatinine and urine-output criteria. If the two criteria indicate different stages, use the more severe stage.

Creatinine example

A rise from a baseline of $0.8\ \text{mg/dL}$ to $1.7\ \text{mg/dL}$ is $2.125$ times baseline:

This meets Stage 2 by creatinine criteria, which includes a rise to $2.0$–$2.9$ times baseline. In this example, urine output of $0.7\ \text{mL/kg/hour}$ does not change the creatinine-based classification.

Urine-output criteria

The criteria include:

• Stage 1: Output below $0.5\ \text{mL/kg/hour}$ for at least $6$ hours.

• Stage 2: Output below $0.5\ \text{mL/kg/hour}$ for at least $12$ hours.

• Stage 3: Output below $0.3\ \text{mL/kg/hour}$ for at least $24$ hours, or no urine output for at least $12$ hours.

For a patient weighing $70\ \text{kg}$ who produces $20\ \text{mL/hour}$ for $7$ hours, the rate is about $0.29\ \text{mL/kg/hour}$:

This is below $0.5\ \text{mL/kg/hour}$ for at least $6$ hours, meeting Stage 1 by urine-output criteria.

Assessing the patient

Assessment should help identify the cause, track kidney function, and detect complications. Useful elements include:

• Trends in serum creatinine and urine output.

• Intake and output, weight, and volume status.

• Medication and recent-exposure history.

• Urinalysis when AKI is suspected or detected.

• Evaluation for urinary obstruction when indicated, including when obstruction is suspected.

• Ongoing monitoring of electrolytes and other complications; monitoring should not stop once AKI is diagnosed.

Oliguria together with rising creatinine warrants assessment of the patient’s volume status and potential causes.

Managing AKI and urgent complications

Management addresses the underlying cause and any complications. For suspected volume depletion, such as after several days of diarrhea with low blood pressure and clinical signs of volume depletion, promptly assess and restore circulating volume with appropriate fluid therapy. Monitor the patient’s response, watch for fluid overload, and address the cause of fluid loss.

Fluid treatment must be guided by the patient’s condition. A patient who is already overloaded or has heart failure may need a different approach; do not assume that every patient with AKI needs more fluid. Review medicines and adjust or withhold potentially harmful drugs when clinically appropriate.

Complications needing urgent attention

Urgent clinical assessment and management are warranted for severe hyperkalemia with ECG changes, pulmonary edema with worsening oxygenation, or suspected urinary obstruction with infection. A mildly elevated creatinine in a patient with stable vital signs and no symptoms is not identified as an urgent finding in this comparison. Kidney replacement therapy may be needed when serious complications cannot be managed adequately otherwise.

Reducing AKI risk

Prevention and early recognition are especially important for people at increased risk, including those with chronic kidney disease, heart failure, diabetes, sepsis, hypovolemia, or exposure to potentially kidney-harming medicines.

Practical measures include:

• Monitor serum creatinine and urine output.

• Identify and treat dehydration, infection, or other causes of poor perfusion.

• Review medicines and avoid or minimize nephrotoxic exposures when possible.

• Adjust drug doses when kidney function is impaired.

• Evaluate for obstruction when indicated.

Medication decisions, including temporary changes to prescribed drugs, should be individualized and clinician-directed.