Acute Kidney Injury

Acute kidney injury (AKI) is defined as an acute drop in kidney function. It is diagnosed by measuring the serum creatinine.

 

NICE Criteria For AKI

  • Rise in creatinine of ≥ 25 micromol/L in 48 hours
  • Rise in creatinine of ≥ 50% in 7 days
  • Urine output of < 0.5ml/kg/hour for > 6 hours

 

Risk Factors for Acute Kidney Injury

Consider the possibility of an acute kidney injury in patients that are suffering with an acute illness such as infection or having a surgical operation. Risk factors that would predispose them to developing acute kidney injury include:

  • Chronic kidney disease
  • Heart failure
  • Diabetes
  • Liver disease
  • Older age (above 65 years)
  • Cognitive impairment
  • Nephrotoxic medications such as NSAIDS and ACE inhibitors
  • Use of a contrast medium such as during CT scans

 

Causes

TOM TIP: Whenever someone asks you the cause of renal impairment always answer “the causes are pre-renal, renal or post-renal”. This will impress them and allow you to think through the causes more logically.

Pre-renal Causes

Pre-renal pathology is the most common cause of acute kidney injury. It is due to inadequate blood supply to kidneys reducing the filtration of blood. Inadequate blood supply may be due to:

  • Dehydration
  • Hypotension (shock)
  • Heart failure

Renal Causes

This is where intrinsic disease in the kidney is leading to reduced filtration of blood. It may be due to:

  • Glomerulonephritis
  • Interstitial nephritis
  • Acute tubular necrosis

Post-renal Causes

Post renal acute kidney injury is caused by obstruction to outflow of urine from the kidney, causing back-pressure into the kidney and reduced kidney function. This is called an obstructive uropathy. Obstruction may be caused by:

  • Kidney stones
  • Masses such as cancer in the abdomen or pelvis
  • Ureter or uretral strictures
  • Enlarged prostate or prostate cancer

 

Investigations

Urinalysis for protein, blood, leucocytes, nitrites and glucose.

  • Leucocytes and nitrites suggest infection
  • Protein and blood suggest acute nephritis (but can be positive in infection)
  • Glucose suggests diabetes

Ultrasound of the urinary tract is used to look for obstruction. It is not necessary if an alternative cause is found for the AKI.

 

Management

Prevention of acute kidney injury is important. This is achieved by avoiding nephrotoxic medications where possible and ensuring adequate fluid input in unwell patients, including IV fluids if they are not taking enough orally.

The first step to treating an acute kidney injury is to correct the underlying cause:

  • Fluid rehydration with IV fluids in pre-renal AKI
  • Stop nephrotoxic medications such as NSAIDS and antihypertensives that reduce the filtration pressure (i.e. ACE inhibitors)
  • Relieve obstruction in a post-renal AKI, for example insert a catheter for a patient in retention from an enlarged prostate

In a severe acute kidney injury or where there is doubt about the cause or complications, input from a renal specialist is required. They may need dialysis.

 

Complications

  • Hyperkalaemia
  • Fluid overload, heart failure and pulmonary oedema
  • Metabolic acidosis
  • Uraemia (high urea) can lead to encephalopathy or pericarditis

 

Last updated April 2019
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