Acute Kidney Injury (AKI)

What happens in acute kidney injury

The kidneys filter about 180 liters of blood plasma a day, removing waste, balancing salt, potassium and acid, and controlling fluid. In AKI, filtration falls suddenly. Creatinine, a waste product from muscle, rises in the blood, and urine output often falls.

Doctors use standard KDIGO criteria. AKI is present if any of these occur:

• creatinine rises by 0.3 mg/dL (26.5 µmol/L) or more within 48 hours
• creatinine rises to 1.5 times baseline or more within 7 days
• urine output falls below 0.5 ml per kg of body weight per hour for 6 hours

Stages of acute kidney injury

Higher stages carry greater risks and slower recovery.

Causes of acute kidney injury

Doctors group causes by where the problem is.

Pre-renal: not enough blood reaching the kidneys

The most common type. The kidneys themselves are healthy but under-perfused.

• Dehydration from vomiting, diarrhea, poor intake or heat
• Bleeding, burns, or severe fluid loss
• Low blood pressure from sepsis, heart attack or heart failure
• Liver cirrhosis
• Medicines that reduce kidney blood flow, especially combined: NSAIDs (such as ibuprofen), ACE inhibitors or ARBs, and diuretics

Intrinsic: damage within the kidney

• Acute tubular necrosis: injury to kidney tubules from prolonged low blood flow, sepsis or toxins. The most common intrinsic cause in hospital.
• Medicines and toxins: some antibiotics (aminoglycosides, vancomycin), contrast dye in some high-risk people, chemotherapy, lithium, and some herbal remedies.
• Muscle breakdown: rhabdomyolysis after crush injury, extreme exercise, long immobility, seizures or some drugs.
• Glomerulonephritis: immune inflammation of the filters, for example after strep infection, lupus or vasculitis; may cause blood and protein in the urine and high blood pressure.
• Interstitial nephritis: an allergic-type reaction, often to medicines such as proton pump inhibitors, penicillins or NSAIDs.

Post-renal: blocked urine flow

• Enlarged prostate
• Kidney stones blocking a single working kidney or both ureters
• Bladder, prostate or pelvic cancers
• Blocked urinary catheter

See urinary tract obstruction.

Risk factors

• Age over 65
• Chronic kidney disease, diabetes, heart failure or liver disease
• Severe illness, sepsis, major surgery (especially heart surgery)
• Taking medicines that affect the kidneys
• Previous AKI

Symptoms

AKI is often found on a blood test before symptoms appear. As it worsens:

• passing less urine, or none
• swelling of the legs, ankles and around the eyes
• tiredness, weakness and loss of appetite
• nausea and vomiting
• confusion or drowsiness
• shortness of breath from fluid in the lungs
• palpitations from high potassium
• itching and, in severe cases, seizures

Some forms of AKI produce normal or even increased urine, so a normal urine volume does not rule it out.

Diagnosis

1. Blood tests: creatinine compared with a previous result, urea, potassium, sodium, bicarbonate, and blood count.
2. Urine output monitoring, sometimes with a catheter in hospital.
3. Urine dipstick and microscopy: blood and protein suggest glomerulonephritis; muscle pigment suggests rhabdomyolysis.
4. Kidney ultrasound: looks for blockage (hydronephrosis) and kidney size.
5. Medicine review.
6. Further tests: autoimmune blood tests or a kidney biopsy if inflammation is suspected.

Treatment

There is no single drug that reverses AKI. Treatment focuses on the cause and on protecting the body while the kidneys recover.

• Fluids: IV fluids for dehydration or low blood volume, given carefully to avoid overload.
• Treating infection and low blood pressure, including medicines to support blood pressure in sepsis.
• Stopping or pausing harmful medicines: NSAIDs, ACE inhibitors and ARBs, diuretics, metformin and some antibiotics; adjusting doses of others.
• Relieving blockage: a urinary catheter, or tubes (stents or nephrostomies) to drain blocked kidneys.
• Managing potassium, acid and fluid: diet changes, medicines, and diuretics for fluid overload.
• Specific treatments such as steroids or immunosuppressants for some types of glomerulonephritis.
• Dialysis: temporary dialysis is needed for dangerous potassium, severe acid build-up, fluid in the lungs not responding to treatment, or very high waste levels. Most people who need dialysis for AKI do not need it permanently.

Complications

• Hyperkalemia (high potassium) causing dangerous heart rhythms
• Fluid overload and fluid on the lungs
• Metabolic acidosis
• Uremia: confusion, nausea, inflammation around the heart
• Increased risk of death in seriously ill patients
• Later chronic kidney disease and high blood pressure

Recovery and outlook

Recovery usually starts within days to weeks once the cause is treated. Pre-renal AKI often recovers fully within a few days. Acute tubular necrosis can take 1 to 3 weeks or longer. Not everyone recovers completely: AKI increases the long-term risk of chronic kidney disease, and repeated episodes add up. After AKI, a kidney function blood test is recommended within about 3 months, with blood pressure checks.

Prevention

• Stay hydrated, especially during illness, heat or exercise.
• Sick day rules: if you have vomiting, diarrhea or fever and cannot keep fluids down, ask your doctor or pharmacist whether to pause medicines such as ACE inhibitors, ARBs, diuretics, metformin, SGLT2 inhibitors and NSAIDs until you recover.
• Avoid regular NSAID use if you have kidney disease, heart failure or take blood pressure medicines.
• Tell staff about kidney problems before scans with contrast or surgery.
• Treat prostate problems and urinary retention promptly.

Frequently asked questions

What is the most common cause of acute kidney injury?

The most common cause is reduced blood flow to the kidneys from dehydration, low blood pressure or severe infection (sepsis). In hospital, sepsis, major surgery and medicines that affect the kidneys are the leading triggers. Urine blockage, often from an enlarged prostate, is another common cause.

Is acute kidney injury reversible?

Often, yes. When the cause is found and treated quickly, kidney function frequently returns to normal or near normal within days to weeks. Severe or repeated AKI may leave lasting damage and increase the risk of chronic kidney disease, so follow-up blood tests are important.

What are the first signs of kidney injury?

Early AKI often has no symptoms and is found on blood tests. The first noticeable signs are usually passing less urine, swelling of the legs or around the eyes, tiredness, nausea and confusion. Anyone very unwell with vomiting or infection should watch their urine output.

What is the difference between acute kidney injury and chronic kidney disease?

Acute kidney injury develops suddenly, over hours to days, and is often reversible. Chronic kidney disease is a gradual loss of kidney function over months to years that is usually permanent. AKI can happen on top of chronic kidney disease and can speed its progression.

Will I need dialysis for acute kidney injury?

Most people with AKI do not need dialysis. It is used when the kidneys cannot control potassium, acid or fluid, or waste levels become dangerous. Dialysis for AKI is usually temporary, until the kidneys recover.

Is acute kidney injury the same as a kidney injury from trauma?

No. AKI is a loss of kidney function from any cause, usually medical. A blow or wound that damages the kidney itself is called a traumatic kidney injury, although it can sometimes lead to AKI.