Acute Kidney Injury: Causes, Warning Signs, and How It Differs From Chronic Kidney Disease

Acute kidney injury (AKI) happens when something suddenly stops your kidneys from filtering waste out of the blood as well as they should [1][5]. It usually develops over a few hours to a few days [5][8], and for some people it can be very serious [1]. KDIGO, an international kidney guideline group, describes AKI as potentially treatable and says early detection and treatment may improve outcomes [4]. You may still hear the older name for it, acute renal failure [5].

What Happens in Acute Kidney Injury

When the kidneys suddenly struggle, waste products can build up in the blood, and it becomes harder for the body to keep the right balance of fluid and minerals [5]. The National Kidney Foundation (NKF) says that for most people AKI develops within 48 hours, although sometimes it can take as long as 7 days [5]. NIDDK, part of the U.S. National Institutes of Health, describes AKI as a sudden drop in kidney function that usually lasts a short time, but notes that it can leave lasting effects even after the underlying problem has been treated [6].

Common Causes

The NHS explains that the problem may start in the kidneys themselves or somewhere else in the body [1]. The NKF adds that AKI usually happens because of several factors acting together, especially in older adults [5]. The main causes fall into three broad groups.

Less blood reaching the kidneys. Dehydration, severe diarrhea or vomiting, blood loss, very low blood pressure, sepsis and serious heart problems can all reduce blood flow to the kidneys [1][5].

Harm to the kidneys themselves. Some infections, inflammatory and autoimmune conditions, some medicines and the dye sometimes injected for some scans, such as a CT scan, can affect the kidneys [1][5][8]. The NKF explains that a medicine may not cause AKI by itself, but the risk goes up significantly when it combines with other factors, such as a serious illness or dehydration [5]. A kidney infection is one of the causes the NKF lists [5]; for more on urinary infections, see Urinary Tract Infection: Symptoms, Causes, and When to See a Doctor.

A blockage. Conditions that block the flow of urine, such as a kidney stone or an inflamed prostate, can also lead to AKI [1]. For more on stones, see Kidney Stones: Symptoms, Causes, and When to Seek Care.

Warning Signs

Many people with AKI have no symptoms, especially when it is mild [1]. The NHS says it is often found when someone is having tests or is in hospital for another reason [1], and the NKF notes that in some cases it is only picked up through other tests [5].

When symptoms do appear, the NHS and NKF lists overlap but are not identical. They include:

• passing much less urine than usual, or none at all [1][5]
• swelling in the legs, ankles or feet [1][5]
• feeling tired or confused [1][5], or very thirsty, dizzy or sleepy [1]
• nausea or vomiting [1][5], or a poor appetite [5]
• feeling breathless [1][5], especially when lying down [1]
• pain in the side of the back between the ribs and hips, or chest pain or pressure [5]

The NKF notes that seizures or coma can happen in severe cases [5].

Who Is More Likely to Get It

Anyone can develop AKI, but risk lists differ a little between health bodies. Taken together, the NHS, NICE (which writes clinical guidance in England), KDIGO, NIDDK and the CDC point to a higher risk with:

• age 65 or over [1][2]
• chronic kidney disease [2][4][7]
• diabetes, heart failure or other heart disease, and liver disease [2][4][8]
• high blood pressure [8]
• a previous episode of AKI [1][2]
• recent surgery, or a serious illness that needs hospital care [1][2][8]
• needing help from someone else to drink enough fluids [2]

A 2018 CDC report found that people with diabetes in the US were nearly four times as likely as people without diabetes to be in hospital with AKI, based on data from 2000 to 2014 [8].

How Doctors Find It

The main test is a blood test for creatinine, a waste product made by your muscles and normally cleared by your kidneys. A higher level than usual means the kidneys are not working as well as they should [1]. Doctors compare the result with your usual level, and a rise over hours to days, or a drop in how much urine you pass, points to AKI [2][4].

Your doctor may also test a urine sample, ask how often you have been peeing, and check your pulse and blood pressure [1]. Depending on the likely cause, you may have other tests, such as an ultrasound or other scan to look for a blockage [1][2]. The NKF advises telling your clinician about every medicine you have taken recently, including over-the-counter products and herbal supplements [5].

Doctors also grade AKI by how severe it is [4]. In someone who already has chronic kidney disease, NICE points out that a rise in creatinine may be a new AKI rather than the chronic disease getting worse [2].

How It Is Treated

Treatment depends on what is causing AKI and how severe it is [1][5], which is why finding the cause matters so much. The NKF says most people with AKI need to spend some time in hospital to be monitored [5].

Depending on the cause, treatment may include fluids into a vein, treatment for an infection, raising low blood pressure, a thin tube called a catheter to drain the bladder, or a procedure to remove a blockage [1][5]. Your care team may also stop or change a medicine that could be affecting your kidneys [1][5]. If AKI is severe, dialysis may be used for a short time, usually days or weeks, until the kidneys recover [1][5].

In England, NICE says a kidney specialist is usually not needed when the cause is clear and AKI is improving quickly with treatment. A specialist is usually involved when the cause is unclear, treatment is not working or AKI is severe [2].

Recovery and Outlook

The NHS says many people recover fully within a few weeks with no long-term effects, while more serious AKI can take several months [1]. KDIGO adds a note of caution: even mild AKI that gets better can still have important consequences for health [4].

Some people are left with lasting kidney damage, which means chronic kidney disease (CKD). The NHS says this is more likely when AKI was severe or when other long-term health problems are present [1]. After AKI, the NKF notes a higher risk of CKD, heart disease, stroke and another episode of AKI [5].

How AKI Differs From Chronic Kidney Disease

AKI and CKD both describe kidneys that are not working as well as they should, but they differ in important ways.

The two are linked in both directions. AKI can leave lasting damage that becomes CKD [1], and having CKD raises the risk of AKI [7]. KDIGO also uses the term acute kidney disease for kidney problems that last longer than AKI but less than 3 months [4]. For CKD tests, stages and screening, see Chronic Kidney Disease: Early Signs and Why Screening Matters.

Follow-Up After AKI

The NHS says you will have regular blood tests and check-ups after you recover [1]. Guidance on timing differs:

• In England, NICE says adults leaving hospital after AKI should have a clinical review within 3 months, or sooner if they are at higher risk. The review covers your medicines, a kidney function check and a discussion about keeping your kidneys healthy [3].
• KDIGO recommends checking people 3 months after AKI to see whether the kidneys have recovered [4], and the NKF suggests follow-up tests ideally within 3 months of finishing treatment [5].
• A 2018 CDC report cited a US national objective of a kidney check 6 months after a hospital stay for AKI [8].

The NKF suggests asking your healthcare professional when you should follow up to check your kidney health [5].

Lowering Your Chances of Another Episode

If you have had AKI before, or are at higher risk because of your age or medicines, the NHS suggests [1]:

• drinking enough fluids that your pee is pale and regular throughout the day, especially in hot weather or if you are vomiting or have diarrhea
• speaking to a doctor before taking any new medicines or supplements
• knowing the symptoms to watch for and when to get medical help

Before a non-urgent CT scan that uses dye, NICE says you may be asked whether you have kidney disease or a kidney transplant, and the benefits and risks of the dye should be discussed with you. In an emergency, the dye should not be delayed if waiting is likely to cause harm [2]. For more on how CT scans work, see MRI vs CT Scan: Differences, Uses, and Risks.

The NKF suggests questions to ask your healthcare team [5]:

• What are my biggest risk factors for AKI?
• What can I do to help lower my risk?
• Are there any medicines I should avoid because of my kidneys?
• Was the cause of my AKI preventable, and how can I stop it happening again?

Children

Kidney disease is not common in children [6]. NIDDK says burns, dehydration, bleeding, injury or surgery can lower blood pressure and blood flow to a child's kidneys, which can lead to AKI [6]. NICE also lists severe diarrhea, particularly bloody diarrhea, as a risk in children [2]. A child with AKI may be referred to a pediatric nephrologist, a doctor who specializes in children's kidney problems [6].

When to Get Medical Help

The NHS and NIDDK both say AKI can be life-threatening if it is not treated [1][6]. In the UK, the NHS advises asking for an urgent GP appointment or contacting NHS 111 if you think you may have symptoms of AKI [1]. In the US, the NKF suggests asking your healthcare professional whether your symptoms need a trip to the emergency room [5].

Conclusion

For most people, AKI is a short episode that is found and treated in a medical setting. What happens afterward matters too. Keeping your follow-up appointments, knowing the warning signs and checking with a doctor before starting new medicines all help protect your kidneys in the years ahead.

Medical Disclaimer

This article is for general information and does not replace advice from a qualified health professional. If you or someone with you is seriously unwell, get emergency help straight away.