How Acute Renal Failure Is Diagnosed

Acute renal failure occurs when the kidneys are suddenly unable to filter wastes from the blood. It is a complication of any number of diseases or disorders, the effect of which leads to the rapid build-up of toxins and a cascade of symptoms ranging from decreased urination and fatigue to chest pains and seizures.

While acute renal failure can often occur without symptoms and only be revealed during lab tests for an unrelated condition, most cases are diagnosed in people who are either critically ill or arrive at the hospital with a serious illness.

If acute renal failure is suspected, blood tests, urine tests, ultrasound, and biopsies may be ordered to confirm and establish the level of impairment. Based on the results, the doctor will be able to stage the disease and take the appropriate action. In the worst-case scenario, end-stage kidney disease may be declared.

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Labs and Tests

Acute renal failure ​(ARF), also known as acute kidney injury (AKI), is primarily diagnosed by blood and urine tests. Among the many lab tests used to evaluate kidney function, there are two key measures central to the diagnosis and management of ARF.

Serum Creatinine

Serum creatinine (SCr) measures the amount of a substance called creatinine in the blood. Creatinine is a by-product of muscle metabolism that is excreted in urine. Because it is produced and excreted at a fairly steady rate, it is a reliable measure of kidney function and is a key indicator of kidney failure.

Normal SCr levels in adults are:

  • Approximately 0.5 to 1.1. milligrams (mg) per deciliter (dL) in women
  • Approximately 0.6 to 1.2 mg/dL in males

Urine Volume

Urine volume simply measures the amount of fluid you urinate over a given period of time. As ARF is defined by the loss of kidney function, the value—measured in milliliters (mL) per kilograms of your body weight (kg) per hour (h)—is central to confirming kidney impairment and measuring your response to treatment.

Oliguria, the production of abnormally small volumes of urine, is defined as anything less than 0.5 mL/kg/h.

Other Lab Tests

Other lab tests used to diagnose ARF include:

  • Blood urea nitrogen (BUN) measures the amount of a waste product in the blood called urea nitrogen. Urea nitrogen is created when the liver breaks down protein and, like serum creatinine, is produced and excreted in the urine if fairly consistent volumes. High BUN levels are indicative of ARF and may also suggest the underlying cause of the kidney failure (such as heart failure, dehydration, or urinary tract obstruction).
  • Creatinine clearance measures creatinine level in both a sample of blood and urine sample collected over 24 hours. The combined results can tell us how much creatinine is being cleared from the blood through urination as measured by mL per minutes (mL/min). A normal creatinine clearance is 88 to 128 mL/min in women and 97 t0 137 mL/min in men.
  • Estimated glomerular filtration rate (eGFR) is a blood test that estimates how much blood is passing through the natural filters of the kidneys, called glomeruli. The speed by which this happens can tell us how much the kidneys have been damaged from stage 1 (minimal to no loss of kidney function) right through stage 5 (kidney failure).
  • Serum potassium is used to determine whether there is excess potassium in the blood (a condition known as hyperkalemia). Hyperkalemia is characteristic of ARF and, if left untreated, can lead to severe and potentially life-threatening dysrhythmia (abnormal heart rate).
  • Urinalysis is simply a lab analysis of the make-up of your urine. It can be used to detect whether there is excess protein in the urine (​proteinuria), considered a key feature of ARF. It can also detect blood in the urine (hematuria) which may occur if the ARF is caused by some sort of kidney damage or urinary tract obstruction.

Diagnostic Criteria

Acute renal failure is diagnosed based on the result of the serum creatinine and urine volume tests.

The criteria for diagnosis was established by Kidney Disease: Improving Global Outcomes (KDIGO), a non-profit organization that oversees and implements clinical practice guidelines for kidney disease.

According to KDIGO, acute renal failure can be diagnosed if any one of the following is present:

  • An increase in SCr by 0.3 mg/dL or more within 48 hours
  • An increase in SCr of at least 150 percent within a seven-day period
  • A urine volume of less than 0.5 ml/kg/h over a six-hour period

Imaging Tests

In addition to blood and urine tests, imaging tests may be used to detect if there is any sort of kidney damage or if there is an impairment to either the flow of blood to the kidney or the excretion of urine from the body.

Among some of the tests used:

  • Ultrasound is the preferred method of imaging testing and can be used to measure the size and appearance of the kidneys, detect tumors or kidney damage, and locate blockages in the urine or blood flow. A newer technique called a Color Doppler can be used to assess clots, narrowing, or ruptures in the arteries and veins of the kidneys.
  • Computed tomography (CT) ​is a type of X-ray technique that produces cross-sectional images of an organ. CT scans can be useful in detecting cancer, lesions, abscesses, obstructions (such as kidney stones), and the accumulation of fluid around the kidneys. They are standardly used in obese people in whom an ultrasound may not provide a clear enough picture.
  • Magnetic resonance imaging (MRI) ​uses magnetic waves to produce high-contrast images of the kidneys without radiation.

Kidney Biopsy

A biopsy involves the removal of organ tissue for examination by the lab. The type typically used to assess kidney disease is called a percutaneous biopsy in which a needle is inserted into the skin and guided into a kidney to remove a sampling of cells.

Biopsies are most often used to diagnosed intrinsic ARF (acute renal failure caused by damage to the kidneys). The biopsy can quickly diagnose some of the more common causes of kidney damage, including:

  • Acute interstitial nephritis (AIN), the inflammation of tissue between kidney tubules
  • Acute tubular necrosis (ATN), a condition in which kidney tissues die due to the lack of oxygen
  • Glomerulonephritis, the inflammation of glomeruli in blood vessels of the kidneys

Differential Diagnosis

As a complication of an underlying disease or disorder, acute renal failure can be caused by many different things, including heart failure, liver cirrhosis, cancers, autoimmune disorders, and even severe dehydration.

At the same time, there may be situations where lab tests suggest ARF but other conditions are, in fact, to blame for the elevated blood levels. Among them:

  • Chronic kidney disease (CKD), often undiagnosed, may have all the serological signs of ARF but will ultimately persist for more than three months. With CKD, the only explanation for the elevated SCr will be an impaired glomerular filtration rate. A 24-hour creatinine clearance test can usually different between the two conditions.
  • Certain medications, like the H2 blocker Tagamet (cimetidine) and the antibiotic Primsol (trimethoprim), can cause an elevation of creatinine. Discontinuation of the suspected drug will usually be enough to make the differentiation.

Frequently Asked Questions

What does acute renal failure mean?

It means that your kidneys have abruptly stopped working, usually within the course of two days. Also known as acute kidney failure or acute kidney injury (AKI), it is a very serious condition requiring immediate medical treatment.

How is acute renal failure diagnosed?

Acute renal failure is primarily diagnosed based on the amount of urine you produce over a period of time (urine volume) and the accumulation of a waste product called creatinine in your blood (serum creatinine) that your kidneys are usually able to eliminate. If you have acute renal failure, your urine volume will decrease and/or your serum creatinine will increase to abnormal levels.

How is urine volume used to diagnose acute renal failure?

If acute renal failure is suspected, your doctor will collect and measure the amount of urine you produce over a six-hour period (called a urine volume test). Based on your weight, the doctor can estimate the functional status of your kidneys. A normal urine output for an adult is 0.5 milligrams per kilogram per hour (mg/kg/h) or more. With acute kidney failure, the output will drop below 0.5 mg/kg/h.

How is creatinine used to diagnose acute renal failure?

Creatinine is a byproduct of muscle metabolism excreted in urine. If acute renal failure is suspected, a serum creatinine (SCr) blood test can measure how much and how quickly this byproduct is accumulating. Acute renal failure can be diagnosed either when the SCr increases by 150% or more within seven days or the SCr increases by at least 0.3 milligrams per deciliter (mg/dL) within 48 hours.

How is the glomerular filtration rate (GFR) used for acute renal failure?

Glomeruli are tiny structures in the kidneys that filter blood. The glomerular filtration rate (GFR) is a blood test used to calculate how effectively your glomeruli are working based on your age, height, weight, sex, and race. The GFR plays an important role in the staging of acute renal failure.

What other blood tests are used for acute renal failure?

In addition to serum creatinine, blood tests can detect if there are abnormal levels of other substances in the bloodstream. This includes a blood urea nitrogen (BUN) test that measures a waste product called urea nitrogen, and a serum potassium test that can detect excessive and potentially dangerous accumulations of potassium.

How is ultrasound used in the diagnosis of acute renal failure?

Ultrasound can help diagnose the cause of acute renal failure. It is the preferred method of imaging that can detect things like kidney damage, blood clots, ruptured vessels, blood or urine obstruction, tumors, and other causes of kidney failure.

What does staging mean in relation to acute renal failure?

Staging is used to determine the severity of acute renal failure. The staging can be based on the RIFLE criteria, which categorizes the severity by class R (risk of failure), class I (kidney injury), or class F (kidney failure). A similar system called AKIN categorizes it by stages 1, 2, and 3. The determination is based on the results of SCr, urine volume, or GFR tests.

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