Your doctor will begin to evaluate your acute renal failure with a medical history and physical exam. Your doctor will review your health history and hospital chart (if you are currently in the hospital), check all the medicines you are taking, and ask about any other illnesses you have. A complete evaluation is needed to:

• Find the cause of kidney failure. This is an extremely important part of the diagnosis. Identifying the cause is sometimes difficult and may require many tests.
• Know how to treat your condition. How it will be treated and your chances for recovery usually depend on what is causing your acute renal failure.
• Learn how much kidney function remains.
• Detect and treat any complications that may have developed.

Lab tests

Doctors routinely use blood and urine tests to evaluate acute renal failure. These tests can detect a buildup of waste products in the blood and chemical imbalances in the body and may help diagnose an underlying disease or infection.

Blood and urine tests may include:

• Serum creatinine. An increase in the amount of creatinine in the blood (serum creatinine) is usually the first sign of acute renal failure. Repeated tests of serum creatinine can help monitor the progress of renal failure and can help determine whether treatment has been successful. In cases caused by reduced blood flow (prerenal acute renal failure), serum creatinine levels will rapidly return to normal once blood flow or fluid volume is restored.
• Blood urea nitrogen (BUN). BUN measures the amount of nitrogen in your blood that comes from the waste product urea. If your kidneys are not able to remove urea from the blood normally, your BUN level increases.
• Blood electrolyte tests, such as calcium, phosphate, potassium, and sodium.
• Complete blood count (CBC). A CBC provides important information about the red blood cells, white blood cells, and platelets. It can be used to check for diseases or infections that could be causing renal failure.
• Other blood tests, such as an erythrocyte sedimentation rate (ESR, or sed rate) or antinuclear antibodies (ANA) test. These may be used to screen for infection, autoimmune disease, and other disorders if your medical history and symptoms suggest that one of these conditions might be present.
• Urinalysis, which examines a sample of your urine. The results can provide information about urine sediment, which is useful for evaluating kidney damage (intrinsic acute renal failure). It also looks for:
  • Urine eosinophils (a type of white blood cell). The presence of eosinophils in the urine may be a sign that an allergic reaction is damaging the kidneys. Often the allergic reaction is caused by a medication.
  • Fractional excretion of sodium (Fe_Na) in the urine. It measures the kidneys' ability to process sodium (Na) based on the levels of sodium and creatinine in both the urine and the blood. This test can help distinguish prerenal acute renal failure, where there has been no damage to the kidney itself, from intrinsic acute renal failure, caused by damage to the kidneys.
• 24-hour urine collection. Urine output is measured over a 24-hour period. You may have a small tube (catheter) inserted into your bladder to collect all of the urine you produce. Reduced urine output may or may not occur with acute renal failure, depending on the cause. Careful measurement of urine output over time can also help monitor fluid balance in a person who has renal failure.

Imaging tests

Imaging tests provide pictures of your kidneys. They can help determine whether kidney failure is acute or chronic and check for a blockage in the urinary tract. The imaging tests most commonly used for these purposes are:

• Abdominal ultrasound.
• Computed tomography (CT) scan.
• Abdominal X-ray or a spiral CT scan, which may be used if a kidney stone is suspected.

If a blockage is believed to be causing acute renal failure (postrenal acute renal failure), you may need more extensive tests to determine the location and cause of the obstruction. These tests may include the imaging tests listed above, as well as:

• Retrograde pyelography. During this test, the doctor inserts a thin, lighted tube (cystoscope) into your urethra. A catheter is then put through the cystoscope and into a ureter. Dye is injected through the catheter and X-rays are taken.
• Magnetic resonance imaging (MRI). For this type of MRI, your abdomen is positioned inside a strong magnetic field. The MRI can detect changes in the structure of the kidneys and urinary tract.
• Kidney scan (renal scintigraphy). A kidney scan may also help evaluate whether blood flow to the kidney is normal or whether a blockage is present.

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