Exams and Tests

Many people with acute renal failure notice no symptoms. Even with symptoms, they are nonspecific, meaning they could be caused by many different conditions. A physical examination typically reveals few, if any, abnormal findings.

Kidney failure is often detected from blood or urine tests. These tests might be ordered because the patient is in the hospital for another reason, because they don't feel well and can't tell why, or as part of a routine health screening.

• Levels of urea (blood urea nitrogen [BUN]) and creatinine are high in kidney failure of prerenal origin. This is called azotemia.
  
  
• Electrolyte levels in the blood may be abnormally high or low because of improper filtering.
  
  
• When the duration and severity of kidney failure is severe, the red blood cell count may be low. This is called anemia.

The amount of urine produced over a period of hours may also be measured for quantity and quality or the amount of wastes being excreted. When kidney tissue is injured, protein and desirable substances may be inappropriately excreted in the urine. In some cases, the amount of urine remaining in the bladder after urination will be measured by inserting a catheter (a thin, rubber tube) that drains the bladder.

• Urine retained in the bladder after urinating suggests postrenal failure, usually due to prostate enlargement in men.
  
  
• The urine may be dark, indicating that creatinine and other substances are concentrated.
  
  
• The urine will be examined under a microscope to detect signs of specific kidney problems. Some of these signs include blood, pus, and solid materials called casts.
  
  
• Electrolyte levels in the urine may help pinpoint the exact cause of the kidney failure.

If the diagnosis is not certain after laboratory tests, an ultrasound of the kidneys and bladder may be done. These can help reveal signs of specific causes of kidney failure.

In some cases, tissue samples of the kidneys are taken (biopsy) to find the cause of the renal failure.