Prerenal azotemia is an abnormally high level of nitrogen-type wastes in the bloodstream. It is caused by conditions that reduce blood flow to the kidneys.

Prerenal azotemia is relatively common, especially in hospitalized patients.

The kidneys normally filter the blood. When the volume or pressure of blood flow through the kidney drops, blood filtration also drops drastically, and may not occur at all. Waste products remain in the bloodstream and little or no urine is formed, even though the internal structures of the kidney are intact and functional.

Lab tests show that nitrogen-type wastes, such as creatinine and urea, are accumulating in the body (azotemia). These waste products act as poisons when they accumulate, damaging tissues and reducing the ability of organs to function. The build-up of nitrogen waste products and accumulation of excess fluid in the body are responsible for most of the symptoms of prerenal azotemia and acute renal failure.

Prerenal azotemia is the most common form of kidney failure seen in hospitalized patients. Any condition that reduces blood flow to the kidney may cause it -- including loss of blood volume, which may occur with dehydration, prolonged vomiting or diarrhea, bleeding, burns, and other conditions that allow fluid to escape from circulation.

Conditions where the volume is not lost, but where the heart cannot pump enough blood, or the blood is pumped at low volume, also increase risk for prerenal azotemia. These conditions include shock (such as septic shock), heart failure, and conditions where the blood flow to the kidney is interrupted, such as trauma to the kidney, surgery of various types, renal artery embolism, and other types of renal artery occlusion.

• Decreased or no urine produced
• Fatigue
• Decreased alertness
• Confusion
• Pale skin color
• Rapid pulse
• Dry mouth
• Thirst
• Swelling

• Excessive urination at night
• Abdominal pain

An examination may show signs of low heart function or signs of hypovolemia. Blood pressure may be low or may drop when the person stands up. The pulse pressure (difference between systolic blood pressure and diastolic blood pressure) may be reduced. The heart rate may be rapid.

Skin turgor may be poor, and mucous membranes may be dry. The neck veins may be collapsed. There may be little or no urine in the bladder even when drained by a catheter. If the condition is prolonged, other signs of acute kidney failure may be present.

A urinalysis may show decreased kidney function preserving the ability of the tubules. Nitrogen wastes and electrolytes continue to be excreted, but at abnormally low rates.

• Urine sodium may be low, with fractional excretion at less than 1%.
• Urine creatinine to serum creatinine ratio is high.
• Urine urea to serum urea (BUN) ratio is high.
• Fractional excretion of urea is low.
• Osmolality and specific gravity show concentrated urine.

• Increased BUN
• Increased creatine
• Increased BUN/creatinine ratio