Azotemia is a condition in which the blood accumulates higher than normal and potentially harmful concentrations of nitrogen containing compounds, such as urea, creatinine, and other compounds. It is related to renal disease, as the primary cause of azotemia is the inadequate filtering of these waste materials by the kidneys. As with many kidney-related disorders, azotemia is divided into three categories: prerenal azotemia, primary renal azotemia, and postrenal azotemia.

These terms refer to whether the cause of the azotemia arises before the kidneys themselves in the renal-function process (prerenal azotemia), arises from kidney damage specifically (primary renal azotemia), or is due to obstruction of the urinary tract after the kidneys (postrenal azotemia). The three types of azotemia indicate different causes of the problem and different types of treatment.

Prerenal azotemia results from decrease in blood flow to the kidneys. With prerenal azotemia, there is normally no kidney disease as such, although damage to the kidneys can also result from decreased blood flow, so that renal disease can occur as an outcome.

The drop in blood flow to the kidneys can result from blood loss, as from severe bleeding or burns; from dehydration due to many causes ranging from diarrhea to simply not drinking enough water; to shock, which results in inability of the heart to pump blood in adequate volume (and which also affects other vital organs); to congestive heart failure; to narrowing of the renal artery (the artery delivering blood to the kidneys), and for a number of other reasons.

BUN-Creatinine Ratio And Prerenal Azotemia

When azotemia is diagnosed, one reliable test differentiating prerenal from primary renal azotemia is the BUN-creatinine ratio. This derives from two blood tests, one for serum urea nitrogen (also called blood urea nitrogen, or BUN) and the other for creatinine.

Serum urea nitrogen is a waste product produced by the liver and filtered from the blood by the kidneys. Creatinine is another waste product produced by the action of the muscles and also filtered from the body by the kidneys.

Because of reactions within the kidney itself in response to decreased blood volume, the filtering of blood urea nitrogen is reduced compared to the filtering of creatinine in such a situation.

For this reason, in cases of prerenal azotemia the BUN-creatinine ratio is high. A ratio above 20:1 is common in prerenal azotemia, compared to 15:1 or less with normal kidney function or lower still with primary renal azotemia.

Symptoms

Signs and symptoms of prerenal azotemia include oliguria or anuria (reduced urinary flow or absence of urine), asterixis (a tremor of the wrist when the wrist is extended, described as similar to a bird flapping its wing), loss of alertness or mental confusion, thirst and dry mouth, tachycardia (rapid pulse rate), pallor of the skin, orthostatic blood pressure (that is, blood pressure that rises and falls depending on  position), and uremic frost, a condition in which urea is excreted through the skin in sweat, leaving urea compounds on the skin that resemble a frost.

Treatment

Treatment for prerenal azotemia will depend to some extent on the cause of the condition as well as its severity. In all cases, the goal is to restore normal blood flow to the kidneys, and to do so quickly, before renal damage occurs.

Where the loss of blood to the kidneys is caused by blood loss, treatment appropriate to restoration of blood volume is undertaken. Where the cause is dehydration, reestablishing fluid balance in the body is the proper treatment, as well as treatment of the underlying cause of the dehydration, which may be any of several infectious diseases as well as other non-infectious conditions.

Where the problem is a blockage, constriction, or other problem specific to the renal artery, procedures to relieve the blockage and restore normal blood flow to the kidney are required. Once normal fluid volume is restored, medications may be used to raise blood pressure or to increase output from the heart. These may include dopamine or other heart medications.

In all cases, prerenal azotemia should be regarded as a serious condition requiring medical attention. Loss of proper blood flow to the kidneys can result in severe renal damage, necessitating dialysis and, in the case of irreversible damage, a kidney transplant.