The nuts and bolts of azotemia (Proceedings)
Azotemia is defined as increased concentrations of urea and creatinine (and other nonproteinaceous nitrogenous substances) in the blood. The interpretation of serum urea nitrogen and creatinine concentrations as a measure of renal function requires a knowledge of the production and excretion of these substances. Urea is synthesized in the liver from ammonia, which is in turn generated from the catabolism of ingested and endogenous proteins. Urea production is increased in the settings of a high dietary protein intake, upper gastrointestinal tract hemorrhage, and occasionally catabolic states that result in the breakdown of body proteins (e.g., corticosteroid administration). Conversely, urea production is decreased in the settings of a low dietary protein intake, decreased hepatic function, or decreased delivery of ammonia to the liver (e.g., portosystemic shunt). Urea has a small molecular weight (60 daltons) and is a permeate solute that readily diffuses throughout all body fluid compartments; its concentration is similar in intracellular and extracellular fluid and in plasma, serum, and blood. Urea that diffuses into the intestinal lumen is degraded by enteric organisms to ammonia, which is then reabsorbed into the portal circulation and again converted to urea by the liver. Urea is principally excreted by the kidneys; it is freely filtered through the glomeruli and passively resorbed by the renal tubules. The tubular resorption of urea is increased when tubular flow rates and volumes are decreased. Decreased tubular reabsorption of urea results in increased excretion and over time, decreased blood concentrations of urea. Conversely, the tubular resorption of urea is decreased and excretion increased in the presence of diuresis. Decreased renal blood flow (prerenal causes, e.g., dehydration or decreased cardiac output) and decreased excretion of urine (postrenal causes, e.g., urethral obstruction or ruptured bladder), as well as primary renal dysfunction, will result in decreased excretion of urea.
Creatinine is irreversibly formed by the nonenzymatic metabolism of creatine and phosphocreatine in muscle. Creatinine production is relatively constant and proportional to muscle mass; animals with a large muscle mass produce more creatinine each day than do animals with a small muscle mass. For example, serum creatinine concentrations would be expected to be relatively low in an older cat with loss of muscle mass and decreased body condition score. Muscle trauma and inflammation do not increase the production of creatinine. In comparison with the urea nitrogen concentration, the creatinine concentration is relatively unaffected by the dietary protein level; however, serum creatinine concentrations can increase after the ingestion of meat and the subsequent increased absorption of creatinine from the gastrointestinal tract. The molecular weight of creatinine is 113 daltons; therefore it diffuses throughout body fluid compartments more slowly than urea does. Some creatinine diffuses into the intestinal lumen, is degraded by enteric bacteria, and is excreted from the body in the feces; however, most creatinine is excreted by the kidneys. Creatinine is freely filtered by the glomeruli and is not significantly resorbed or secreted by the renal tubules. Because the production of creatinine is relatively constant, an increase in the serum creatinine concentration is indicative of decreased renal excretion. It is important to remember, however, that prerenal and postrenal factors influence renal function, and therefore, the excretion of creatinine. Disproportionate increases in BUN relative to creatinine can be caused by high protein diets, upper gastrointestinal hemorrhage, and increased tubular reabsorption of urea nitrogen associated with prerenal azotemia. Conversely, a disproportionately low BUN can be observed with decreased liver function, portosystemic shunts, low protein diets and prolonged diuresis.