CHAPTER 22 Renal Disorders 539
made of protein or fats and can either be benign or signify kidney disease. Blood Urea Nitrogen Azotemia is the increase of blood urea nitrogen (BUN) within the bloodstream. The normal level for BUN is 5 to 20 mg/dL. An elevated BUN can occur when there is a decrease in the GFR, which leads to accumulation of nitrogenous waste products in the blood. However, a high BUN level is not always an indicator of kidney dysfunction; it can result from dehydration, which highly concentrates the urea in the urine. A high BUN level can also occur in any condition that elevates the amount of nitrogen waste in the bloodstream. Extremely muscular individuals will have a high nitro- gen level in the bloodstream because of high muscle breakdown. The muscle cell proteins break down into amino acids, which are nitrogen compounds. High BUN levels also occur in persons on high-protein diets, as the large load of protein breakdown into amino acids raises nitrogen in the bloodstream. CLINICAL CONCEPT Because of the possible elevation of BUN with nonrenal conditions such as dehydration, the clinician should not rely on BUN alone as an indicator of renal dysfunction. Serum Creatinine Creatinine is a muscle breakdown product that is fil- tered almost completely at the glomerulus. The nor- mal range of serum creatinine is approximately 0.5 to 1.5 mg/dL. After being filtered out of the bloodstream, it is not reabsorbed by the nephron tubules.
Creatinine Clearance Creatinine clearance (CrCl) is sometimes used to assess the GFR. The test requires measurement of both blood and urine creatinine and 24-hour urine volume. CrCl can also be estimated using a mathe- matical formula. The amount of creatinine filtered at the glomerulus is the total amount of creatinine that appears in the urine. A decreased creatinine clearance indicates decreased GFR and impaired renal function. This can be caused by conditions such as renal disease or can result from lack of circulation to the kidney, which occurs in hypotension, heart failure, and shock. Increased creatinine clearance indicates there is more creatinine in the urine than normal. This can be seen in pregnant women, patients with DM, patients with large muscle mass, or those with high protein intake. Imaging Studies Visualization of the kidneys through various imaging studies can provide valuable information about renal size and function. Renal ultrasound is used to deter- mine the size of both kidneys. It can also be used in the diagnosis of hydronephrosis, renal cysts, tumors, and kidney stones. Abdominal x-rays can sometimes visualize radio-opaque stones or nephrocalcinosis. Computed tomography (CT) scan or magnetic reso- nance imaging (MRI) can also visualize kidney stones and abnormalities. Renal biopsy can be performed if imaging tests do not reveal sufficient information.
ALERT! IV contrast-enhanced imaging studies should be avoided in patients with renal impairment because radiopaque dye can cause renal failure. Dehydration markedly increases this risk.
CLINICAL CONCEPT Serum creatinine is a reliable indicator of kidney function.
Treatment Regardless of the etiology, all of the functions regu- lated by the kidney must be maintained when treat- ing renal disease. It is important to maintain fluid, electrolyte, and acid–base levels; to control blood glucose; to control blood pressure; and to monitor RBC production. To accomplish this, patients usually need multiple medications to maintain physiological homeostasis; sodium bicarbonate can help control metabolic acidosis, whereas beta blocker medications can control blood pressure. Epogen is a synthetic form of erythropoietin that can be used to stimulate RBC production. Diuretics can be used to stimulate water loss from the body. However, when these medica- tions cannot reverse the imbalances of renal failure, dialysis is necessary. Indications for dialysis include persistent hyperkalemia, uncompensated metabolic acidosis, and fluid volume excess that is unrespon- sive to diuresis.
Accumulation of serum creatinine indicates decreased filtering of creatinine at the glomerulus. There are exceptions to this rule in extremely mus- cular individuals and very frail individuals. Because serum creatinine is based on muscle tissue breakdown, serum creatinine can vary depending on the patient’s muscle mass. A person who has an increased amount of muscle breakdown daily may have an abnormally high serum creatinine, whereas a frail individual will have a low amount of serum creatinine daily. ALERT! Nephrotoxic antibiotics include ami- noglycosides. Whenever these are administered, serum levels of the medication and serum creatinine levels must be monitored.
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