Renal Diagnostic Tests Nurs ing Jobs P lus As you review your patient's labs, most will have values that reflect how well the kidneys are performing. Many of us were taught that the holy grail of renal function is the blood urea nitrogen (BUN) and creatinine levels, but weren't told the essential differences between these two tests. Urine, a vital substance, provides many clues about homeostasis, yet many nurses don't fully appreciate all the information this humble specimen provides. Read on and add to your growing body of knowledge about renal lab analysis. Urinalysis: Think fluid laden with clues The first morning urine void is the preferred specimen for a urinalysis due to its greater acidity and concentration, which preserves cellular elements. After obtaining the specimen, you must send it immediately to the lab where it should be analyzed within 1 hour to prevent bacterial overgrowth. Using a dipstick, the characteristics of urineappearance, pH, specific gravity, and presence of protein, glucose, ketones, red blood cells (RBCs), and white blood cells (WBCs)-can be detected. Calculation of a urine albumin-to-creatinine ratio is often used to screen for kidney disease in patients at high risk, particularly those with diabetes and hypertension. Using a first morning specimen, the amount of protein and creatinine in the urine is analyzed. A normal ratio is less than 0.2 g of protein per gram of creatinine. If your lab analyzes the specimen using micrograms, the standard values are less than 17 mcg/mg for men and less than 25 mcg/mg for women. Healthy individuals excrete less than 30 mg of protein/day, an amount not detectable by urine dipstick. When protein is identified in urine, it's vital to determine its origin. Albumin, the major plasma protein, is quite large and unable to pass through the glomerular membrane. When albumin is detected either by dipstick or by performing an albumin-to-creatinine ratio, its presence usually signals trouble. When glomeruli develop significant disease and become sievelike, albumin passes freely through the pores, resulting in urinary losses. A small amount of albumin (between 30 to 299 mg/day) is termed microalbuminuria; a loss of greater than 300 mg/day is called macroalbuminuria. Both conditions are a sign of deteriorating kidney function. Many individuals with type 1 diabetes develop glomerular damage, resulting in passage of albumin into urine. Diabetic nephropathy frequently results in losses of 1 g/day. Losses occurring with nephrotic syndrome are 1 to 2 g/day in early disease, advancing to 20 to 30 g/day with severe disease. Figure. No caption available. Glucose will appear in the urine when the renal plasma threshold for glucose is exceeded. Carriers in the renal tubule become saturated around glucose levels of 200 mg/100 mL of blood, and glycosuria appears. Ketones, which are acidic byproducts of fat metabolism, will show up when the patient is utilizing excess adipose tissue for energy. Both of these Renal Diagnostic Tests developments signal diabetes. Leukocyte esterase, an enzyme produced by WBCs and nitrite (some Gram-negative bacteria produce nitrite through the reduction of nitrates to nitrites), is a useful indicator of urinary tract infection (UTI). The presence of urobilinogen, a product of conjugated bile, suggests either RBC hemolysis or biliary and hepatic dysfunction. Microscopic analysis is done using a high-power field (HPF). RBC structure is examined and, if irregular, points to a kidney disorder. If observed, WBCs are typically neutrophils, which indicate infection if high levels of bacteria are found (bacteriuria). Casts are aggregates of protein that take the shape of a renal structure, such as the tubule, and suggest kidney disease. There are RBC and WBC casts, as well as hyaline, granular, fatty, waxy, and broad casts that may not reflect pathology or may be indicative of pyelonephritis, glomerular or renal tubular disease, or nephrotic syndrome. Crystals formed from calcium oxalate, uric acid, struvite, or cystine suggest metabolic renal disorders such as gout or kidney stones. BUN: Think hydration The liver forms urea, a nitrogen-containing compound, when protein is broken down. Because there are many sources of protein, including dietary intake, cellular turnover, intestinal bacteria that degrade protein in food, and protein from decomposing blood when there's internal bleeding, the levels of urea will shift according to a variety of conditions. Urea is a small molecule that diffuses freely in all fluids, and levels in the blood represent the balance between hepatic production and renal excretion. Values range between 5 and 20 mg/dL, with lower levels suggesting liver disease, in which urea isn't able to be synthesized from ammonia in the blood, or low dietary protein intake. Given that particles of urea contribute significantly to blood osmolality, changes due to either expansion of plasma volume or dehydration will affect BUN levels. When plasma is diluted due to overhydration with I.V. fluids or secondary to an endocrine disorder, urea levels will drop. Conversely, when a patient is dehydrated and there's a reduction of plasma volume, urea levels rise. When a patient isn't adequately hydrated, the kidneys respond by retaining sodium and water. During this volume balancing process, there's a parallel increase in urea reabsorption from the proximal tubules. BUN provides valuable information on the hydration status of your patient, as well as kidney function. Creatinine: Think muscle Creatinine is a waste product of muscle metabolism. Skeletal muscle consumes a great deal of energy in the form of creatine phosphate. With the help of creatine kinase, these high-energy phosphate bonds produce creatine, which is then metabolized into creatinine. It's important to know that creatinine is generated at a constant rate and that all creatinine filtered by the kidneys is excreted in the urine. Renal Diagnostic Tests Creatinine levels don't shift with changes in diet like BUN levels do, and the amount of creatinine produced in an individual is proportional to that person's skeletal muscle mass. Men have creatinine levels between 0.8 and 1.2 mg/dL; for women the range is 0.6 to 1 mg/dL. Older patients and lower extremity amputees may have lower serum creatinine levels due to decreased muscle mass. These factors, as well as gender, weight, body surface area, and ethnicity, are considered in estimating kidney function. Glomerular filtration rate: Think filtration pump The glomerular filtration rate (GFR) provides an approximate measure of the number of functioning nephrons. Recall that each kidney has 800,000 to 1.5 million filtering units, which work to remove waste products from the blood. Collectively, they filter 180 L of plasma/day at a rate of 125 mL/minute. Age, gender, ethnicity, and body size affect the GFR, with lower values in women, the elderly, patients with decreased muscle mass, and individuals with small stature. The National Kidney Foundation recommends using the Modification of Diet in Renal Disease Study equation for adults to calculate GFR, which includes serum creatinine, age, gender, body surface area, and ethnicity. The online calculator is easily accessed (http://www.nkdep.nih.gov/professionals/gfr_calculators/idms_con.htm ) and there are downloadable PDA and smart phone applications. Another useful equation that accounts for serum creatinine, weight, age, and gender is the Cockcroft-Gault equation. However, it doesn't adjust for body surface area and may overestimate a patient's GFR. Obtaining this useful value is helpful in examining trends (see Stages of chronic kidney disease). Is the GFR stable or is it declining, reflecting fewer functioning nephrons? Prescribers use GFR levels to adjust the dosage of drugs, manage transplant patients, and include therapies to prevent kidney deterioration such as the use of angiotensinconverting enzyme inhibitors, which are known to delay glomerular damage. BUN/creatinine ratio: Think source of the problem BUN and creatinine are two labs on the basic metabolic panel that are ordered to assess renal function. Now that you appreciate their subtle yet important characteristics, you'll find it useful to understand how the BUN/creatinine ratio is used to distinguish various causes of uremia. The normal range is 12 to 20, and this value is obtained by dividing the BUN value by the creatinine value. For instance, if your patient's BUN level is elevated at 24 mg/dL and the creatinine level is normal at 0.6 mg/dL, the ratio is 40, which is high. Conversely, a low BUN level of 6 mg/dL and a creatinine level of 0.6 mg/dL yield a ratio of 10, which is low. Values below 12 may alert you to hepatic disease, acute tubular necrosis, or low protein reserves when a patient is consuming a low-protein diet or is in a starvation state. A high ratio of greater than 20 with a normal creatinine value suggests a catabolic state, high- Renal Diagnostic Tests protein diet, or gastrointestinal hemorrhage. When creatinine values are elevated, generating a high ratio, the most likely cause is renal failure. Putting it all together The best way to integrate this information into your practice is to routinely view labs related to renal function so you can recognize normal variations. After you're familiar with the normal values, focus on abnormal values and aim to come up with a plausible explanation as to why the value is out of range. You'll start to see patterns develop among patients with diabetes, older patients experiencing dehydration, or young female patients with a UTI, which will guide your physical assessment. Use the many resources available on the Internet to explore this topic and take advantage of free programs that may support you in your practice. cheat sheet Normal urinalysis * Appearance: clear * Color: light yellow to amber * Specific gravity: 1.005 to 1.030 * pH: 4.6 to 8 * Protein: Negative * Glucose: Negative * Ketones: Negative * Bilirubin: Negative * RBCs: Less than 3 cells/HPF * Nitrites: Negative * Leukocyte esterase: Negative * WBCs: Less than 5 cells/HPF * Casts: Negative Renal Diagnostic Tests cheat sheet Normal BUN and creatinine levels * BUN: 5 to 20 mg/dL * Creatinine: 0.8 to 1.2 mg/dL for men; 0.6 to 1 mg/dL for women * BUN/creatinine ratio: 12 to 20 Learn more about it Holcomb SS. Evaluating chronic kidney disease risk. Nurse Pract. 2005;30(4):12-14, 1718, 23-25. Kessenich CR. Urosepsis in the elderly. Nurse Pract. 2010;35(11):10-11. National Kidney and Urologic Diseases Information Clearinghouse. http://kidney.niddk.nih.gov/index.htm . National Kidney Disease Education Program. GFR MDRD calculator for adults. http://www.nkdep.nih.gov/professionals/gfr_calculators/idms_con.htm . Nephron Information Center. Cockcroft-Gault calculator. http://www.nephron.com/cgibin/CGSI.cgi .
