A Brief Review of Creatinine Concentration

Shannon Cook Miller, DVM; Bruce E. LeRoy, DVM, PhD; Heather L. Tarpley, DVM; Perry J. Bain, DVM, PhD, Kenneth S. Latimer, DVM, PhD

Class of 2004 (Miller) and the Department of Pathology (LeRoy, Tarpley, Bain, Latimer), College of Veterinary Medicine, University of Georgia, Athens, GA 30602-7388

Introduction

Creatinine concentration is most commonly used in veterinary medicine as an indirect indicator of renal glomerular filtration rate and to thereby estimate renal function. It may also be measured in peritoneal fluid for the evaluation of uroabdomen, because it is concentrated in the urine and slow to equilibrate between fluid compartments, taking over 4 hours to do so.

Background

Creatinine (Fig. 1) is a byproduct of the breakdown of creatine (Fig. 2) and phosphocreatine (Fig. 3), an energy storage compound in muscle, and has a molecular weight of 113 daltons. Creatinine, in actuality, has poor sensitivity in diagnosing renal disease in the dog and cat because three-fourths of renal function must be lost before abnormalities in creatinine concentration are seen. In addition, elevations in creatinine concentration do not correlate with a proportionate loss of renal function, further complicated by the body’s diversion pathways (discussed in the following paragraphs) to metabolize an overabundance of creatinine. Despite these facts, creatinine clearance is a reliable test of renal glomerular filtration rate (GFR) due to specific elimination of creatinine by the kidney and general lack of metabolism elsewhere in the body. GFR is currently considered the best overall indicator of renal function.⁹

Figure 1. Creatinine is a waste product derived from the enzymatic conversion of creatine by creatine amidohydrolase.

Figure 2. Creatine can be derived from phosphocreatine by the action of creatine kinase or from the enzymatic conversion of guanidinoactetate by guanidinoacetate-N-methyltransferase.	Figure 3. Phosphocreatine is an energy source in muscle that gives rise to creatine, that is subsequently converted to creatinine.

Creatinine is water-soluble and distributes throughout the body water, equilibrating between various fluid regions in approximately 4 hours with the help of Na/Cl transporters on cell membranes. This equilibration time is longer than that for urea, which equilibrates within 1½ hours. Because creatinine is water soluble, it is present in small concentrations in sweat (0.1-1.3 mg/dl). Creatinine also has been detected in peritoneal fluid, synovial fluid, bronchoalveolar lavage fluid, and aqueous and vitreous humor.⁸ Research in the 1950s and 1960s discovered small quantities of creatinine in the vomitus and feces of uremic human beings.³ Similar research findings in animal species have not been reported. Scientific technology has changed dramatically since those times, with more specific enzyme mechanisms available now for testing purposes that were not available at the time the research was performed. However, interspecies data is lacking making meaningful comparisons impossible at this time.

Creatinine is freely filtered by the renal glomerulus. Tubular reabsorption does not occur. Studies indicate that a miniscule amount of proximal tubular secretion may occur in male dogs, but that the amount is not significant and does not interfere with creatinine’s value as an indicator of GFR. This tubular secretion does not occur in cats and ponies, but is considerable in the goat.³ A small amount of creatinine is also excreted in the small intestine and is degraded by the enteric flora; a negligible amount of recycling most likely occurs but is not considered to significantly alter serum creatinine concentrations. Alimentary tract excretion is believed to occur in the dog, cat, and horse, as creatinine is diffusible across most cell membranes.²

Because of its slow equilibration, dialysis is less effective in lowering the serum creatinine concentration than it is for BUN concentration. It takes at least 4 hours for the creatinine to passively make its way along the diffusion gradient from the serum to the ultrafiltrate that is urine, if the glomerulus is not functioning properly. Therapeutically, diuresis of the patient is clinically important to lower the serum creatinine concentration, but it also is imperative to improve renal bloodflow in order to create the maximum reduction in serum creatinine concentration.

Elevations in creatinine concentration do not cause uremia. Uremia is defined as the clinical signs of illness due to renal failure. Past studies in humans have shown that a patient with an elevation in BUN concentration is most likely to have clinical signs of uremia as compared to a patient with the same creatinine value and a lower BUN concentration.³

Variations in Creatinine Concentration

The serum creatinine concentration can vary based on a number of factors including an animal’s diet, muscle mass, and gender. Diets that contain high concentrations of muscle offer a large pool of creatine and creatinine that are absorbed in the small intestine and contribute to the serum concentration of creatinine. Muscle mass harbors the precursor of creatinine, phosphocreatine (Fig. 3), as an energy source. This compound is often mistakenly referred to as "phosphocreatinine." A constant amount of phosphocreatine is spontaneously, irreversibly and nonenzymatically converted to creatinine daily and utilized by the body. This amount is directly proportional to the individual’s muscle mass. Therefore, a stable amount of creatinine is presented to the kidneys daily for excretion. Muscle disease or wasting decreases the amount of phosphocreatine available for conversion and thereby decreases the serum creatinine concentration. Conversely, an increasing muscle mass from conditioning or exercise will result in an increase of phosphocreatine and serum creatinine. Males often have higher creatinine values than females, as well.¹ This finding is most likely due to their typically increased muscle mass as compared to females.

Serum creatinine values also depend on the kidney’s ability to excrete creatinine. An elevation in creatinine is called azotemia and usually occurs simultaneously with an increase in blood urea nitrogen, a compound that is also freely filtered by the glomerulus. This can be due to prerenal, renal or postrenal processes causing a decrease in GFR. Examples of prerenal processes that could cause azotemia include dehydration and some medications, such as gentamicin, oxytetracycline, amphotericin B, trimethoprim-sulfadiazine, and furosemide.⁸ Renal processes could encompass anything from congenital or breed abnormalities like Cocker Spaniel familial nephropathy or Greyhound glomerular vasculopathy, to acquired renal failure from such causes as amyloidosis or intoxications by sodium arsenate or vitamin D.⁸ Postrenal causes include urinary tract obstruction or rupture.

There are other non-renal diseases that can create an elevation in serum creatinine concentration from secondary renal involvement. In the dog, infestation with Dirofilaria immitis, Babesia canis, Ehrlichia canis, Leishmania sp., or Leptospira sp. can create significant increases in creatinine concentration that may mistakenly indicate the kidneys as part of the disease process. Disease processes like pyometra, gastric dilatation / torsion, diabetes mellitus and hypercalcemia of malignancy also can create secondary renal injury and elevate serum creatinine levels.⁸

A decreased serum creatinine value is not recognized as being clinically significant and may be the normal value for the patient, as individual variations are seen based on gender, age, muscle mass and many additional factors.

Creatinine Quantitation

The creatinine concentration is most commonly determined by Jaffe’s reaction, a process that takes advantage of the color change present when alkaline picrate solution is exposed to creatinine. This is the methodology used in the UGA CVM Clinical Pathology Laboratory. The reaction, however, is not specific and can be falsely influenced by almost 50 compounds other than creatinine. These interfering compounds are referred to as noncreatinine chromagens. False-high test values in serum can result from the presence of these noncreatinine chromagens, such as ketones, glucose, fructose, ascorbic acid, protein, urea and ascorbic acid.³ Balint and Visy proved that noncreatinine chromagens were absent from the urine of the dog in 1965, which implies that there are alternate elimination pathways for these compounds. This implies that the Jaffe reaction is an accurate method for determining urine creatinine concentration, as there are no compounds present to interfere with the test result.³

More specific methods are now available for creatinine measurement, but are expensive and limited in their availability. A clay that specifically absorbs creatinine prior to reacting with alkaline picrate is available, but is difficult to manipulate and is often messy. Enzyme-specific methods are available for various domestic species but are limited in their availability. These methods are based on the use of creatinine amidohydrolase or creatinine inimohydrolase. However, they may be inaccurate when serum bilirubin concentrations are greater than 50 µmol/L.⁸

Creatinine is remarkably stable in blood samples and is unaffected by hemolytic or lipemic samples. Creatinine also is relatively stable in stored serum and heparinized plasma. Thoresen and colleagues⁷ attempted to mimic the storage temperatures that would most likely be present for many samples submitted by mail to a diagnostic laboratory. Samples for their test were refrigerated overnight and kept at room temperature for the remainder of the study. Creatinine values were shown to remain constant for approximately 3 days in non-refrigerated heparinized plasma and up to 7 days in stored, non-refrigerated whole blood.⁷ Serum creatinine concentration has been shown to be higher than that found in plasma. The majority of reference intervals have been determined on serum samples.⁸

Creatinine Clearance Tests

Creatinine is used to evaluate GFR because it is almost exclusively cleared by glomerular filtration and is neither secreted nor absorbed by the renal tubules. Creatinine is not significantly metabolized or excreted anywhere else in the body (insignificant but amounts are excreted in sweat and feces). Creatinine clearance provides a reasonable estimation of GFR in domestic animals, but is not equivalent to GFR. A decrease in clearance indicates a decrease in GFR, but it cannot differentiate causes of decreased GFR due to prerenal, renal or postrenal factors. When performed correctly, a creatinine clearance test provides the benefit of detecting approximately 20% decrease in renal function, as compared to the 75% functional loss that is necessary before elevations in BUN and creatinine concentrations are detected.³ Two methods are most commonly used to evaluate GFR. These are the endogenous and exogenous creatinine clearance tests. These tests may only be performed in well hydrated, non-azotemic animals.

The endogenous creatinine clearance test involves a measurement of the starting serum creatinine concentration, complete evacuation of the bladder, collection of all urine created in a set amount of time (20 minutes to 24 hours) and determination of a final serum creatinine concentration. The urine creatinine value is determined. The following calculation is used to compute the endogenous creatinine clearance:

Creatinine clearance = [urine creatinine (mg/dl) x urine volume (ml/min) ÷ serum creatinine (mg/dl)] ÷ body weight (kg)

The endogenous creatinine clearance test is based on the principle that serum creatinine is a constant value for that patient because of the irreversible nonenzymatic catabolism of phosphocreatine from the muscle. Inaccuracies with this method are possible due to incomplete evacuation of the bladder both before or after the procedure, tubular secretion of creatinine in some male dogs, possible increased extrarenal secretion of creatinine (such as in the gastrointestinal tract) and measurement of noncreatinine chromagens like ketones that occur in the serum sample but not in the urine sample, as the chromagens are not present in the urine. Patients that undergo this test usually are suspected of having renal disease because they are polyuric, not azotemic.

The plasma exogenous creatinine clearance test (PECCT) is very similar to the iohexol clearance test, currently considered the gold standard in human medicine. The PECCT is useful for evaluating renal function in cases of known renal disease, detecting early renal dysfunction in predisposed breeds, evaluation of GFR during or after certain drug protocols that are nephrotoxic (like aminoglycoside therapy) and monitoring continued GFR failure in cases of known renal failure.⁶ The PECCT has been shown to be an accurate indicator of GFR in healthy dogs and in dogs with an estimated 60% or greater decrease in their GFR.⁹ A pre-injection serum creatinine sample is collected, the bladder emptied and an injection of creatinine is administered subcutaneously, intramuscularly or intravenously. After a set period of time, 20 minutes to 24 hours, all of the urine created is collected and a post-injection serum creatinine value is determined. The same calculation as described above is used to determine this value. This test is believed to present an increased challenge to the kidney because of the presence of the exogenous creatinine in addition to the endogenous creatinine concentration and is thought to better represent the true GFR value. Also, with the increased amount of creatinine present, the error from the presence of noncreatinine chromagens in the serum becomes less significant. Serum and urine creatinine values measured over a 10 hour period give the practitioner the most accurate picture of renal health, although reliable information can be gained with fewer samples in a shorter time period, if desired. However, there is a lack of standardization of methods between practioners that makes comparisons difficult. Also, it is unknown whether some of the exogenous creatinine is shuttled to the proximal tubules and excreted there in larger amounts as the kidney copes with the elevation in creatinine challenge.²

Other Methods for Evaluating Creatinine

Urine creatinine / serum creatinine ratios have been suggested by some to aid in differentiation of prerenal from renal azotemia. These ratios are of questionable value as they are based on single measurements and do not take into consideration urine flow rate or volume during the test window. An example of this includes electrolyte clearance as compared to the endogenous creatinine clearance to evaluate renal function. Fractional clearance of sodium is used most commonly. A single serum sample is taken to measure sodium and creatinine concentrations and is compared to the concentrations seen in a urine sample taken at the same time. The following formula is used to calculate these "spot" measurements:

Fractional clearance of sodium (%) = (urine Na/serum Na) ÷ (urine creatinine/serum creatinine) x 100

A percentage >1 may indicate tubular failure, prerenal azotemia and concurrent administration of diuretics or renal azotemia. A percentage <1 is likely to indicate prerenal azotemia. This function test is of questionable use in veterinary medicine.¹

Serum blood urea nitrogen to creatinine ratios also have been suggested as a method to differentiate between prerenal and renal azotemia. Comparisons of these two values are not significant enough to differentiate prerenal from renal disease or acute from chronic renal failure.²

Urine concentrating tests are often indicated in situations with renal dysfunction. The patient typically has polydipsia and polyuria with submaximally concentrated urine. In most species, polyuria precedes azotemia during the progression of renal disease.³ This is not true in the cat, which may maintain superior concentrating abilities even with extreme azotemia. However, urine concentration tests in polyuric animals are not indicated if azotemia or dehydration is present (and diuretics are not being administered) since the combination of polyuria with either azotemia or evidence of dehydration indicates renal disease.

Uroabdomen

As mentioned previously, creatinine may be useful in the diagnosis of uroabdomen. Uroabdomen is most frequently associated with rupture of the bladder or urethra due to abdominal or pelvic trauma.⁵ Many of these animals have concurrent pelvic fractures or penetrating abdominal wounds. Uroabdomen also may be caused by non-traumatic processes, such a urinary tract obstruction. Regardless of cause, uroabdomen results in clinical signs of severe dehydration with or without the presence of urination, life-threatening hyperkalemia that can cause bradycardia, severe azotemia, metabolic acidosis with or without respiratory compensation, and chemical peritonitis. Other electrolyte disorders, such as hyponatremia and hypochloremia also may exist. Creatinine is a relatively large molecule (113 daltons) that is primarily concentrated in the urine bathing the abdomen and is slow (over 4 hours) to diffuse into the blood. Creatinine also acts as an osmotic agent, pulling fluid from extracellular and intracellular spaces while dehydrating the animal (a prerenal factor). As the creatinine value in the blood stream increases, so will the BUN concentration. This will eventually lead to clinical signs of uremia, such as vomiting, which will worsen dehydration. The potassium in the urine will rapidly diffuse into the bloodstream as well, creating dangerously high concentrations that can initiate cardiac arrhythmias and damage myocytes. The hydrogen ions normally excreted in the urine also will be retained creating a state of metabolic acidosis. The dehydration will decrease the glomerular filtration rate, thereby decreasing the excretion of creatinine and urea. The retention of the urine inside the abdomen creates a continual source of creatinine, urea and corrosive excretory products. These corrosive products will soon create a chemical peritonitis, which results in functional ileus and abdominal pain.

Uroabdomen is diagnosed by a combination of patient history, physical examination findings, complete blood cell count, serum chemistry results, abdominal radiographs, abdominal fluid analysis and/or contrast studies of the urinary system. A definitive diagnosis of uroabdomen is obtained by determining the creatinine concentration in the abdominal fluid and comparing it to the serum creatinine value. A uroabdomen sample will have a creatinine value that varies from slightly to markedly greater than that of the serum. Usually a value greater than two times serum creatine concentration is diagnostic for uroabdomen.

Additional Species Information

With the popularity of canine rescue and adoption, it is necessary to become familiar with breed variations in creatinine concentration. Greyhounds adpoted from the racetrack are seen more commonly in private practice. These dogs often have blood and biochemical values that are outside of most canine reference ranges, but these values are considered normal for the breed in racing condition. Studies by Porter and Canaday⁴ show that elevations in creatinine, mean red blood cell count, packed cell volume, mean corpuscular hemoglobin concentration, and serum sodium, chloride and bilirubin concentrations are normal for this breed as compared with laboratory reference intervals which are often based on samples from mixed breed dogs. This elevated baseline creatinine concentration of racing Greyhounds can make it difficult for a practicioner to evaluate renal function, if presented with a patient showing signs of polyurina, polydipsia or uremia.⁴

Creatinine concentration is more specific than BUN with regard to renal function in the cow and the horse, because gastrointestinal excretion or absorption do not occur.¹ The rumen microflora metabolizes a larger percentage of urea than the enteric flora in monogastrics, often preventing BUN concentrations from increasing proportionately with creatinine levels in cattle. Once anorexia occurs, there is often a rapid elevation in serum BUN concentration to match the elevated creatinine concentration. This often occurs in the final stages of renal failure.

Creatinine also is used as a rough indicator of GFR in birds. In addition, reference intervals are published for many species.¹⁰

References

1. Latimer KS, Mahaffey EA, Prasse KW (eds). Duncan & Prasse's Veterinary Laboratory Medicine: Clinical Pathology, 4th ed., Iowa State Press, Ames, IA, 2003.

2. Stockham SL , Scott MA. Fundamentals of Veterinary Clinical Pathology, Iowa State Press, Ames, IA, 2002.

3. Kaneko JJ, Harvey JW, Bruss ML. Clinical Biochemistry of Domestic Animals, 5th ed. Academic Press, San Diego, CA, 1997.

4. Feeman WE, Couto CG, Gray TL. Serum creatinine concentrations in retired racing Greyhounds. Vet Clin Pathol 32:40-42, 2003.

5. Gannon K, Moses L. Uroabdomen in dogs and cats. Compend Contin Educ Pract Vet . 24:604-611, 2002.

6. Watson AD, Lefebvre HP. Assessment of renal function using creatinine: New insights into an old question. Proc of the Am College Vet Intern Med, 2003.

7. Thoresen SI, Havre GN, Morberg H, Mowinckel P. Effects of storage time on chemistry results from canine whole blood, heparinized whole blood, serum and heparinized plasma. Vet Clin Pathol 21:88-94, 1992.

8. Braun JP, Lefebvre HP, Watson AD. Creatinine in the dog: A review. Vet Clin Pathol 32:162-179, 2003.

9. Watson AD, Lefebvre HP, Concordet D, Laroute V, Ferre JP, Braun JP, Conchou F, Toutain PL. Plasma exogenous creatinine clearance test in dogs: Comparison of other methods and proposed limited sampling strategy. J Vet Intern Med 16:22-33, 2002.

10. Fudge A. Avian and Exotic Pets Laboratory Medicine, WB Saunders Co, Philadelphia, PA, 2000.

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