Microalbuminuria in the Dog: Detection and Significance

Crystal B. Fricks, DVM; Bruce LeRoy, DVM, PhD; Kenneth S. Latimer, DVM, PhD, and Holly Moore, DVM

Class of 2005 (Fricks) and Department of Pathology (LeRoy, Latimer, and Moore), College of Veterinary Medicine, University of Georgia, Athens, GA

Introduction

Veterinarians commonly use blood urea nitrogen and serum creatinine concentrations, coupled with urine specific gravity, to assess renal function. Unfortunately, these laboratory values only become abnormal when approximately 25 to 30% of renal function remains. By that time, renal disease is often too advanced for medical intervention to significantly help the condition. Recently, a semiquantitative, in-house test strip to identify microalbuminuria (MA) in canine and feline urine has been marketed. This test has the ability to detect MA before it can be detected by conventional urine dipsticks. Previous research in humans has suggested that MA is a good predictor of overt proteinuria and early renal disease. This project reviews previous studies done on MA and proteinuria, the significance of early detection of urinary protein loss as an early marker of renal disease, and its treatment.

Proteinuria and Microalbuminuria

In dogs and cats, the glomerular capillary wall is normally selectively permeable and excludes most proteins greater than 60,000 daltons from of the urine .¹ Albumin has a molecular weight of 69,000 daltons and thus should be excluded from the glomerular filtrate. In addition, the negative charge of the capillary wall excludes negatively charged proteins from the urine. Those small proteins that do pass the glomerular capillary wall are largely resorbed by the renal tubular epithelial cells. When the glomerulus is damaged or tubular epithelial cells are presented with amounts of protein that exceed resorptive capacity, proteinuria can occur. Furthermore, proteins can sometimes be secreted into the urine by the tubular epithelial cells.¹

Proteinuria can be transient due to physiological factors (seizures, fever, strenuous exercise, etc.) or pathologic changes. Non-transient proteinuria is most often categorized as prerenal, renal, or postrenal. Prerenal proteinuria occurs when increased concentrations of low molecular weight proteins present in the plasma overwhelm tubular resorptive capacity. Renal causes of proteinuria are numerous and include renal failure and certain inflammatory, infectious, and metabolic diseases to be discussed later. Postrenal proteinuria occurs due to lower urinary tract infection or hemorrhage. The presence of persistent proteinuria with a benign sediment is most often indicative of glomerular disease.

MA in the dog is defined as a urinary albumin concentration between 1.0 and 30.0 mg/dl.² This concentration is undetectable with conventional urine dipsticks. In humans, it has been shown that MA is a consistent predictor of proteinuria and is used to predict early renal disease associated with diabetes mellitus³ and essential hypertension.¹ Diabetic nephropathy is a leading cause of renal failure in humans,⁴ and efforts have been made to detect the early stages of renal disease in such patients. The Bayer Corporation has a semiquantitative test strip called Clinitek® Microalbumin to detect MA in human urine. One study compared these test strips to the gold standard, timed-overnight urine collection to test for MA in pediatric patients with Type 1 diabetes. ⁴ The study found that the Clinitek® Microalbumin strips were a reliable screening test for MA as a predictor of diabetic nephropathy. However, the test showed a somewhat low specificity prompting the authors to recommend that a positive Clinitek® Microalbumin test be verified by the timed-overnight method of measuring MA.

Testing for Canine Microalbuminuria

The most reliable and trustworthy measurement of MA and overt proteinuria is quantitative analysis of a 24-hour collection of urine. However, this method is too costly and time-consuming for most private veterinary practices. Traditional urinary dipsticks only detect overt proteinuria and can have false positive test results if the urine is alkaline or concentrated.⁵ False negative test results can occur if the urine is dilute, acidic, or if there is less than 30 mg/dl of albumin in the urine. A sulfosalicylic acid tests may also be used to detect proteinuria.¹ This test is more sensitive to albumin than dipsticks and will also detect Bence-Jones proteins, but can have false positive and false negative test results as well. Urine protein/creatinine (UPC) ratios are performed by reference laboratories to detect proteinuria and have been used to determine if the protein is of tubular or glomerular origin. Like other semi-quantitative tests, the UPC ratio does detect nonglomerular protein such as that from lower urinary tract infections. All semiquantitative tests for urinary protein should be interpreted with knowledge of the urine sediment findings and urine specific gravity, as inflammation may elevate protein concentrations in the urine. Heska Corporation offers a quantitative test in their laboratory to detect MA as well.

In 2002, a study evaluated the accuracy of the Bayer Clinitek® strips in detecting MA in canine urine.⁶ The objective of this study was to provide veterinarians with a quick, in-house system to dependably evaluate urine specimens for MA. The results of the Bayer Clinitek® strips did not wholly match the quantitative ELISA results enough for these strips to be a reliable screening test for MA in dogs.

In early 2002, Heska introduced a new semi-quantitative point-of-care test, the E.R.D. HealthScreen^TM canine and feline urine tests.⁷

This kit is marketed to detect MA within 5 minutes and identify patients in the earliest stages of renal disease. The test uses an anti-canine (or anti-feline) albumin antibody which, upon binding to albumin causes blue lines to appear. The intensity of the blue color is proportional to the concentration of albumin in the urine. Apparently, there is no cross-reactivity with microscopic hematuria, but some cross-reactivity does occur with pyuria and bacteriuria.⁸ One study showed that this assay was a more sensitive indicator of proteinuria than dipstick protein measurement and urine protein:creatinine ratios.⁹ Even though the detection of MA with a semiquantitative, in-house test is more sensitive than traditional methods of detecting proteinuria, its specificity still needs to be determined. In humans, as stated earlier, the Clinitek® Microalbumin strips did not show a reliable degree of specificity,⁴ and it was suggested that a positive test result should be investigated further.

Some Reported Causes of Microalbuminuria in the Dog

Strenuous exercise often causes a transient increase in MA. One study found that mild to moderate exercise did not increase MA in healthy dogs.¹⁰ This study measured MA using the E.R.D. HealthScreen^TM test and compared the results with a quantitative ELISA. However, the authors did mention that another study found that swimming did increase MA in dogs.

Some dogs, such as Soft-Coated Wheaten Terriers that develop glomerular disease, can be genetically predisposed to renal failure. A preliminary study found that these dogs and their cross-bred progeny had a higher prevalence of MA than clinically normal dogs.¹¹ In addition, dogs with X-linked hereditary nephropathy had persistent albuminuria approximately 4 weeks before overt proteinuria developed.¹²

There are many infectious and metabolic diseases that can cause MA in dogs as well. It has been shown that dogs experimentally infected with heartworms have a higher occurrence of MA than healthy dogs,¹³ but it has yet to be determined if these dogs develop overt proteinuria. Other diseases that have been reported to cause MA in the dog include leptospirosis, tick-born diseases, diabetes mellitus, Cushing’s disease, hyperthyroidism, hypertension, and some neoplasms.⁵

When conventional urine dipstick testing is performed, the protein colorimetric pad will react in patients with hematuria and/or pyuria. A study evaluating MA in these dogs demonstrated that many dogs with pyuria did not have albuminuria, but albuminuria was more likely in dogs with hematuria, bacteriuria, and pyuria.¹⁴ There was no cross-reactivity with microscopic hematuria, but albuminuria did occur with macroscopic hematuria. Urine samples with gross hematuria had normal urine protein:creatinine ratios. The authors of this study recommended that if a dog with pyuria is positive for MA, it should have follow-up evaluations for MA following treatment for the pyuria. More research needs to be done in such patients to determine if the positive MA test truly detects albumin or possibly cross-detects other proteins. It should be noted, however, that the dogs with pyuria, bacteriuria, and hematuria were not rechecked for the presence of MA after these conditions were resolved.

Microalbuminuria in Healthy Dogs?

Heska published a manuscript concerning a study of veterinary staff-owned dogs. ¹⁵ Approximately 25% of apparently healthy dogs tested positive for MA using the E.R.D. HealthScreen^TM test. Heska asked the participants to perform further diagnostics in the MA-positive patients to identify disease that may have been affecting the kidneys. Fifty-six percent of the respondents diagnosed some type of disease in their dogs (please see figures below). This indicates that approximately 50% of the dogs that had MA also had a concurrent (not necessarily underlying) inflammatory, infectious, neoplastic, or metabolic disease. It is unlikely that all of the above mentioned diseases will eventually result in a glomerulopathy or renal failure in most dogs. In addition, the severity and duration of the diseases in the MA-positive dogs is not known. Currently, other follow-up data has not been published concerning these MA-positive dogs. The authors comment that ongoing studies are being performed to determine the relationship of the diseases listed in Table 2 to MA.

Courtesy of http://www.heska.com/erd/includes/erd_data572.pdf

Conclusions

The Heska E.R.D. HealthScreen^TM test is still a relatively new product and the premise of predicting overt proteinuria and renal failure with MA measurements still needs further study. Large-scale research studies are currently being conducted to follow apparently healthy, MA-positive dogs longitudinally to see if the MA is a predictor of more serious renal problems. Once the significance that MA has on the overall health of a patient is fully understood, better clinical decisions can be made for the management and treatment of patients with a positive MA test result.

Persistent proteinuria is abnormal. Current research indicates that it is medically prudent to further investigate animals with persistent MA. However, the benefit of including the semiquantitative MA measurement in routine, geriatric, or wellness examinations is unclear at this point. The presence of a positive MA test does not necessarily mean the patient is going to develop overt renal failure or that treatment should be instituted. Some studies listed in this above indicate that there is a somewhat low specificity for these point-of-care, semiquantitative, MA tests. Until further research is conducted, a positive test result in an apparently healthy animal should be re-evaluated at a later date and persistent MA should be cautiously explored to reveal a potential underlying disease.

References

1. Nelson RW, Couto CG (eds.): Renal Disease. In: Small Animal Internal Medicine, 3rd ed., St. Louis, Mosby, 2003, pp. 579-586.

2. Vaden S: Microalbuminuria: What is it and how do I interpret it? Proc 21st ACVIM Forum, 2003.

3. Arun C, Stoddart J, Mackin P, MacLeod J, New J, Marshall S: Significance of microalbuminuria in long-duration Type 1 diabetes. Diabetes Care 26:2144-2149, 2003.

4. Meinhardt U, Ammann RA, Fluck C, Diem P, and Mullis PE: Microalbuminuria in diabetes mellitus: Efficacy of a new screening method in comparison with timed overnight urine collection. J Diabetes Complic 17:254-257, 2003.

5. Cleland P: Microalbuminuria testing: Why, who, when. Proc 41st Annual Veterinary Conference, University of Georgia, 2004.

6. Pressler B, Vaden S, Jensen W, and Simpson D: Detection of canine microalbuminuria using semiquantitative test strips desgined for use with human urine. Vet Clin Pathol 31:56-60, 2002.

7. News release. http://www.prnewswire.co.uk/cgi/news/release?id=80504. 2002.

8. Frank J: An update on the diagnosis of proteinuria in dogs. DVM Magazine. http://www.dvmnewsmagazine.com/dvm/article/articleDetail.jsp?id=73072. 2003.

9. Whittemore JC, Jensen WA, Prause L, Radecki S, Gill V, and Lappin MR: Comparison of microalbuminuria, urine protein dipstick, and urine protein creatinine ratio results in clinically ill dogs. J Vet Intern Med 17:437, 2003 (abstract).

10. Cary A, Cohn L, Kerl M, and Jensen W: The effects of exercise on urinary albumin excretion in dogs. J Vet Intern Med 18:52-55, 2004.

11. Vaden SL, Jensen WA, Longhofer S, and Simpson DF: Longitudinal study of microalbuminuria in Soft-Coated Wheaten Terriers. J Vet Intern Med 15:300, 2001 (abstract).

12. Less G, Jensen W, Simpson D, and Kashtan C: Persistent albuminuria precedes onset of overt proteinuria in male dogs with X-linked hereditary nephropathy. J Vet Intern Med 16:353, 2002 (abstract).

13. Grauer GF, Oberhauser EB, Basaraba RJ, Lappin MR, Simpson DF, and Jensen WA: Development of microalbuminuria in dogs with heartworm disease. J Vet Intern Med 16:352, 2002 (abstract).

14. Vaden S, Pressler, B, Lappin M, Jensen W: Effects of urinary tract inflammation and sample blood contamination on urine albumin and total protein concentrations in canine urine samples. Vet Clin Pathol 33:14-19, 2004.

15. Jensen WA, Cleland WP, Donnelly R, and Stinchcomb J: New data: Identification of underlying disease in dogs that test positive with the E.R.D.-HealthScreen^TM Canine Urine Test. Heska Corporation, Fort Collins, CO. www.heska.com/erdscreen, 2003.

Acknowledgement

The image detail of "Sammy" is from Dan and Rebecca Collins website artpaw.com and is used with permission.

^  Top of page