The prevalence of chronic kidney disease (CKD) in dogs may be as high as 1.5% of dogs presenting to general practice. Glomerular disease may be the cause of renal injury in 50% or more of dogs with CKD. Because glomerular diseases are common in dogs and the outcomes are not always favorable, there has been an enhanced effort to improve early recognition, diagnosis and treatment of all renal diseases in dogs, with particular emphasis on proteinuria renal diseases. In support of this effort, the WSAVA Renal Standardization Project developed a classification system for glomerular pathologic findings in dogs. At the same time, the International Renal Interest Society (IRIS) developed clinical guidelines for the management of dogs with glomerular disease. The latter initiative used a formal consensus method for developing several sets of recommendations, including one set about the clinical evaluation of dogs with protein losing nephropathy. The purpose of this presentation is to discuss those recommendations, while bringing in additional information about monitoring proteinuria and performing renal biopsy.

Diagnostic Recommendations

When proteinuria is detected, there are three key elements that need to be assessed: localization, persistence, and magnitude. Although proteinuria is the hallmark of glomerular disease, it can also occur secondary to pre-renal, post-renal, or renal tubular causes. The magnitude of proteinuria is generally assessed via the urine protein: creatinine ratio (UPC). Persistent renal proteinuria that is of high magnitude (i.e., UPC >2) is most likely of glomerular origin; however, there is no range of numbers that is diagnostic for any one renal disease and the overlap in expected ranges is too broad to be clinically reliable. In fact, glomerular lesions have been seen in dogs without proteinuria. Urine albumin can also be used to assess proteinuria. Microalbuminuria may develop before the UPC is increased. A dog with persistent microalbuminuria of increasing magnitude should be assessed as having an injurious process to the glomerular filtration barrier and may eventually develop overt proteinuria.

Clinicopathologic findings in dogs with various forms of glomerular disease are diverse and with findings being widely variable between individual dogs. Likewise, secondary complications of glomerular disease are variably present in individual dogs. Because of this, the consensus recommendations are to separate dogs with glomerular disease into different tiers based on their clinical manifestations. The tiers group dogs based on major clinical manifestations of disease and help direct the diagnostic recommendations. Each test or procedure should serve one of three purposes: 1) Identify an underlying disorder that might be causing the glomerular injury; 2) Assess the presence and severity of the different possible sequela of glomerular disease (e.g., azotemia, hypertension, hypercoagulability); and, 3) Characterize the pathologic changes in the kidney to better understand the diagnosis (and pathophysiologic development of that diagnosis), prognosis, and therapeutic options. The specific tests and procedures should be assigned a priority based on severity of disease, resources, expertise, and client willingness and finances. Tests that are considered essential (i.e., the minimum diagnostic assessment) for all dogs with glomerular disease include a history, physical examination, complete blood count, biochemical profile, urinalysis (including culture if indicated), UPC and regional infections disease testing. In most situations blood pressure measurement should also be considered essential. Additional testing may be classified as essential, recommended (i.e., what should always be done if resources permit) or potentially helpful (i.e., those that might be performed in specific circumstances or for a complete evaluation) dependent upon the tier the dog is in. These additional tests include abdominal ultrasound, chest radiographs, renal biopsy, antithrombin, thromboelastography, and SDS-PAGE.

Renal Biopsy

Renal biopsy provides a definitive diagnosis of glomerular disease, but may not be needed if treatment of a potential underlying disease leads to resolution of the proteinuria or end-stage renal disease is already present. Renal biopsy is generally not indicated in patients with CKD (stage IV and possibly late stage III). Results are unlikely to alter the prognosis, therapy, or outcome in these patients. When evaluated appropriately, renal biopsy specimens can provide important clinical information about the type and severity of lesions in dogs and cats with glomerular disease. In fact, obtaining an accurate histologic diagnosis may be one of the more important factors in successful management of the dog or cat with glomerular disease. Clinical decisions regarding the diagnosis, treatment, and prognosis can be made from the information obtained through renal biopsy.

The renal biopsy procedure requires expertise and should be performed only by experienced personnel. Hypertension and coagulation disorders should be controlled prior to biopsy. Moderate to severe thrombocytopenia, uncontrolled systemic hypertension, administration of a NSAID within the previous 5 days and operator inexperience may all increase the risk of post-procedural hemorrhage. Other contraindications to biopsy include renal cystic disease, moderate to severe hydronephrosis, pyelonephritis, perirenal abscess, severe anemia, and pregnancy. A relative contraindication to renal biopsy is the lack of access to a qualified and experienced renal diagnostic pathology center.

Renal biopsy samples can be obtained by one of several techniques:

When a specimen is to be used for evaluation of glomerular disease, only cortical tissue should be obtained; biopsy of the medulla is not needed and is associated with a greater risk of hemorrhage, infarction, and fibrosis. The use of general anesthesia is associated with an ability to obtain better-quality specimens and reduces the risk of severe hemorrhage. An adequate sample of cortex has a minimum of five glomeruli when examined by light microscopy, although one glomerulus may be all that is needed to make a definitive diagnosis in animals in which the disease is diffuse (i.e., one in which most glomeruli are involved) and easily recognizable (i.e., amyloidosis).

If a percutaneous method is used to obtain a renal biopsy specimen from a patient with glomerular disease, at least two quality samples of renal cortex (i.e., each >10 mm long) should be obtained, using either a 16- or 18-gauge needle. A dissecting microscope can be used to verify that adequate biopsy samples have been obtained. One sample should be placed in formalin, and the other should be divided into two smaller pieces containing glomeruli. One piece is put into a fixative suitable for transmission electron microscopy {TEM) (e.g., 4% formalin plus 1% glutaraldehyde in sodium phosphate buffer), and the other piece is frozen for immunofluorescent microscopy (IFM). An alternative to freezing is to immerse the tissue in ammonium sulfate-N­ethylmaleimide (i.e., Michel’s solution), which preserves tissue-fixed immunoglobulins. Wedge biopsies should be divided in a similar fashion; tissue for TEM should be minced appropriately. Tissue for TEM should be put into the fixative within 5 minutes of biopsy.

References

1.  IRIS Canine GN Study Group Diagnosis Subgroup et al. Consensus recommendations for the diagnostic investigation of dogs with suspected glomerular disease. J Vet Intern Med. 2013;27:S19–S26.

2.  Vaden SL, et al. Renal biopsy. In: Elliot J, Grauer G, eds. Manual of Canine and Feline Nephrology and Urology, 4th ed. British Small Animal Veterinary Association. In press.