Evaluation of Renal Biopsy Samples

Nephropathology is unique among the subspecialties of anatomic pathology.

It routinely involves ultrastructural (transmission electron microscopic) examinations, histologic (light microscopic) findings, and immunostaining (immunofluorescence microscopy) for the diagnosis of common diseases, especially those affecting glomeruli (Figure 1).

Pathologic findings must be carefully integrated with information derived from the clinical investigation of the patient’s illness to formulate the correct diagnosis.

Images from the histologic (A and B), ultrastructural (C), and immunostaining (D) examinations that are routinely performed by the Texas Veterinary Renal Pathology Service. The images are from a dog with membranous glomerulonephropathy: A and B show histologic sections with special stains—glomerulus stained with periodic acid–Schiff stain (A; original magnification, 400×)

and a portion of the same glomerulus stained with Masson’s trichrome (B; original magnification, 1000×). With the trichrome stain, red granular immune deposits can be seen along the capillary walls. Transmission electron microscopy (C) shows multiple electron-dense deposits under the visceral epithelial cells (podocytes) on the outer surface of the glomerular basement membrane of a single capillary loop (original magnification, 15,000×). Fluorescence microscopy (D) shows granular labeling for IgG distributed on all capillary walls (original magnification, 400×).

Renal Pathology Service
The Texas Veterinary Renal Pathology Service (TVRPS) was established in 2005 at Texas A&M University. This service gives veterinarians in North America access to the technical capabilities and expertise required to perform thorough pathologic evaluations of renal biopsy specimens.

The mission of TVRPS is to improve health care for individual patients with kidney disease and generate new knowledge about renal diseases in animals by expertly performing thorough pathologic evaluations of kidney specimens obtained from dogs and cats, as well as other animals.

Specific Goals

• Perform histologic, ultrastructural, and immunohistologic examinations of the kidney that will help veterinarians, especially nephrologists and specialists in academic and private practices, evaluate patients with kidney disease.
• Effectively communicate in a timely manner with each patient’s attending clinician about the pathologic findings and their implications.
• Collect detailed clinical information about the features, course, response to treatment, and outcome of pathologically well-defined kidney diseases in animals and integrate the information with the pathologic findings.
• Generate and maintain a database containing clinical and pathologic information collected about pathologically well-defined kidney diseases in animals.
• Use the resources of TVRPS, including the database and technology for gathering and managing the data, to facilitate retrospective and prospective studies that will advance understanding of the causes, treatment, and prevention of renal diseases in dogs, cats, and other animals.

Indications
The general indications for renal biopsy are the topic of Ask the Expert: Should You Perform a Renal Biopsy?.

The comprehensive evaluation of renal biopsy samples obtained from TVRPS is indicated most often for proteinuric dogs or cats that are suspected to have glomerular disease. This indication accounts for approximately two thirds of biopsy samples submitted. However, renal biopsy specimens are accepted for evaluation regardless of the category of kidney disease for which they were obtained, and the methods of pathologic examination used in each case are tailored to the condition being evaluated.

Advantages

• Upon request, users are sent renal biopsy kits containing the materials and instructions needed to submit satisfactory specimens for comprehensive evaluations, including the fixatives and preservatives for electron microscopy and immunostaining (Figure 2).
• Proficiency in performing and interpreting the specialized pathologic examinations that are required is maintained by evaluating a sufficiently large caseload (> 100 cases/year) and focusing only on pathologic evaluation of renal disease.
• Evaluations are performed by a team of veterinary nephrologists and pathologists who have a thorough knowledge of renal medicine as well as renal pathology.
• Pathology reports routinely include photomicrographs showing the pathologic changes and are distributed electronically (usually as email attachments).
• A limited-access website is available to authorized users who wish to use it to provide clinical information about submitted cases or to review their reports (including the photomicrographs) online.

Contents of a renal biopsy kit as provided by TVRPS: The container with the green cap and white label is for 10% formalin, which is to be supplied by the user and is the fixative used for samples to be examined by light microscopy. The vial with the white cap and red label contains 3% glutaraldehyde, which is the fixative used for samples that will be examined by transmission electron microscopy. The vial with the black cap and blue label contains Michel’s transport media, which is a preservative used for samples that are intended for immunostaining. Other kit contents include plastic pipettes, single-edged razor blades (still in their cardboard sheaths), and large glass slides, which are useful platforms on which to work with biopsy specimens. The forceps shown, which are not included in the kit, have smooth (not toothed or serrated) tips and are suitable for delicate handling of small, fragile tissue specimens.

Disadvantages

• It is necessary to contact the TVRPS directly at least 2 to 3 days before performing a renal biopsy to obtain the necessary materials and instructions for submitting suitable specimens (ie, to request and receive a renal biopsy kit).
• Some components of the biopsy kits must be kept refrigerated and have a limited shelf-life. As a result, users who choose to keep a stock of kits on hand (for use on short notice) must monitor the expiration dates for those components and request replacements in a timely manner.

Reliability of Results

• At TVRPS, renal biopsy samples are examined by a team of veterinary nephrologists and pathologists who have substantial experience (> 350 cases since 2005) with comprehensive evaluations of naturally occurring renal diseases in dogs and cats. However, the development of evidence-based information about interpreting renal biopsy findings and applying them effectively to improve clinical care of patients is still in its infancy in veterinary medicine. The current strategy is to cautiously apply what is currently known while learning more as quickly as possible.
• The TVRPS is a component of a larger, international group of veterinary nephrologists and pathologists who are focused on improving pathologic evaluation and diagnosis of renal diseases in companion animals (the International Veterinary Renal Pathology Initiative) under the auspices the World Small Animal Veterinary Association.

Economic Impact

• Shipping costs to submit specimens can be greater than for other biopsy samples because the samples should be kept chilled in transit (packaged in an insulated box with “wet ice”­ cold packs) and designated for next-morning delivery.
• Provision of the biopsy kit (delivered) is included in the fees charged for comprehensive pathologic evaluations, which are as much as $600 (fee schedule available upon request).