Reading Your Kidney Biopsy Report

A kidney biopsy is the most direct way to diagnose Fibrillary Glomerulonephritis (FGN) or Immunotactoid Glomerulopathy (ITG) ^[1]. Because these conditions are so rare, your nephrologist sends a small sample of kidney tissue to a specialized pathology lab where it is examined using three different methods.

It is important to know that pathologists do not look at any single test in isolation. The diagnosis is integrated by a renal pathologist and nephrologist, who look at all three methods together, along with your bloodwork and clinical history.

The Three Pillars of Your Report

Your pathology report is generally divided into three main sections. Each provides a different “zoom level” on your kidney tissue:

1. Light Microscopy (LM): The pathologist uses a standard microscope to look at the overall structure of the glomeruli (filters). In FGN and ITG, this often shows scarring, inflammation, or thickening, but it cannot see the tiny protein strands themselves ^[2].
2. Immunofluorescence (IF): The pathologist applies glowing dyes that stick to specific proteins, like antibodies (immunoglobulins). This reveals the composition of the proteins stuck in your kidney ^[1].
3. Electron Microscopy (EM): This uses a powerful beam of electrons to see structures thousands of times smaller than a human cell. This allows the pathologist to measure the size and shape of the protein deposits ^[1][3]. (Occasionally, a biopsy sample might not contain enough tissue for EM, requiring doctors to rely heavily on the other tests.)

Electron Microscopy: Fibrils vs. Microtubules

The EM section describes the shape, size, and arrangement of the deposits. While there can be overlap, pathologists use these general patterns to help distinguish FGN from ITG:

• Fibrillary (FGN): Typically shows fibrils that are solid (no hollow center) and randomly arranged ^[4][3]. They are usually small, approximately 12 to 24 nanometers in diameter ^[1].
• Immunotactoid (ITG): Typically shows microtubules. These are usually larger (often greater than 30 nanometers) and have a hollow center (a lumen) ^[4][1]. These tubes are often neatly lined up in parallel bundles ^[1].

Note: These are approximate ranges. The size alone is not an absolute rule, which is why other stains are needed.

The DNAJB9 Stain: A Strong Indicator for FGN

A test called DNAJB9 immunohistochemistry has become a vital tool. DNAJB9 is a specific protein that is highly sensitive and specific for FGN; it is very rarely found in amyloidosis or ITG ^[5]. If your biopsy shows a positive stain for DNAJB9, it strongly supports an FGN diagnosis ^[5]. However, a missing DNAJB9 stain does not make a report “incomplete,” especially if ITG or another condition is suspected.

The Congo Red Stain: The Amyloid Screen

Because FGN and amyloidosis both involve thread-like “fibrils,” pathologists use a red-orange dye called Congo Red.

• Usually Negative: Classically, FGN and ITG do not pick up this stain ^[3].
• Congophilic FGN: Rarely, FGN can test positive for Congo Red. If this happens, DNAJB9 testing, expert review, or mass spectrometry is used to ensure it is FGN and not amyloidosis ^[6].

Light Chains: Polyclonal vs. Monoclonal

In the Immunofluorescence (IF) section, the pathologist looks at the “light chains” of the antibodies, called Kappa and Lambda.

• Polyclonal (Mixed): Both Kappa and Lambda are present. This means the proteins come from a variety of immune cells. This is typical for most cases of FGN ^[7].
• Monoclonal (Restricted): Only one type (either Kappa or Lambda) is present. This is very common in ITG and may suggest an underlying blood disorder, known as a “monoclonal gammopathy.” ^[7][8].

Note: Exceptions exist. Some ITG is polytypic, and some FGN can appear monotypic. Apparent restrictions on IF are always correlated with your blood tests.

Biopsy Discussion Guide

When reviewing the report with your doctor, you can ask about how these pieces fit together:

• Electron Microscopy (EM): What was the approximate size and shape (solid fibrils vs. hollow microtubules) of the deposits?
• DNAJB9 Stain: Was this performed, and did it support FGN?
• Congo Red Stain: Was it negative as expected, or positive requiring more tests?
• Immunofluorescence (IF): Did the light chains show a mixed (polytypic) or restricted (monoclonal) pattern?