Thin basement membrane nephropathy cannot be diagnosed reliably in deparaffinized, formalin-fixed tissue

Area of Science:

• Renal Pathology
• Diagnostic Techniques
• Electron Microscopy

Background:

• Electron microscopy (EM) is crucial for diagnosing renal pathologies, typically using glutaraldehyde-fixed tissue (EM-G).
• When glomeruli are absent in EM-G samples, formalin-fixed, paraffin-embedded tissue (EM-F) can be reprocessed as a salvage technique.
• The diagnostic utility of EM-F for thin basement membrane nephropathy (TBMN) remains uninvestigated.

Purpose of the Study:

• To systematically evaluate the usefulness of reprocessing formalin-fixed, paraffin-embedded tissue (EM-F) for diagnosing thin basement membrane nephropathy (TBMN).
• To compare glomerular basement membrane (GBM) thickness measurements obtained by EM-F versus standard glutaraldehyde-fixed electron microscopy (EM-G).

Main Methods:

• Compared GBM thickness in 21 renal biopsies using both EM-G and EM-F.
• Biopsies included cases of TBMN, normal controls, minimal change disease (MCD), and diabetic nephropathy (DN).
• Analyzed GBM thickness reduction and diagnostic thresholds between the two EM preparation methods.

Main Results:

• EM-F demonstrated a significant reduction in GBM thickness across all diagnostic categories compared to EM-G.
• Mean GBM thickness reduction was 23% for TBMN, 40% for normal/MCD, and 34% for DN cases.
• Four MCD cases showed mean GBM thickness below the TBMN diagnostic threshold using EM-F; TBMN diagnosis by EM-F was unreliable, with a 99th percentile of 194 nm.

Conclusions:

• Reprocessing formalin-fixed tissue for electron microscopy (EM-F) introduces artifactual GBM thinning.
• This artifactual thinning precludes accurate diagnosis of thin basement membrane nephropathy (TBMN).
• Renal pathologists must be aware that EM-F is unsuitable for TBMN diagnosis due to unreliable GBM thickness measurements.