Formalin Fixation

• poor morphology
  • struggle to make diagnosis, if at all possible
• IHC
  • primary diagnosis
  • prognostic / predictive factors
    • ER, PR HER2/Neu
    • Mismatch repair proteins
    • PD-L1 and other biomarkers

Fixation

• stability of tissue components
• increases avidity for dyes

Poor Fixation

• degradation/sloughing of mucosa layer (like in colon)
• contour and detail

Irreversible Fixation

• covalent bonding by methylene bridges, altering structure
• over-fixation makes this worse, irreversible altered protein shape
• standardized IHC tests rely on a certain degree of how altered the protein shape gets
• Newfoundland breast cancer scandal Cameron Inquiry https://www.cihrt.nl.ca/transcripts.html
• particularly important for membranous pattern stains, like CD20 (as opposed to nuclear or cytoplasmic)

Decalcification

• destroys RNA, DNA
• particularly important to note for metastases (Primary diagnosis)