Formalin Fixation

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  • 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


  • 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
  • particularly important for membranous pattern stains, like CD20 (as opposed to nuclear or cytoplasmic)


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