Preanalytic Variables and Specimen Handling: Fixation, Processing, Sectioning, and Antigen Preservation
Why preanalytics dominate IHC
No antibody can rescue a specimen that was allowed to autolyze, dry out, or over-fix. The candidate must be able to trace the entire chain: warm ischemia at procurement, cold ischemia time (time from excision to fixative, ideally under 1 hour), fixative choice (neutral buffered formalin 10% is the standard; alcohol-based fixatives preserve DNA but alter many protein epitopes), fixative pH and osmolarity, fixation duration, gross section thickness, processing solvents, embedding, section thickness, and slide storage.
Fixation kinetics
Formalin penetrates at roughly 1 mm per hour and cross-links progressively. Under-fixation leaves the center of a thick block unfixed, giving strong edge staining and weak centers plus poor morphology. Over-fixation masks epitopes and demands harsher retrieval, which in turn increases background. For most surgical specimens 6-24 hours in 10% NBF is a practical target; for breast HER2 testing the accepted window is 6-72 hours. Decalcification with strong acids (hydrochloric, nitric) destroys many epitopes, so EDTA or formic acid is preferred when IHC is planned.
Sectioning and storage
Cut paraffin sections at 3-5 µm, mount on charged or adhesive slides for retrieval-heavy protocols, and bake at about 56-60 °C. Avoid prolonged high-heat baking and avoid leaving sections to dry before staining, both of which can create edge artifacts and reduce signal. Cut-and-baked slides should be stained promptly; antigen decay is slow but real for some markers.
Worked rule of thumb
A 2 cm solid tumor dropped whole into a cassette will be unfixed at its center even after overnight processing. Trim to 3-4 mm thick, orient, and fix 6-24 hours. If staining is strong at edges and absent in the middle, suspect fixation before you suspect the antibody.
Traps in questions
"Longer fixation is always better" is false. "Overnight is always sufficient for any specimen" is false for thick or fatty tissue. Also remember that necrotic and autolyzed areas stain nonspecifically and should not be read as true positivity.