Which fixative is preferred for tissue prepared for Warthin-Starry staining?

• A. 10% NBF
• B. 70% ethanol
• C. Formalin
• D. Bouin's solution

Answer: (A)

Fixation method affects how silver-based stains perform. Warthin-Starry relies on silver reduction to deposit metallic silver at the sites of microorganisms, so the tissue must be preserved without introducing residues or pH changes that would alter the staining chemistry. Ten percent neutral buffered formalin is preferred because it preserves tissue morphology while keeping the pH stable and avoiding reactive additives that can interfere with silver deposition. This leads to more uniform, specific staining with less background. In contrast, Bouin’s solution contains picric acid and can leave residues that interfere with the silver reaction and create background color; ethanol fixation can cause dehydration and poor penetration, compromising stain quality; unbuffered formalin can cause pH shifts during fixation that affect the staining.