Which stain may give false-positive results after glutaraldehyde fixation?

• A. PAS
• B. Alcian Blue
• C. Congo Red
• D. Thioflavin T

Answer: (A)

Glutaraldehyde fixation leaves reactive aldehyde groups in tissue. The PAS method relies on converting certain carbohydrates into aldehyde groups that then react with Schiff reagent to give magenta. But those fixation-derived aldehydes can react directly with Schiff reagent, producing a magenta color even when carbohydrates aren’t present. That makes PAS staining give false-positive results after glutaraldehyde fixation. Other stains use different chemical targets (Alcian Blue for acidic mucopolysaccharides, Congo Red and Thioflavin T for amyloid) and aren’t driven by aldehyde groups from the fixative, so they’re not as susceptible to this specific artifact.