Acid-fast stains are satisfactory on tissue fixed in Carnoy fixative.

• A. True
• B. False
• C. Only for frozen tissue
• D. Not fixed with Carnoy ever

Answer: (B)

Acid-fast staining depends on the lipid-rich, waxy cell wall of acid-fast organisms to resist decolorization. Carnoy fixative (ethanol, chloroform, acetic acid) is highly dehydrating and extracts lipids from tissues and bacterial walls. This lipid removal disrupts the mycolic acid–containing surface that makes acid-fast staining possible, so the organisms no longer retain the red dye after the acid-alcohol step. Therefore, tissue fixed in Carnoy fixative does not yield satisfactory acid-fast staining. For reliable results, use fixatives that preserve lipids and the cell wall, such as formalin-based fixatives, rather than Carnoy.