Formalin pigment AFH may be removed from tissue by which method?

• A. Alcoholic picric acid
• B. Sodium thiosulfate
• C. Acetic acid
• D. Hydrogen peroxide

Answer: (A)

Formalin pigment is an acid formalin–hematin pigment that can form in tissues fixed with formalin. It looks like a brown to black deposit and, if not removed, can be mistaken for true pigments. The way to reliably remove it is by using alcoholic picric acid. In ethanol with picric acid, the formalin pigment forms a soluble picrate complex, allowing it to be washed away without harshly affecting the tissue or other stains. This makes alcoholic picric acid the best choice for clearing this pigment before staining.

The other reagents don’t provide the same dissolving effect on this pigment. Acetic acid, hydrogen peroxide, and the other listed options don’t form the necessary soluble complex with the pigment, and thus aren’t considered effective for removal in standard protocols.