Formalin pigment formation is promoted by acidic fixation; which adjustment reduces pigment formation?

• A. Keep fixative pH above 6.0
• B. Lower fixative pH below 4.0
• C. Increase fixation temperature
• D. Decrease fixation time

Answer: (A)

Formalin pigment forms more readily in acidic environments because formaldehyde reacts with tissue components to form acid hematin under low pH. Using a neutral buffered fixative with a pH above 6.0 keeps the environment away from strong acidity, so the reaction that creates acid hematin is minimized. That’s why keeping the fixative pH above 6.0 reduces pigment formation.

Lowering pH would increase pigment formation; increasing fixation temperature can accelerate pigment formation rather than prevent it; decreasing fixation time might help somewhat but does not address the underlying pH issue. Using neutral buffered formalin at about pH 7.0 is the most direct and reliable way to limit pigment formation.