Which is an example of a noncoagulant fixative?

• A. Formaldehyde
• B. Acetone
• C. Ethanol
• D. Picric acid

Answer: (A)

Understanding how fixatives affect proteins helps explain why formaldehyde is the noncoagulant choice. Noncoagulant fixatives stabilize tissue by cross-linking proteins rather than precipitating them. Formaldehyde fixes by forming methylene bridges between amino groups in proteins, locking structures in place without causing widespread protein precipitates. This preserves overall architecture and cellular details better under light and immunohistochemical review.

In contrast, the other agents listed act by coagulating or precipitating proteins, which hardens and sometimes distorts tissue and can mask details: acetone and ethanol cause protein precipitation, and picric acid behaves as a strong coagulating fixative in many preparations. Therefore, formaldehyde is the noncoagulant fixative.