Ratio of fixative to the specimen?

• A. 10:1
• B. 5:1
• C. 20:1
• D. 15:1

Answer: (C)

The essential idea is that enough fixative must be present to surround and penetrate the tissue so fixation occurs uniformly. Fixatives like formaldehyde work by diffusing into the specimen and crosslinking proteins, so diffusion limitations mean too little fixative allows inner areas to remain unfixed, risking autolysis and poor preservation.

A fixative-to-tissue volume around twenty to one provides ample fixative, maintains the fixative concentration during penetration, and helps ensure complete and consistent fixation for routine-sized specimens. Smaller ratios, such as five to one or ten to one, can be insufficient for larger or thicker pieces, increasing the chance of uneven fixation. Therefore, twenty to one is the best choice.