What is in Carnoy fixative?

• A. Absolute alcohol, acetic acid, chloroform
• B. 95% alcohol, acetic acid, formaldehyde, picric acid
• C. Formaldehyde, mercuric chloride, potassium dichromate
• D. Acetic acid, copper acetate, formaldehyde

Answer: (A)

Carnoy fixative is a rapid, non-aqueous fixative that preserves cellular detail by precipitating proteins rather than cross-linking them. Its classic recipe combines absolute ethanol, acetic acid, and chloroform. The ethanol dehydrates and precipitates proteins, the acetic acid helps preserve and reveal nuclear detail by causing slight swelling, and the chloroform aids tissue penetration and lipid removal to speed fixation and improve sectioning. This mix fixed quickly and preserved nuclei well, which is why it’s used for cytology and certain histology preparations.

Other mixtures listed use components not part of Carnoy’s formula, such as formaldehyde-containing fixatives that cross-link proteins or Bouin-like compositions with picric acid and metals, which produce different fixation characteristics.