10% formalin is approximately equal in fixation strength to which of the following?

• A. 4% Paraformaldehyde
• B. 2% Glutaraldehyde
• C. 70% Ethanol
• D. 95% Methanol

Answer: (A)

Fixation strength depends on how much aldehyde activity is available to form cross-links with tissue proteins. A 10% neutral buffered formalin solution contains about 3.7–4% formaldehyde in practice, which is the active fixing agent. When paraformaldehyde is used at 4%, it depolymerizes to formaldehyde in solution and provides a similar amount of reactive aldehyde, yielding a comparable level of cross-linking and preservation. That’s why 4% paraformaldehyde behaves similarly to 10% formalin in routine tissue fixation. The other options fix tissues by different mechanisms—alcohols precipitate and dehydrate proteins, while glutaraldehyde, though also a cross-linker, has a different reactivity and is typically used for special applications—so they don’t match the fixation strength of 10% formalin.