What is the minimum fixation time in NBF before processing?

• A. At least 45 minutes
• B. 15 minutes
• C. 60 minutes
• D. 90 minutes

Answer: (A)

Fixation time in neutral buffered formalin must be long enough for the fixative to diffuse into the tissue and begin cross-linking proteins, stabilizing cellular structures before processing. The minimum practical time often used for routine small specimens is around 45 minutes. This provides enough fixation to prevent rapid surface degradation and autolysis while still allowing processing to proceed without excessive over-fixation. A shorter time, such as 15 minutes, wouldn’t adequately fix deeper tissues and can lead to artifacts from un-fixed areas. Longer times like 60 or 90 minutes aren’t necessary as a minimum and can push toward over-fixation for some tissues, which can hinder staining and later analyses. Always tailor fixation to specimen size, but 45 minutes is the commonly cited minimum prior to processing.