Fixation on Histology

It has been nearly two months since the COVID-19 pandemic hit the US in full force, and histology labs are beginning to receive samples that are COVID positive, or suspected cases. How should you handle these specimens?

How long should COVID-19 specimens be left in formalin for fixation?

You can safely fix COVID-19 specimens for the normal length of time that you would any other specimen of a similar tissue type and size. A general recommendation from the CDC from their autopsy protocol is 72 hours. Anything over 72 hours may affect the antigenicity when running IHC testing. They also state for autopsy samples, that prolonged immersion in formalin can “diminish the sensitivity of virus detection assays” so they recommend shipping specimens to the CDC as soon as possible following complete fixation. See the full autopsy COVID-19 guidelines.

The 72-hour recommendation is to ensure complete fixation, so for diagnostic surgical specimens like biopsies, you can reduce accordingly based on the size, knowing that 10% NBF fixes at a rate of approximately 1 mm per/hr.

Should we perform frozen sections on COVID cases?

The article, Coronavirus Disinfection in Histopathology, published in March in the Journal of Histotechnology, suggests that labs refrain from frozen sectioning possible COVID cases, unless they are confident that they can contain the aerosols in the cryostat. There is of course the possibility that other routine frozen sections may be positive for COVID without the lab’s knowledge, so some labs have stopped frozen sections altogether or are taking enhanced precautions in PPE. At the very least remember to follow universal precautions for frozen sections, such as face shield, face mask, gloves, and lab coat. You should also definitely not use a freezing spray. Sprays create aerosols which you want to avoid for COVID. Also keep in mind routine disinfection of the cryostat as you would for TB or HIV cases. Frozen sectioning always involves risks of exposure, so you should always be following universal precautions.