Fixation

Author: Mikael Häggström, M.D.

Immersion

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Adipose tissue with crumpling artifact due to insufficient fixation.

Within an hour after removal from the body, tissue samples should generally be placed in vessels with enough fixative to allow them to lie freely in the solution. The standard fixation fluid is generally 10% neutral buffered formalin, which is roughly equivalent to 4% formaldehyde. The ratio of tissue:formalin should be 1:5 to 1:10.

Duration

The duration depends on tissue thickness, where formalin will penetrate and fix the tissue at ~1 mm/hour.

When not to use formalin

The main exceptions to using formalin are mainly:

• A tophus or other specimen suspicious for gout versus pseudogout should be sent in alcohol or dry, since formalin will dissolve the crystals.
• Lymph nodes (or other lymphoid aggregates) with a suspicion of lymphoma, where samples are generally put in a special solution for flow cytometry.
• Products of conception with a need to take samples for genetic testing.

General notes

Further reading: Starting pathology (handbook) Learning pathology Emergency pathology Fixation Gross processing Evaluation: Suspected malignancies Immunohistochemistry Consultation Reporting Secrets

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