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.
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.
- . Breast pathology grossing guidelines. UCLA Health. Retrieved on 2021-09-09.
- Katarzyna Lundmark, Krynitz, Ismini Vassilaki, Lena Mölne, Annika Ternesten Bratel. Handläggning av hudprover – provtagningsanvisningar, utskärningsprinciper och snittning (Handling of skin samples - Instructions for sampling, cutting and incision. KVAST (Swedish Society of Pathology). Retrieved on 2019-09-09.
- . Paraformaldehyde, Formadehyde and Formalin. Duke University. Retrieved on 2019-12-17.
- . Fixation of Tissues. Approval Date: August 2016, August 2020. Review Date: August 2024|website=Royal College of Pathologists of Australia
- Buesa RJ, Peshkov MV (2012). "How much formalin is enough to fix tissues? ". Ann Diagn Pathol 16 (3): 202-9. doi:10.1016/j.anndiagpath.2011.12.003. PMID 22483550. Archived from the original. .
- . How to Submit Tissues for Embedding. University of Pittsburgh, Starzl Transplantation Institute. Revised 04/19/21