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Author: Mikael Häggström, M.D. [note 1]


Adipose tissue with crumpling artifact due to insufficient fixation.

Within an hour after removal from the body,[1] tissue samples should generally be placed in vessels with enough fixative to allow them to lie freely in the solution.[2] The standard fixation fluid is generally 10% neutral buffered formalin, which is roughly equivalent to 4% formaldehyde.[3] The ratio of tissue:formalin should be 1:5[4] to 1:10[5].[5]


The duration depends on tissue thickness, where formalin will penetrate and fix the tissue at ~1 mm/hour.[6]

When not to use formalin

The main exceptions to using formalin are mainly: edit

  • Intraoperative consultation.  
  • Suspected crystals, such as a tophus or other specimen suspicious for gout versus pseudogout. These should be sent in alcohol or dry, since formalin will dissolve the crystals.
  • Suspected lymphoproliferative disorders, such as lymph nodes (or other lymphoid aggregates) with a suspicion of lymphoma, where samples are generally put in a special solution for flow cytometry.
  • Need for genetic testing, such as some cases of products of conception.
  • Cytology specimens, which are preferably sent fresh (such as in red top tubes) to be processed within a few hours. If processing may be after a few hours, put tubes on ice, or add 50% alcohol.[7]
  • Need for microbiology evaluation, mainly bacterial culture. For potentially infectious workups, check with the microbiology lab if they have the tissue they need before putting the specimen in formalin.
  • Need for immunofluorescence, such as immune complex-mediated disease, where specific preservation will give better test sensitivity.[8]

If you don't know, and if you cannot soon get in touch with anyone who can guide you, specimens can generally be stored in a fridge in the meantime, even overnight if it is late (but make sure to follow-up as soon as possible in the morning). Until then, don't put the specimen in formalin and don't freeze the specimen.

General notes edit

Further reading:


  1. For a full list of contributors, see article history. Creators of images are attributed at the image description pages, seen by clicking on the images. See Patholines:Authorship for details.

Main page


  1. . Breast pathology grossing guidelines. UCLA Health. Retrieved on 2021-09-09.
  2. 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.
  3. . Paraformaldehyde, Formadehyde and Formalin. Duke University. Retrieved on 2019-12-17.
  4. . Fixation of Tissues. Approval Date: August 2016, August 2020. Review Date: August 2024|website=Royal College of Pathologists of Australia
  5. 5.0 5.1 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. . 
  6. . How to Submit Tissues for Embedding. University of Pittsburgh, Starzl Transplantation Institute. Revised 04/19/21
  7. . How to send fluid and make good cytology slides. Tufts University.
  8. Mubarak M, Kazi Javed I, Kulsoom U, Ishaque M (2012). "Detection of immunoglobulins and complement components in formalin fixed and paraffin embedded renal biopsy material by immunoflourescence technique. ". J Nephropathol 1 (2): 91-100. doi:10.5812/nephropathol.7518. PMID 24475396. PMC: 3886135. Archived from the original. . 

Image sources