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:
- 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.
- 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.
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.
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