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 exception to using formalin are mainly:
- Intraoperative consultation. If a specimen has several parts, and you only want intraoperative consultations on some of them, hold the rest back to avoid potential delays. For all fresh specimens, communicate clearly (such as on a requisition form with the specimen) whether intraoperative consultation is requested or not.
- 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 in cases where there is a need to take samples for genetic testing.
- 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.
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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