Tissue processing begins with fixation. The fixative of choice in most histology laboratories is 10% neutral buffered formaldehyde (NBF). That is: this solution is made by diluting stock formaldehyde to a 10% concentration with phosphate buffer in the pH range of 7.0 to 7.4. Since stock formaldehyde is actually a 40% solution, the final chemical concentration of formaldehyde used in fixation is 4% formaldehyde.
Let’s review tissue processing for a moment, so we’re all on the same page. Tissue is received in the grossing room, cut into smaller pieces, placed in cassettes, and then placed in a fixative (in this case NBF) until placed on the tissue processor. Usually there is an extra one or two containers of NBF on the processor, to allow more fixation time during processing.