Do needle biopsies need to follow the 6-48 hour requirement for formalin fixation or is shorter fixation adequate due to their smaller size?
Table 5 in the Guidelines specifies a minimum one-hour formalin fixation time for needle biopsies, but longer fixation is strongly recommended for these specimens. Formalin penetrates tissues at the rate of about 1mm/hour, but penetration is not the same as fixation and the minimum fixation time needed for core biopsy specimens has not yet been determined. A number of studies have shown that the biochemical cross-linking that represents formalin fixation requires up to 24 hours. Because of the adverse effects of underfixation, which cannot be overcome by antigen retrieval, testing on specimens fixed for only one or two hours is strongly discouraged.
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