Usually the results of immunohistochemisty (IHC) and Fluorescent In Situ Hybridization (FISH) agree. When they do not agree it is important to determine the reason. Doing only one of the tests means that a false-positive or false-negative result may not be recognized.
False-negative IHC:
(1) monoclonal antibody
(2) technical problem
False-positive IHC:
(1) technical problem
False-negative FISH:
(1) region of gene targeted by probe deleted
(2) technical problem
False-positive FISH:
(1) technical problem
where:
• Technical problems may include tissue fixation, tissue handling, tissue processing, testing errors, or observer errors.
• If the probe set is suspect then it may be appropriate to sequence the entire gene.
Other reasons for a discrepancy:
(1) pseudodiscrepancy based on cut-offs (at the borderline)
(2) different sites of the same tumor (reflecting different clones)
(3) tumor biopsied at different times (reflecting change in tumor)
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