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For decades anatomical pathologists have established and managed diagnostic immunohistochemistry (IHC) laboratories to assist in accurate tissue diagnosis. Although test validation, quality control (QC), and quality assurance (QA) procedures have always been a part of routine operations, test results were always viewed in the context of clinical, morphological, and other IHC test results. However, there has been a shift in the use of these tests. Now, targeted treatments are being determined based on the results of a stand alone IHC test. Specifically, eligibility for traztuzamab is determined by IHC testing (in the UK) or by IHC and/or fluorescent in situ hybridisation (FISH) testing in the USA. Because this represents a fundamental change in the use of IHC in the clinical laboratory, additional attention to the performance and interpretation of this IHC test is warranted. In the current issue of the Journal of Clinical Pathology, Ellis and colleagues detail guidelines for HER2 testing in the UK. (1)
The guidelines can be briefly summarised as follows:
(1) Formalin fixation should be used. In the USA, testing is only approved by the Federal Drug Administration for formalin fixed tissue. Other fixatives such as Bouin's type fixatives and, in our experience, some of the newer formalin alternative fixatives preclude FISH testing.
(2) A relatively high case load is recommended because this is correlated with better performance.
(3) A two tiered system is recommended in which clear IHC results (either 0-1+ or 3+) can be accepted. An equivocal result (2+) should be …