33rd Annual Scientific Meeting proceedings

The use of immunohistochemistry (IHC) to assist in histopathological diagnosis is now routine for most veterinary diagnostic laboratories. IHC is a technically complex process requiring skill to perform successfully and crucially to interpret the results. Although used for detection and identification of infectious agents in tissues, IHC is most widely used for tumour identification (cell of origin) and histopathological grading.

This lecture will discuss sample requirements for IHC, stressing the need for a thorough histopathological evaluation of routinely stained (H&E) sections first. Very small, poorly fixed, excessively bloody or necrotic tissues are poor candidates for IHC and liable to result in incorrect and potentially diagnostically misleading labelling. An overview of the IHC process will highlight the various stages of the process and points where problems can occur which can have an impact on the final interpretation and thus clinical diagnosis. The use of tissue controls (both positive and negative, internal and external) is essential, especially when interpreting unexpected results (e.g. where IHC might be at odds with the favoured diagnosis based on H&E). Also important is knowledge of the expected cellular labelling pattern e.g. Ki67 is largely restricted to the nucleus where as CD79a, a B-cell marker, should be cytoplasmic.

There are a number of very useful scenarios where IHC can provide a transformative diagnosis such as poorly differentiated tumours e.g. some soft tissue sarcomas, or give crucial information for prognosis e.g. Ki67 labelling of mast cell tumours. IHC should always be offered as panels e.g. multi-keratin with vimentin to differentiate carcinomas from sarcomas, CD3 with CD20 to differentiate T- vs B-cell lymphomas. Obviously cost can be prohibitive in some scenarios, this is not a ‘cheap’ assay, but careful discussion between surgeon, oncologist and pathologist can determine the optimum range of antibodies for a particular lesion.

Finally, it is important to know the limits of IHC. It may seem obvious but when a biopsy report is inconclusive as to the presence of neoplasia vs reactive/hyperplastic tissue, IHC will not help. As mentioned above, nothing can replace thorough routine histopathological evaluation, assessing cellular features, patterns, the interface with normal microanatomical structures and across multiple tissue sections. Although the range of markers (antibodies) commercially available that have been validated for use in domestic species (cats, dogs and horses predominantly) continues to grow each year, these may not be available in every laboratory. Knowing the limits of reaching a final refined diagnosis, hopefully having ruled out some possible differentials, with IHC is important and sometimes ‘poorly differentiated soft tissue sarcoma’ may be as good as we can do.