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A positive control preparation for every antigen should be included in every test run. This will be evidence that the various layers were applied in the correct order and that all the solutions were functional. Any diversion from the normal appearance for the positive control should be an alert that something may be wrong. Ideally, a positive control should be a preparation of the same type that is being tested, but this may not always be possible in the context of cytopathology. Therefore a paraffin section of positive control tissue could be used (de-paraffinised and brought to water before the procedure). Sections from suitable cell blocks of cytological preparations would be useful as controls but if these are not available, histological tissue sections will do. A negative control preparation using the antibody diluent without primary antibody should be included for each diagnostic specimen to show that any reactions are specific, due to the primary antibodies.
Cross-reactions
(See Burry, 2011;Taylor,2014)
There is always a possibility that the primary antibody may bind specifically to tissue antigens that share amino acid sequences or a particular molecular configuration with the antigen being sought. This is unlikely in the context of diagnostic immunocytopathology if tried and tested primaries are obtained from reputable companies, but any unexpected results should be investigated with this in mind.