Positive control slides are used to ensure that all techniques were executed properly and resulted in successful staining. They verify that antigen retrieval was utilized correctly, all reagents were mixed and applied properly, antibody dilutions and the method of staining were correct, and the incubation times and temperatures were optimal. Tests on patient tissue should not be considered valid if positive control tissues fail during staining and results are negative.
Each time an antibody is run by IHC, a positive tissue control must also be run. Known positive tissue may be found in the performing laboratory or commercially purchased slides may be used. However, it is optimal to use tissue found in the performing laboratory to ensure control tissues were fixed, processed, and cut under the same conditions as the patient tissue.
As with the negative control tissue, the primary antibody data sheet usually lists the best positive tissue to use as quality control. Always read antibody data sheets thoroughly to prevent misuse of the antibody. The manufacturers have specific information listed on these data sheets. They will also have contact numbers for technical help and advice.
A positive control section is a MUST with every antibody that is used. Ideally, the positive control section should be mounted on the same slide as the patient tissue section to ensure that all tissues are treated equally. Having a positive control tissue on each slide can help identify uneven staining if you are using an automatic stainer that may not be leveled properly. It will also help to identify inconsistency in the mixing of the reagents that appear as spotty staining on the control tissue.