Positive tissue controls in IHC are used to assess the quality of staining based on the expected staining pattern of the antibody used (nuclear versus cytoplasmic). Negative tissue controls are used to detect an antibody's lack of specificity and may also indicate nonspecific staining. A multi-tissue control block that may contain 40 to 100 different tissues is a preferred type of control slide to use. It can provide a positive and negative tissue control that is ideal for use in an antibody dilution panel (titer) of new primary antibodies. However, this is not always possible for every laboratory.
If a multi-tissue control section is not available, a known positive and negative tissue slide should be produced. Both tissues may be on the same slide or they can be three separate slides (positive tissue, negative tissue and patient tissue), as long as:
- All three slides are stained together in the EXACT same manner and methodology.
- All tissues (controls and specimens to be tested) are fixed and processed in an IDENTICAL manner.
- All IHC staining treatments MUST be subjected to ALL of the control slides in the SAME manner as the testing tissue section in order to compare equivalent results.
Precautions should be taken when cutting control slides ahead of time and storing them for future use. There are numerous methods that technicians use to preserve the antigenicity of those slides, such as refrigeration after cutting with monitoring of dates cut. These must be monitored closely to ensure consistent quality results.
It may be best to perform a literature search on the various techniques available for preserving quality control (QC) slides in order to find one that works best for your laboratory. The ideal policy is one that requires the control slides be cut the same day and under the same conditions as the patient’s slides. Paraffin-embedded control tissue should be stored in such a way to preserve antigenicity, such as freezing.
