Positive Controls - Histology

What are Positive Controls?

Positive controls are essential elements in the field of histology that are used to validate the accuracy and reliability of staining procedures and other diagnostic tests. By using a sample known to produce a positive result, researchers and clinicians can ensure that the histological techniques employed are working correctly.

Why are Positive Controls Important?

The primary importance of positive controls lies in their ability to guarantee that the staining protocols or assays are functioning as intended. This is crucial for several reasons:

They help in identifying potential errors in the procedure.
They provide a benchmark to compare test results.
They ensure the reproducibility and reliability of the results.
They assist in the quality control of histological studies.

How to Select a Positive Control?

Choosing an appropriate positive control involves several steps:

Select a tissue or sample known to express the target antigen or marker.
Ensure the tissue is processed under similar conditions as the test samples.
Verify the positive control is consistent and reproducible across different batches.

An example of a positive control could be using human tonsil tissue for immunohistochemical staining of lymphocyte markers.

Types of Positive Controls

There are different types of positive controls depending on the application:

Internal Positive Controls: These are within the same sample being tested. For instance, using endogenous tissue elements that are known to react positively.
External Positive Controls: Separate from the test sample, these are tissues or cells known to express the target antigen.
Synthetic Positive Controls: These are artificially created, such as plasmids or recombinant proteins, used when natural controls are unavailable.

Common Issues with Positive Controls

Despite their importance, positive controls can sometimes present challenges:

Non-specific staining can occur, leading to false positives.
Variability in expression levels of the target antigen can affect consistency.
Technical errors in tissue processing may lead to unreliable results.

To mitigate these issues, it is essential to optimize the conditions and validate the positive controls regularly.

Examples of Positive Controls in Histology

Different histological techniques use various types of positive controls:

In Immunohistochemistry (IHC), a positive control tissue like human tonsil or spleen is often used for lymphocyte markers.
For In Situ Hybridization (ISH), positive control probes that bind to known sequences are used.
In Hematoxylin and Eosin (H&E) staining, a known tissue sample with distinguishable cellular structures can act as a control.

Conclusion

Positive controls are indispensable in histology, providing crucial validation and ensuring the reliability of diagnostic and research findings. By carefully selecting, validating, and regularly reviewing these controls, histologists can maintain high standards of accuracy and reproducibility in their work.