What are External Positive Controls?
External positive controls are tissue samples or slides known to express the target antigen or marker being tested in a histological assay. These controls are essential for validating the accuracy and reliability of staining procedures, whether it involves immunohistochemistry (IHC), in situ hybridization (ISH), or other histological techniques.
Verify the specificity and sensitivity of the staining reagents.
Detect any potential errors in the staining procedure.
Provide a benchmark for comparing staining intensity.
Without these controls, it would be challenging to distinguish between a true negative result and a potential technical failure.
Tissue Type: The control tissue must be similar to the test tissue in terms of cellular composition and antigen expression.
Antigen Specificity: The control must contain the specific antigen or marker that is being tested.
Staining Consistency: The control should consistently demonstrate the expected staining pattern.
Common sources for external positive controls include previously validated tissue samples, commercial control slides, and cell lines.
How to Utilize External Positive Controls?
When performing a histological assay, external positive controls are processed alongside the test samples. They undergo the same staining procedures, including fixation, sectioning, and staining. The results from the controls are then compared to those of the test samples to ensure the validity of the assay.
Common Issues with External Positive Controls
Despite their importance, there are some challenges associated with external positive controls: Variability: Differences in tissue preparation, fixation, and handling can lead to variability in staining results.
Degradation: Over time, control samples may degrade, leading to diminished staining quality.
Availability: Obtaining high-quality, consistent control tissues can sometimes be difficult.
Regular validation and proper storage of control tissues are crucial to mitigate these issues.
Best Practices for External Positive Controls
To ensure the effectiveness of external positive controls, the following best practices should be adhered to: Regular Validation: Periodically validate control tissues to ensure they still express the target antigen.
Proper Storage: Store control tissues under appropriate conditions to prevent degradation.
Documentation: Maintain detailed records of control tissue sources, preparation methods, and validation results.
Consistent Protocols: Use consistent staining protocols for both controls and test samples to minimize variability.
Conclusion
External positive controls are indispensable in the field of histology. They play a critical role in ensuring the accuracy and reliability of staining procedures, thereby contributing to the validity of diagnostic and research findings. By carefully selecting, validating, and maintaining these controls, histologists can significantly enhance the quality of their work.
