Introduction
- Immunohistochemistry is one of the most important diagnostic techniques used in modern pathology for identification of specific cellular antigens in tissue sections by antigen–antibody interaction.
- It combines principles of histology, immunology, and biochemistry to localize proteins within cells and tissues.
- Immunohistochemistry plays a major role in:
- Tumor diagnosis
- Classification of neoplasms
- Identification of infectious agents
- Determination of prognostic markers
- Prediction of therapeutic response
- Because IHC directly influences final histopathological diagnosis, every stage of testing must be carefully controlled.
- Even minor technical variation can produce:
- False positive staining
- False negative staining
- Weak staining
- Background staining
- Non-reproducible results
- Therefore, strict quality assurance is essential in every IHC laboratory.
Quality Assurance in Immunohistochemistry
Quality assurance includes two major systems:
- Internal Quality Control (IQC)
- External Quality Assessment (EQA)
Purpose of Quality Assurance
- Maintain diagnostic accuracy
- Ensure reproducibility
- Standardize laboratory procedures
- Detect technical errors early
- Improve reliability of reports
Internal Quality Control of Immunocytochemistry
- Internal quality control consists of all procedures performed within the laboratory to monitor daily performance of immunohistochemical staining.
- It ensures that each staining run produces valid and reproducible results before patient reporting.
Internal Quality Control
- Verify antigen detection accuracy
- Ensure staining consistency
- Detect reagent failure
- Identify technical problems immediately
- Prevent incorrect interpretation
Components of Internal Quality Control
Main Components
- Control tissues
- Reagent quality control
- Technique control
- Equipment monitoring
- Documentation
Control Tissues in Internal Quality Control
Positive Control
- A tissue known to contain the target antigen is stained together with patient sample.
Purpose
- Confirms primary antibody is functioning correctly
- Confirms antigen retrieval is adequate
- Confirms detection system is active
Characteristics of Good Positive Control
- Stable antigen expression
- Moderate staining intensity
- Similar fixation as test sample
Examples
- Tonsil for lymphocyte markers
- Breast carcinoma for estrogen receptor
- Liver for cytokeratin
Negative Control
- Same tissue processed without primary antibody or with non-immune serum.
Purpose
- Detect nonspecific staining
- Detect endogenous enzyme activity
- Exclude false positivity
Importance
- Very important when background staining is suspected.
Internal Quality Control of Reagents Used in IHC Assay
Importance
- Reagents directly determine staining quality.
- Poor reagent quality is one of the commonest causes of staining failure.
Primary Antibody Quality Control
Important Points
- Correct antibody clone selection
- Proper dilution
- Storage at recommended temperature
- Avoid repeated freeze-thaw cycles
- Check expiry date
Problems Due to Improper Primary Antibody Use
- Weak staining
- Excessive background
- False negative result
Secondary Antibody Quality Control
Important Points
- Species compatibility
- Proper dilution
- Storage conditions
- Avoid contamination
Detection System Quality Control
Detection Systems Commonly Used
- Polymer-based systems
- Biotin-streptavidin systems
Important Monitoring
- Sensitivity
- Background reaction
- Shelf life
Chromogen Quality Control
Common Chromogen
- Diaminobenzidine
Points to Control
- Fresh preparation
- Correct incubation time
- Proper color intensity
Problems
- Weak color
- Excessive precipitate
Buffer Quality Control
Buffers Used
- Phosphate buffer saline
- Tris buffer
Importance
- Maintain pH stability
- Prevent nonspecific reaction
Antigen Retrieval Reagent Quality Control
Importance
- Restores antigen masked during fixation.
Common Solutions
- Citrate buffer
- EDTA buffer
Monitoring
- Correct pH
- Correct heating temperature
- Uniform heating time
Internal Quality Control of Immunocytochemical Techniques
Tissue Fixation Control
Importance
- Fixation preserves tissue morphology and antigenicity.
Standard Fixative
- 10% neutral buffered formalin
Errors Due to Improper Fixation
- Under-fixation → antigen loss
- Over-fixation → weak staining
Tissue Processing Control
Importance
- Proper dehydration and embedding preserve tissue integrity.
Problems
- Poor paraffin infiltration
- Antigen destruction
Section Thickness Control
Standard Thickness
- 3–5 micrometers
Effects of Incorrect Thickness
- Thick sections → heavy background
- Thin sections → weak staining
Slide Adhesion Control
Importance
- Tissue should remain attached during antigen retrieval.
Common Adhesive Slides
- Poly-L-lysine coated slides
Antigen Retrieval Technique Control
Methods
- Heat-induced epitope retrieval
- Enzymatic digestion
Monitoring
- Temperature
- Time
- Buffer type
Incubation Control
Importance
- Correct incubation time for antibody reaction.
Problems
- Short incubation → weak staining
- Prolonged incubation → nonspecific staining
Washing Step Control
Importance
- Removes excess reagent.
Inadequate Washing Causes
- Background staining
- Nonspecific deposits
Counterstaining Control
Common Counterstain
- Hematoxylin
Importance
- Provides nuclear contrast.
Equipment Quality Control
Instruments Requiring Monitoring
- Microtome
- Water bath
- Incubator
- Autostainer
- Microscope
Monitoring Includes
- Temperature calibration
- Mechanical maintenance
- Cleaning schedule
Documentation in Internal Quality Control
Records to Maintain
- Antibody lot number
- Reagent expiry date
- Control slide results
- Temperature records
- Instrument maintenance logs
Common Internal Errors in IHC
Causes of False Positive Results
- Endogenous enzyme activity
- Nonspecific antibody binding
- Inadequate blocking
Causes of False Negative Results
- Antigen destruction
- Wrong antibody dilution
- Poor antigen retrieval
External Quality Assessment (EQA)
- External Quality Assessment is independent evaluation of laboratory staining performance by outside agencies.
Purpose
- Compare laboratory with standard reference laboratories
- Detect hidden technical problems
- Improve inter-laboratory consistency
Process of EQA
| Step | Process |
|---|---|
| 1 | External agency sends unknown tissue sections |
| 2 | Laboratory performs staining |
| 3 | Slides returned for review |
| 4 | Expert panel evaluates staining |
Parameters Evaluated in EQA
- Staining intensity
- Specificity
- Background staining
- Morphological preservation
- Interpretation accuracy
Benefits of EQA
- Standardization
- Improved credibility
- Staff training
- Error identification
Common EQA Programs
National Programs
- National pathology quality schemes
International Programs
- International IHC quality assurance networks
Difference Between IQC and EQA
| Feature | Internal Quality Control | External Quality Assessment |
|---|---|---|
| Performed by | Laboratory itself | External agency |
| Frequency | Every run | Periodic |
| Purpose | Immediate monitoring | Comparative evaluation |
Clinical Importance
- Quality-controlled IHC is essential for:
- Accurate tumor diagnosis
- Hormone receptor reporting
- Targeted therapy decisions
- Prognostic marker evaluation
Common Errors in Immunohistochemistry
1. Poor Fixation
Cause
- Delayed fixation
- Under-fixation
- Over-fixation
Effect
- Antigen destruction
- Weak staining
- Uneven staining
Troubleshooting
- Use 10% neutral buffered formalin
- Fix tissue for recommended time
- Avoid prolonged fixation
2. Improper Tissue Processing
Cause
- Incomplete dehydration
- Inadequate paraffin infiltration
Effect
- Poor section quality
- Irregular staining
Troubleshooting
- Ensure correct dehydration schedule
- Maintain proper paraffin temperature
3. Thick or Uneven Sections
Cause
- Incorrect microtome adjustment
Effect
- Background staining
- Poor antigen penetration
Troubleshooting
- Maintain section thickness at 3–5 µm
4. Tissue Section Detachment
Cause
- Poor slide adhesion
- Excessive antigen retrieval heat
Effect
- Loss of tissue during staining
Troubleshooting
- Use poly-L-lysine coated slides
- Dry slides properly before staining
5. Inadequate Antigen Retrieval
Cause
- Incorrect buffer
- Insufficient heating
- Excessive heating
Effect
- Weak staining or antigen loss
Troubleshooting
- Use correct retrieval buffer
- Standardize temperature and time
6. Incorrect Antibody Dilution
Cause
- Too concentrated
- Too diluted
Effect
- Strong background or weak staining
Troubleshooting
- Use manufacturer-recommended dilution
- Perform antibody titration
7. Inadequate Incubation Time
Cause
- Short incubation
- Excess incubation
Effect
- Weak signal or nonspecific staining
Troubleshooting
- Standardize incubation time
8. Inadequate Washing
Cause
- Insufficient buffer washing
Effect
- Background staining
- Nonspecific deposits
Troubleshooting
- Wash thoroughly between steps
9. Expired or Poor Reagents
Cause
- Old reagents
- Improper storage
Effect
- Weak or failed staining
Troubleshooting
- Check expiry regularly
- Store at correct temperature
10. Endogenous Enzyme Activity
Cause
- Endogenous peroxidase not blocked
Effect
- False positive staining
Troubleshooting
- Use hydrogen peroxide blocking step
Troubleshooting in Immunohistochemistry
Weak Staining
Causes
- Poor fixation
- Inadequate antigen retrieval
- Low antibody concentration
Correction
- Improve fixation
- Optimize retrieval
- Increase antibody concentration
Excess Background Staining
Causes
- High antibody concentration
- Poor washing
- Inadequate blocking
Correction
- Dilute antibody
- Increase washing
- Use blocking serum
No Staining
Causes
- Wrong antibody
- Reagent failure
- Antigen destroyed
Correction
- Verify antibody specificity
- Check control slide
Uneven Staining
Causes
- Uneven reagent distribution
- Drying of sections
Correction
- Cover tissue completely with reagent
- Prevent drying during procedure
Controls Used in IHC
Importance
- Controls are essential for validating staining results.
Positive Control
Definition
- Tissue known to contain target antigen.
Purpose
- Confirms antibody and detection system are working.
Examples
- Tonsil tissue
- Breast carcinoma tissue
Negative Control
Definition
- Tissue processed without primary antibody.
Purpose
- Detects nonspecific staining.
Internal Tissue Control
Definition
- Normal cells within patient section acting as internal control.
Example
- Lymphocytes within tumor section
Reagent Control
Purpose
- Checks reagent performance.
Stepwise IHC Technique
Step 1: Tissue Fixation
- Fix tissue in 10% neutral buffered formalin.
Step 2: Tissue Processing
- Dehydrate, clear, embed in paraffin.
Step 3: Section Cutting
- Cut 3–5 µm sections.
Step 4: Slide Mounting
- Use adhesive slides.
Step 5: Deparaffinization
- Remove paraffin with xylene.
Step 6: Rehydration
- Pass through descending alcohol series.
Step 7: Antigen Retrieval
- Heat-induced or enzymatic retrieval.
Step 8: Blocking Endogenous Activity
- Use hydrogen peroxide.
Step 9: Primary Antibody Application
- Incubate for specific time.
Step 10: Secondary Antibody Application
- Apply secondary antibody.
Step 11: Detection System
- Apply detection reagent.
Step 12: Chromogen Development
Common Chromogen
- Diaminobenzidine
Step 13: Counterstaining
- Use hematoxylin.
Step 14: Dehydration and Mounting
- Alcohol → xylene → mounting medium
Clinical Importance
- Correct IHC technique ensures:
- Accurate tumor diagnosis
- Proper marker interpretation
- Correct therapeutic decision