Allred Scoring System for ER/PR in Breast Cancer
The Allred score is calculated by adding a proportion score (0-5, based on percentage of positive tumor cells) and an intensity score (0-3, based on staining strength), yielding a total score of 0 or 2-8, where scores ≥3 are considered positive for hormone receptor expression.
Calculation Method
The Allred scoring system uses two components 1, 2:
Proportion Score (PS)
- 0 = No cells staining
- 1 = <1% of cells staining
- 2 = 1-10% of cells staining
- 3 = 11-33% of cells staining
- 4 = 34-66% of cells staining
- 5 = 67-100% of cells staining
Intensity Score (IS)
- 0 = No staining
- 1 = Weak staining
- 2 = Moderate staining
- 3 = Strong staining
Total Allred Score
Total Score = PS + IS, resulting in scores of 0 or 2-8 (a score of 1 is not possible by this system).
Clinical Interpretation
Scores of 3-8 are considered positive, while scores of 0-2 are considered negative 1. However, current ASCO/CAP guidelines have evolved beyond traditional Allred scoring thresholds 3:
- ER/PR Positive: ≥1% of tumor nuclei staining
- ER Low Positive (new category): 1-10% of tumor nuclei staining (should be reported separately with a comment noting limited data on endocrine therapy benefit)
- ER/PR Negative: 0% of tumor nuclei staining
Key Clinical Considerations
Bimodal Distribution
Research demonstrates that ER staining results are essentially bimodal—99% of breast cancers are either completely negative or show ≥70% positivity, with Allred scores clustering at 0 or 7-8 4. This means weakly positive tumors (1-10% staining) are rare but clinically significant when encountered.
Prognostic Thresholds
While the 1% cutoff defines positivity per current guidelines 3, studies suggest that ER staining >80% and PR >1% correlate with excellent survival outcomes 5. The 10% threshold (Allred PS of 3) showed the most significant differences in disease-free survival for patients receiving endocrine therapy 1.
Quality Control Requirements
For cases with 0-10% staining, laboratories must 3:
- Report the status of controls
- Establish standard operating procedures for confirming/adjudicating low or negative results
- Consider the new "ER Low Positive" reporting category for 1-10% staining
Practical Application
The Allred score correlates strongly with other methods including H-score and automated image analysis 2, 6. Visual Allred scoring remains valid and comparable to automated methods for predicting recurrence-free and cancer-specific survival 2. Both proportion and intensity scores correlate with quantitative assay results (p<0.0001) 1.
Common Pitfalls
- Do not report a score of 1 (mathematically impossible in Allred system)
- Recognize that the traditional Allred cutoff of ≥3 has been superseded by the 1% threshold in current guidelines
- Cases with 1-10% positivity require special reporting attention as "ER Low Positive" with acknowledgment of limited treatment benefit data