How should the estrogen receptor (ER), progesterone receptor (PR), and HER2/neu results be reported in a breast cancer panel?

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Last updated: August 20, 2026 • View editorial policy

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Reporting ER, PR, and HER2/neu in Breast Cancer Panels

ER and PR should be reported with specific percentage of positive tumor nuclei and staining intensity, with ER results categorized as Positive (≥1%), Low Positive (1-10% with mandatory comment), or Negative (<1%), while HER2/neu should be reported separately according to established scoring systems. 1

ER Reporting Algorithm

Primary Classification

  • ER Positive: 1-100% of tumor nuclei staining
  • ER Low Positive: 1-10% of tumor nuclei staining (requires specific comment)
  • ER Negative: <1% or 0% of tumor nuclei staining 1

Required Reporting Elements for ER

  • Percentage of tumor cells with nuclear staining
  • Intensity of staining (weak, moderate, or strong)
  • Control status (especially critical when 0-10% staining) 1

Mandatory Comment for ER Low Positive (1-10%)

When reporting ER Low Positive results, you must include: "The cancer in this sample has a low level (1%-10%) of ER expression by IHC. There are limited data on the overall benefit of endocrine therapies for patients with low level (1%-10%) ER expression, but they currently suggest possible benefit, so patients are considered eligible for endocrine treatment. There are data that suggest invasive cancers with these results are heterogeneous in both behavior and biology and often have gene expression profiles more similar to ER-negative cancers." 1

PR Reporting Algorithm

Classification (Simpler than ER)

  • PR Positive: 1-100% of tumor nuclei staining
  • PR Negative: <1% or 0% of tumor nuclei staining 1

Required Reporting Elements for PR

  • Percentage of tumor cells with nuclear staining
  • Intensity of staining
  • No special "Low Positive" category is required for PR (unlike ER) 1

Key distinction: While the same 1% threshold applies to PR, the special "Low Positive" reporting category with mandatory comment is only required for ER, not PR 1. PR serves primarily prognostic purposes in ER-positive disease rather than predictive purposes for endocrine therapy.

Quality Control Requirements

Pre-analytical Documentation

Document these variables in every report:

  • Fixative type
  • Time to fixation
  • Time in fixation
  • If unavailable, add comment: "Results should be interpreted with caution" 1

Control Status Reporting

Mandatory for cases with 0-10% staining:

  • Report status of internal controls (normal epithelial elements)
  • Report status of external controls
  • If no internal controls present but external controls positive, add: "No internal controls are present, but external controls are appropriately positive. If needed, testing another specimen that contains internal controls may be warranted for confirmation of ER status." 1

HER2/neu Reporting

While the provided guidelines focus primarily on ER/PR, HER2/neu should be reported separately following established ASCO/CAP HER2 testing guidelines, as it represents a distinct therapeutic target and is not part of the hormone receptor reporting framework 1.

Common Pitfalls to Avoid

Critical Interpretation Issues

  1. Do not report ER as negative if 1-10% cells stain - this must be reported as "ER Low Positive" with the mandatory comment 1
  2. Do not apply the "Low Positive" category to PR - PR uses only positive/negative classification 1
  3. Never report results when controls fail - resolve control issues before reporting 1
  4. Do not omit intensity reporting - both percentage and intensity are required elements 1

Sample Adequacy

  • Do not interpret specimens with insufficient invasive cancer
  • Do not interpret specimens with severe processing artifacts
  • Consider these samples "uninterpretable" rather than negative 1

Additional Confirmation Steps for Low/Negative Results

When initial results show 0-10% ER staining, establish laboratory SOPs including:

  • Re-review of controls
  • Second reviewer confirmation
  • Validated quantitative digital image analysis
  • Comparison with prior patient-specific results
  • Retesting same sample if analytic issues suspected
  • Testing different block if no internal controls, preanalytic issues, or unexpected results 1

This algorithmic approach ensures standardized reporting that directly impacts treatment decisions, particularly regarding endocrine therapy eligibility, which significantly affects patient mortality and quality of life outcomes.

References

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