ER vs PR in IHC

The short answer

Estrogen receptor (ER) and progesterone receptor (PR, also called PgR) are separate IHC markers assessed in breast-cancer tissue. Both are scored by nuclear staining in tumor cells, but one result does not substitute for the other. Reports should identify the marker tested and the proportion of tumor nuclei staining.^[1][2]

What the current guideline distinguishes

The current ASCO/CAP breast-cancer guideline interprets 1% to 100% stained tumor nuclei as positive for ER or PR and less than 1% as negative. ER staining in 1% to 10% of cells is reported as 'ER Low Positive' with a recommended comment. That special reporting category applies to ER, not PR.^[1][2]

The guideline notes that evidence is limited for the ER-low group, which is why it asks laboratories to identify that range explicitly. PR is still reported separately with its own percentage and staining intensity rather than being inferred from ER.^[2]

Interpretation depends on assay quality

The guideline emphasizes validated methods, appropriate controls, tissue handling, and concordance with histology. An inadequate specimen or failed controls can make a result uninterpretable. The guideline calls for evaluating each result against assay controls and histology.^[2][1]

Relevant ReagentScope records

The ReagentScope ER marker page and PR marker page link to verified Bio SB ER EP1 product and PR EP2 product records. The manufacturer specifications on those pages are separate from the breast-cancer reporting guideline described above.^[3][4]