The Allred score is based on an assessment of the percentage/proportion of tumor cells in a histopathologic specimen that stain positively and the intensity of the staining.
In 2010, the American Society of Clinical Oncology/College of American Pathologists Guideline Recommendations for Immunohistochemical Testing of Estrogen and Progesterone Receptors in Breast Cancer described how to estimate the percentage/proportion of tumor cells staining positively and how to assess the intensity of staining:
“The percentage/proportion of tumor cells staining positively should
be recorded and reported; all tumor containing areas of the tissue
section on the slide should be evaluated to arrive at this percentage.
The percentage can be arrived at either by estimation or by
quantification, either manually by counting cells or by image
analysis.” [bolded for emphasis]
“The intensity of staining should be recorded and reported as weak,
moderate, or strong; this measurement should represent an estimate of
the average staining of the intensity of the positively stained tumor
cells on the entire tissue section relative to the intensity of
positive controls run with the same batch.” [bolded for emphasis]
So, either using a microscope to look at a slide of a stained specimen or an image of a slide, count all tumor cells and then count positively stained tumor cells. Divide the positively stained tumor cells by the total tumor cells to get the percentage/proportion positively stained. Estimate intensity of staining.
The Allred score is calculated as the sum of the percentage/proportion of tumor cells staining positively and the intensity of staining using the table given here:
Note that the Allred score takes values of 0 or 2-8. There is no Allred score 1 because tumors that have 0 cells that stain (score on first component 0) must have an intensity value of 0 and tumors that have more than 0 and <1% of cells that stain (score on first component 1) must have an intensity of staining with a minimum value of 1.
A 2020 update of the 2010 guideline from the American Society of Clinical Oncology/College of American Pathologists has been published. It does not mention the estimation of the percentage/proportion of tumor cells staining positively or assessment of intensity of staining or describe any changes to the 2010 guidance on this topic.