Need help understanding pathology report

Hi and I'm so sorry to hear of your diagnosis and Stage IV status. I hope you find the support and comfort you need from these forums.

Poorly differentiated is often an interchangeable term with Grade 3. It means that when a pathologist looks at the cells, they are less recognizable than they should be as being breast cells. Grade generally comes in 3 levels for breast cancer: 1-well differentiated, 2-moderately differentiated, and 3-poorly differentiated. The higher the grade, the more quickly the cells are dividing, and hence, more aggressive they are. In essence, the faster that cells divide, the more errors that will occur, and the less they will look like the normal, healthy cells of that particular area. IDC means Infiltrating or Invasive Ductal Carcinoma, the most common form of breast cancer. It means the cancer cells originated in the ducts of the breast and have invaded or infiltrated surrounding breast tissue. 

Combined 2/3 Grade likely means that the tissue tested had both moderately and poorly differentiated cells. I don't have a lot of experience on that part beyond that answer, so someone else may be able to elaborate further.

Stain negative with CK 5/6 means that a form of identifying basal-like cancer cells, called CytoKeratin 5/6 staining was negative. This staining has a prognostic value when determining if a Triple Negative Breast Cancer is also expressing the antigens for basal-like characteristics. This would mean a poorer prognosis if they stained positive. I have Triple Negative that is also basal-like. 

I hope this helps.