Would you test for ESR1 mutation prior to deciding on AI or fulvestrant for metastatic breast cancer?

As with so many things in breast cancer, the answer for me is that it depends.

The argument against testing is that the presence of ER-mutations before treatment with an aromatase inhibitor is low (<10% and in some series <5%, Cancer Discov. 2017, Mol Oncol. 2018, for example), and while PFS with AI ...

I would test if I was considering single agent hormone therapy. I agree with the points raised in @Dr. First Last excellent and thorough answer. I would add that the detection of ESR1 mutations is likely higher in plasma-based tests compared to tissue-based tests (e.g. here, here, and here, among ot...