Here is the latest consensus by experts on what MCAS is and why it's important to treat it.
https://www.degruyter.com/document/doi/10.1515/dx-2020-0005/html
Though some of us have low Immunoglobulins, most do not. For many, it does seem to be related to a disrupted microbiome, commonly from antibiotic use and/or oxalates and/or mycotoxins.
Be persistent. You should not be billed for tests that aren't run or arent run properly. In the US, I've had Quest Diagnostics run MCAS labs, with some prodding - done of the tests are "sendouts."
There are many treatments, see attached.
There are many. See attached.
Quercetin and Neuroprotek are available without prescription, along with H1 and H2 antihistamines. Cromolyn sodium is also useful, but needs a prescription. And Gleevec/imatinib.