@justy Am pretty sure you know my story so will make it very brief. I got pulmonary edema from one liter of saline in Nov 2014 but it turned out that it was infused too quickly. I did not yet know that I had MCAS and the test which showed my histamine was 4x the normal limit was not until the following month.
Long story short, with MCAS, if you are prone to third spacing from fluid (any fluid), it must be infused at a very slow speed (b/c histamine makes the membranes more permeable). I had one liter of saline in 3 hrs in 2014 and it literally almost killed me but I was not actually allergic to it. I am currently doing IVIG infusions of just slightly over 1/4 liter in 7 hrs. It sounds ridiculous but I have no allergic reactions and by spacing it out over three days, I actually do a pretty high dose.
Not sure if this helps but wanted to mention it just in case.
ETA: I re-read what I wrote and not sure if it makes sense. I don't mean spacing the 1/4 over three days. I mean doing 1/4 liter on each of the three days, so in total I am receiving between 3/4 liter to 1 liter of fluid (IVIG) spread over the three day period.