Hi, marymk. I think C3, C4, and CH50 are also used to screen for angioedema (a sort of hive-like skin swelling), RA, vasculitis, etc. It would be nice if HIGH suggests one group of possibilities & LOW suggests another set of possibilities. (Is life ever that tidy? Hope so!)
I recently saw multiple discussions among people with hypothryoidism who get hives. I never knew, before now, that hypothyroidism can cause hives!
Only today, a neighbor showed me her hives. She'd just adopted a long-haired dog, her first doggie ever, and 2 days later broke out in facial hives. She'd told me the story over the phone. Here I was expecting big red lumps like my husband gets when he eats strawberries; instead what I saw looked like a butterfly rash to my untrained eye, startling me! She's had known allergies for years, so her allergist did two patch tests on her back that came back positive for dog dander allergies.
I think my hypothyroidism "report" and my "shaggy dog story"
both illustrate how right you are about lupus & its close cousins being hard to differentiate from other conditions. Too many things look alike, and too few tests are specific to a single condition. I think your PCP and you are smart to investigate multiple things at once.
The particular symptoms people present with certainly can slow or speed answers. I was one of the lucky ones with one symptom that was 100% lupus-specific (an SCLE rash). So I hope you work thru any & all possibilities presented. Actually, in your shoes, I probably wouldn't say NO to any test offered, even one I didn't relish having. Hang in there! Looking forward to more updates & wishing you good luck, with my best, Vee