I don't think it's totally clear yet what causes high C4a. Certainly, I would check for mold in the house. But it's not always just mold causing high levels. In my case, I lived in the desert in a tent for several months and my levels went higher. With high levels like yours, you can certainly conclude your immune system is either dysfunctional or being disturbed by something. But I am not sure you can conclude anything specific. Shoemaker would probably say that the fluctuation has to do with mold exposure and that may be occasionally be the case but I am not sure if anybody really knows in all cases. Mine have been as high as 67,000 but now are typically at 15,000. I haven't gotten them checked for a while. I don't think my insurance covers the test any more.