This link describes the blood tests available from LabCorp (covered by most insurance) that can help distinguish Lyme from mold; though none of them is totally diagnostic, they can be a good clue as to whether to bother investigating the mold angle. https://www.survivingmold.com/diagnosis/lab-tests
An easy test for mycotoxin illness that you can do from your home computer for $15 is the VCS test. It also is not totally diagnostic (i.e. you could score well but still have mold issues) but it can be helpful.
Negative test results are very common with Lyme for a lot of reasons. Like you said, sometimes people don't have the antibodies being tested for because of timing, and sometimes they don't have the antibodies because their immune systems are so dysfunctional from Lyme, and sometimes the antibodies aren't being produced because of treatment, and sometimes the tests are simply insensitive, incorrect, or looking for the wrong species. I have had negative tests for borrelia bergdorferi, including a negative from IGeneX. The only positive for borrelia bergdorferi I had was from the DNA Connexions PCR urine test. However, for borrelia hermsii, I was positive from a standard test. There are at least 30 species of borrelia that can cause very similar illnesses in humans, so you can have one or many species of borrelia without having borrelia bergdorferi (technical Lyme). Plus mycoplasma, ehrlichia, rickettsia, bartonella, babesia.... A good LLMD makes these diagnoses clinically, based on symptoms, not just on test results. My LLMD swears by the DNA Connexions test, saying that it has matched up entirely with the clinical presentations of his patients, but there is some controversy around its reliability. But then I guess that's true for most Lyme tests.
(I am a big fan of RawlsMD.com)
All of that said, mold toxicity and Lyme (especially the co-infection bartonella, of which there also are several species...) can be indistinguishable. Do you ever get jolts that feel like electric shock, or vibrating sensations in your spine or sacrum? Those things plus psych symptoms can indicate mold. Unless the patient also has Bell's palsy or severe pain in large joints, in which case it still could be Lyme and co-infections. Most people think it's isn't quite that simple to distinguish, and lots of people have all of the above, but sometimes it's nice to know an opinion, one way or the other (this opinion being from Neil Nathan's Toxic).
And finally, search the archives, because there are so many threads about
testing, and about
Teasing all of this stuff out is never easy. Sometimes it takes years and absurd amounts of self-advocacy. But I love that we have a forum on which to ask questions and trade information and experience.