OK- here is the explanation I found on the Quest diagnostics site. This is a lab site and is reputable.
Antibodies to Sm are highly specific for systemic lupus erythematosus (SLE) and when present are considered a marker antibody. However, these antibodies are found in only 20% of patients with SLE. RNP antibodies (also known as anti-u1 or ribonucleoprotein antibodies) are found in 45% of SLE patients but are also observed in numerous other disease states such as Sjgren's syndrome, scleroderma and polymyositis. Elevated levels of antibodies to RNP are seen in mixed connective tissue disease. In SLE, RNP antibodies have been associated with a relatively benign disease course with lower incidence of renal and central nervous system involvement. Patients may be considered positive for RNP antibodies when the RNP antibody result is significantly higher than the Sm antibody result.
Yeah, clear as mud! But that is the torture known as autoimmune disorders. Glad you know you actually have a disease and you arent crazy! That is the only good thing about an autoimmune diagnosis! LOL!

Mary Ann

Mary Ann gave you about the best explanation you are likely to find. What I have found is the immune system is just astoundingly complex, and after having this thing for almost 40 years, I sometimes feel like the best doctors (and I have seen some of the best) are just beginning to understand it. The good thing is you know you are not crazy, because that wondering if you are somehow insane when you feel so sick is just miserable. And also, there are a lot of things that can help you. And there is even a chance of remission. I had a nice long one.

Hope you start feeling better soon.

Thanks for the info! It kind of makes sense?!? This is still all so confusing. I guess I never looked at the fact that most doctors are just really learning about it. Sometimes, I have to explain what I have when I see new doctors (urgent care etc.). I guess the reason I'm still confused is I have been told I have SLE and I have MCTD. Someone once asked me what my primary diagnosis was (she also has MCTD). I didn't really know how to answer her. Do I have MCTD with a primary of SLE?? I know most of you probably don't feel comfortable answering this question, I just can't seem to get a straight answer.

Great posts, guys-- I did not know that anti-RNP was associated with a better outcome in SLE.

Ann, great point about the remission, I've had very long ones as well. Also, from the other thread, it's interesting that despite our different opinions about Plaquenil, we both think it's worth consideration for RB.

For what it's worth, the thinking is usually that MCTD can turn into SLE, but I usually don't hear of it going the other way unless it was a misdiagnosis. (I was diagnosed with lupus in the 70s even though I had no positive antibody tests at that time, and the anti-RNP antibody didn't turn up until I was in my late 30s or 40s. So the diagnosis was revised to MCTD, meaning they thought the first dx was incorrect.) Do you guys agree?

I still don't know what the "lupus antibody" test is. I thought it was double-stranded DNA for a while, but that's definitely not it! Maybe they just call it "the lupus antibody."

The exact diagnosis is not as important as the implications, meaning that your disease course may be more benign.

I am getting the impression that what MCTD is is still evolving. I thought it was pretty cut and dried. RNP present- MCTD! But then I read this. I got info from Quest when I was first diagnosed and the info was different than this. Just when you think the water is clearing, it becomes murky!
Mary Ann