I had blood work done and came back positive for lupus. was retested and was negative. why the two different results?

Difficult Testing: Lupus is considered an auto-immune disorder meaning that the body recognizes some of its own tissue as foreign and thus the immune system attacks. Testing for many autoimmune disorders is difficult because the results are not yes/no but rather dilutationary ranges and are best interpreted by a rheumatologist in discussion of your clinical symptoms. It takes symptoms plus blood test for diagnosis.

No simple test: The usual screen for lupus is the anti-nuclear antibody (ana); if the titer is above, say, 1:40 and you have symptoms / signs suggesting lupus, this is suggestive but by no means proof. The large majority of lupus patients will have positive anti-dsdna, but this doesn't settle it. About 1/3 of lupus folks have anti-sm(ith); this clinches if present. Some folks are never diagnosed "for certain".

Depends: There are small random variations in tests that may give discrepant results. The two tests may have been done using different methods. Finally there are a small number of false positives.