I tested positive for lupus and negative for rheumatoid arthritis. How accurate are these tests?

Not accurate for dz. Adding to other answer: so I'm assuming you are ana-positive and rf-negative. ANA is used to screen for lupus and other connective tissue diseases. Everyone with lupus has a positive ana, but not everyone with a positive ANA has lupus. Up to 33% of healthy people without lupus has a positive ana. That's a huge number. So there are many more people with a positive ANA without lupus than have lupus.
Tests are accurate. If you have the right tests theses are quite accurate. You should discuss with your doctor who ordered the tests, because he knows your clinical history and will be able to explain the results to you. I can not answer without knowing the tests which were done.