ANA is positive in 15% of the population [2]
The idea that the tests rule out serious causes is not really correct, people can have seronegative inflammatory arthritis in which case these tests are negative. Peace of mind is dubious as well given that even with a positive test you are still more likely to not have an inflammatory arthritis.
It’s a common misconception that blood tests are binary and provide concrete answers. Sometimes they do. But most blood tests, like many measurements, are far from binary and have a distribution across the normal population, once it passes an arbitrary threshold it does not necessarily mean you have disease X, context (clinical history and examination) are often far more important in making a diagnosis.
Unfortunately some of the population tend to overweight blood tests vs a physicians assessment, I guess they see the former as an objective measure and the latter as subjective. Especially because if you shop around enough clinicians you’ll eventually find one who will say what you want to hear and that one will inevitably be ‘right’.
1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3845430/ 2. https://rheumatology.org/patients/antinuclear-antibodies-ana...