Blood tests could reveal all kinds of things though, they could also be highly suggestive of certain conditions or just of a problem though don't discount that there can be blips, quite often being a reason for repeating bloods.
Rhumatoid arthritis cannot be diagnosed by a blood test. An RF factor can be present in a number of conditions and some people have it without any symptoms or obvious illness. It's a diagnosis based on a number of factors, symptoms, blood tests etc...