When doctors check for a certain virus in your blood , do they disregard any other viruses they see?

Ok, I think that I might of contracted Syphilis from a partner and today my doctor thought that I had the virus Mononucleiosis, so she made me get my blood tested for that and today she called back and said that it came back negative. But my question is that when they test your blood for a certain thing, do they see other viruses in your blood or just for what they're looking for?