Even with testing the liver they cannot rule OUT IBD. They can definitively say the snake HAS IBD... they cannot say definitively that the snake does NOT have IBD. They can always say the inclusion bodies were only in the brain, etc. So getting a blood test would not rule out IBD.
Generally I was told(by the vets at UF) that boas can remain asymtomatic for an unknown period of time, being a carrier. Ball pythons however, die fairly quickly.