Read Jacobson's papers on IBD. He actually wrote two excellent articles in Reptiles Magazine.
After you look those up you will learn:
A liver biopsy can only detect IBD if there are actual inclusion bodies present in the liver.
While the presence of inclusions is a diagnostic for the disease, the absence of these inclusion bodies does not necessarily indicate that the snake is not diseased or is free from IBD.
In other words - a negative biopsy may mean nothing.