<<One way is that the antibody test was not specific for XMRV which means it could have been reacting to another mouse virus. Until we get a better antibody test this must remain in the realm of possibility. >>
Even if it is another virus, other than XMRV, these findingsare still highly significant. They demonstrate infection with SOME virus, specific to CFS/ME patients. And if it not XMRV, then the (presumably monoclonal antibody) reagent tha they are using could be used to isolate and identify it.
But with the isolation of living virus, capable of infecting human cells and multiplying in them, I am firmly convinced that XMRV (or a subtype of XMRV), is involved.