Accuracy of the tests
During the 50 years the direct fluorescent antibody (DFA) test has been used in the United States, there has been no indication it has failed to provide accurate clinical information on the rabies status of an animal for the purposes of treating an exposed person.
Because of its high sensitivity and specificity, in comparison to virus isolation methods, the DFA test is the “gold standard” diagnostic method for rabies and has been rigorously evaluated by international, national, and state health laboratories. The DFA test is currently the only recommended diagnostic method for routine rabies determination in animals in the United States.
During clinical disease, millions of viral particles may be found intermittently in the saliva. In theory, only a single rabies particle or virion is required to result in a productive infection.
The mass of a single virion has been estimated to be approximately 221 thousand kilodaltons. A small proportion of this amount includes viral RNA which accounts for less than 2% of the mass of a single virion.
Any rabies test proposed on living animals would need to be extremely sensitive to detect very minute quantities of protein or nucleic acid. In addition, several repeat tests would be needed over time to ensure that rabies virus excretion is not missed since viral shedding in the saliva is inconsistent. For example, more than 2 billion rabies virions would need to be present in a sample if a test had a sensitivity that was limited to detecting virus in the microgram (1,000,000th of a gram) range.