The autism test was utterly without interpretation. What does 18 mean?

It says at the top:

In the first major trial using the test, the average score in the control group was 16.4. Eighty percent of those diagnosed with autism or a related disorder scored 32 or higher. The test is not a means for making a diagnosis, however, and many who score above 32 and even meet the diagnostic criteria for mild autism or Asperger's report no difficulty functioning in their everyday lives.

but that isn't enough information to calculate P(autism|score=N). There's more information about the test here.