From a technical standpoint these assays are performed in a standardized fashion, as Steven mentioned. The ELISA is formatted to be used as a screening tool so it can be done as a high throughput test—with multiple tests performed at the same time—and it is really designed to be as sensitive as possible. With that screening assay we want to pull in as many of the positives, or hopefully the full range of positive-infected individuals, as possible. When you use a broad approach like that, often you end up pulling in people who have other infections that could be falsely positive. So you cast a very wide net with the ELISA hoping to get as many of the infected individuals in there, but realizing that you may have pulled in some of these people who are not.
That's why we use the second tier test. To remove those falsely positives we've used the western blot,which is supposed to be more specific. It's supposed to be able to detect primarily the individuals who are infected. We often see that when we do the ELISA; say we get a hundred screened positives. But when we do the western blot a percentage of those will come out because they were falsely positive as a result of infection with other disease processes or just reactive antibodies that were non-specific to Borrelia. So, it's a well-established criterion whereby we use a sensitive test at the front end and then re-evaluate those with a western blot. Again, a combination of the two assays provides better depiction of true positives and true negatives than any of the tests run individually.