The diagnosis of aspergillosis has been most extensively studied in humans. Ancillary diagnostic assays used in people include PCR to detect Aspergillus DNA from blood, an ELISA to detect Aspergillus antigen and an ELISA to detect anti-Aspergillus antibody. These studies clearly indicate that even a combination of these three assays will not be adequate to detect many cases of aspergillosis. The problem comes from the fact that most people who contract aspergillosis are immunocompromised. This also may be true in birds. If the infected person's immune system is adequate to contain the disease and the organism is localized in a walled-off granuloma, then these individuals are found to produce antibody. People with generalized disease are generally severely immunocompromised and they do not produce antibody. In these people, Aspergillus antigen and DNA are most likely to be found in the blood, but they are not when the lesion is encapsulated. If the pathophysiology of avian aspergillosis resembles that seen in humans, then none of these assays are likely to detect infection in most infected birds. A combination of these assays may be more specific, but false negatives are to be expected.
Reference: "Clinical Avian Medicine - Vol I and II" by Harrison and Lightfoot