Use of recombinant mitogillin for serodiagnosis of Aspergillus fumigatus-associated diseases.

In a recent article, Weig et al. evaluated an enzyme-linked immunosorbent assay (ELISA), using recombinant mitogillin for the detection of antibodies in patients with Aspergillus fumigatus-associated diseases (3). The authors reported a good correlation between production of immunoglobulin G (IgG) antibody against mitogillin and clinical diseases, with sensitivities of 100, 64, and 60% for aspergilloma (AO), invasive pulmonary aspergillosis (IPA), and invasive disseminated aspergillosis (IDA), respectively, where the corresponding specificity of the ELISA was 95.4%, using a cutoff value of the mean optical density (OD) of 307 normal blood donor sera plus 2 standard deviations (SD). Although the apparently high sensitivities of the ELISA look encouraging, we think that the interpretation of the results was misleading due to the choice of a low cutoff value. For rare diseases such as AO, IPA, and IDA, in order to eliminate the false positives, high specificities (over 99%) are desirable. In our recently published articles on ELISA for antibody and antigen detection in patients with penicilliosis marneffei, a cutoff value of the mean plus 10 SD was chosen, for a specificity of 100% (1, 2). However, in the study of Weig et al., a low cutoff value of the mean plus 2 SD was chosen, leading to a specificity of just 95.4% (3). The ELISA would have much lower sensitivities if the mean plus 3 SD, the mean plus 4 SD, or the mean plus 5 SD were chosen as the cutoff value. Since an index of 0.75 (serum sample no. 96) was regarded as positive and an index of 0.73 (serum sample no. 63) was regarded as negative, the value obtained by dividing the mean plus 2 SD by the mean plus 3 SD should lie between 0.73 and 0.75. If we assume that the mean plus 2 SD divided by the mean plus 3 SD equals 0.74, 0.26 times the mean would equal 0.22 of the SD. Hence, the mean plus 4 SD divided by the mean plus 3 SD and the mean plus 5 SD divided by the mean plus 3 SD can be calculated as 1.26 and 1.52, respectively. Using the IgG OD index values of the serum samples shown on pages 1725 to 1727 of reference 3, a scattergram can be generated (Fig. 1) and the corresponding sensitivities of the ELISA using the mean plus 3 SD, the mean plus 4 SD, and the mean plus 5 SD as the cutoff values can be calculated (Table 1). In order to achieve a high specificity (over 99%), a cutoff value of at least the mean plus 4 SD or the mean plus 5 SD has to be chosen. At these cutoff values, the corresponding sensitivity of the ELISA for AO would fall to below 90%, whereas those for IPA and IDA would be less than 20%. In fact, it is not surprising to have low sensitivities for IPA and IDA, as patients suffering from invasive aspergillosis are usually severely immunocompromised (4) and production of antibodies is greatly impaired.