Discussion of ImmunoCAP testing
Discussion of ImmunoCAP testing
Currently we are using the automated Fluorescent Immuno Enzyme Assay (FIEA) ImmunoCAP to detect IgG antibodies to Avian antigens and to Aspergillus antibodies. Click on the links for a summary of those tests.
The detection of IgG antibodies is not technically difficult, however the interpretation of the significance of the results of such testing can be fraught with difficulty. The ImmunoCAP system is a commercial automated FIEA that has for many years been used to detect IgE antibodies in the diagnosis of allergic reactions. In recent years reagents to detect IgG antibodies to a number of antigens have been produced and many laboratories in the UK and elsewhere have carried out studies on the usefulness of IgG testing using ImmunoCAP. In many cases studies have been aimed at correlating current methods of IgG detection with an ImmunoCAP score without necessarily correlating the results of either test with a well defined clinical entity and this may result in differences in the cutoff values recommended by different laboratories.
Aspergillus IgG detection by ImmunoCAP
Aspergilloma, the formation of a fungal ball in a pre-existing lung cavity is a relatively easy disease entity to diagnose, primarily based on radiological imaging. However, it is often useful to confirm aspergilloma by serodiagnosis as these patients typically have very high levels of anti-Aspergillus antibody (precipitins or IgG).
Another forms of aspergillosis that is seen typically in patients with COPD is chronic necrotising aspergillosis (CNA). This infection is a progressive disease associated with limited tissue invasion and cavitation. Again these patients are characterised by relatively high levels of anti-Aspergillus antibodies.
Although our study and other studies unpublished and in the literature (Van Hoeyveld et al. 2005) do not provide a lot of data by which to determine a suitable cutoff in non-CF patients. However, it is clear that levels of Aspergillus IgG above 40mgA/L are likely to indicate aspergilloma, CNA or other forms of aspergillosis in patients with relevant clinical features.
Diagnosis of Bird Fanciers Lung
Bird fanciers Lung (BFL) is a form of hypersensitivity pneumonitis following exposure to inhaled avian antigens. Detection of precipitating antibody or IgG to bird antigens has been proposed as being an important aspect of the diagnosis of BFL (The Diffuse Parenchymal Lung Disease Group of the British Thoracic Society, 1999). Traditionally precipitin testing has been used, or as previously in our laboratory a combination of ELISA and precipitin testing. However it has been proposed that the ImmunoCAP provides an improved approach to BFL diagnosis and that using the Pigeon serum antigen provides sufficient cross reaction to antibodies to other avian antigens (McSharry et al. 2006). It is proposed that any sera with ³10mgA/L IgG antibody to pigeon serum is strongly suggestive of significant exposure to avian antigens. In patients without symptoms this may indicate an early stage of BFL and that antigen avoidance may enable the patient to avoid serious lung damage. In patients with symptoms of hypersensitivity pneumonitis this finding should enable a diagnosis of BFL to be made.
Many people will be used to using precipitin tests to diagnose bird fanciers lung. When we compared precipitin reactions to the Pigeon serum IgG ImmunoCAP result we found that using a cutoff of 10mg/L the ImmunoCAP result gave a sensitivity of 100%, and specificity of 87% for the detection of pigeon serum precipitin positive sera. Using the pigeon serum ImmunoCAP IgG we found a sensitivity of 94% and specificity of 88% for the detection of budgie serum precipitin positive sera.
References
Greenberger, PA (2002) Allergic bronchopulmonary aspergillosis. J. Allerg. Clin. Immunol. 110: 685-692.
The Diffuse Parenchymal Lung Disease Group of the British Thoracic Society (1999) The diagnosis, assessment and treatment of diffuse parenchymal lung disease in adults. Thorax 54 (Supplement 1): S1-30.
McSharry, C, Dye, GM, Ismail, T, Anderson K, Spiers EM, and Boyd, G (2006) Quantifying serum antibody in bird fanciers hypersensitivity pneumonitis. BMC Pulmonary Medicine 6:16-24
Stevens, D. A., Moss, R. B., Kurup, V. P., Knutsen, A. P., Greenberger, P., Judson, M. A., Denning, D. W., Crameri, R., Brody, A. S., Light, M., Skov, M., Maish, W., Mastella, G., and the Cystic Fibrosis foundation Consensus Conference. (2003) Allergic bronchopulmonary aspergillosis in cystic fibrosis – state of the art: cystic fibrosis foundation consensus conference. Clin. Infect. Dis. 37: (Suppl 3): S225-64
Barton RC, Hobson RP, Denton M, Peckham D, Brownlee K, Conway S, Kerr MA. (2008) Serologic diagnosis of allergic bronchopulmonary aspergillosis in patients with cystic fibrosis through the detection of immunoglobulin G to Aspergillus fumigatus. Diagn. Microbiol. Infect. Dis.
Van Hoeyveld E, Dupont L, and X Bossuyt (2006) Quantification of IgG antibodies to Aspergillus fumigatus and pigeon antigens by ImmunoCAP technology: An alternative to the precipitation technique? Clin. Chem. 52: 1785-93.
