Antibody tests for fungal infection and fungal allergy have firm place in management for a limited number of diseases notably:
- The diagnosis and monitoring of treatment of chronic pulmonary aspergillosis
- Screening for and management of allergic bronchopulmonary aspergillosis (ABPA)
- The diagnosis and monitoring of treatment of allergic, chronic and granulomatous Aspergillus rhinosinusitis
- Supportive information for the diagnosis of Aspergillus bronchitis in non-immunocompromised patients.
View our video demonstrating the immunodiffusion technique: (Kindly produced by Oscar Zaragoza, Mycology Reference Lab, ISCIII, Madrid, Spain, firstname.lastname@example.org).
Several test formats are used for the detection of antibody in blood including double-diffusion (DD) or immunodiffusion (ID), counterimmunoelectophoresis (CIE), enzyme-linked immunosorbent assay (ELISA) and complement fixation (CF). Less commonly used methods include haemagglutination, radioimmunoassay, immunoblotting and co-CIE. The relative strengths and weaknesses of these methods are described in textbooks and are not reviewed here.
Almost all tests detect IgG or IgE antibodies. No utility has been found for the detection of IgA antibodies to fungi. It is possible that IgM antibodies might have more clinical utility than currently realised, especially for aspergillosis but are not routinely available.
Aspergillus antibody testing
There are multiple marketed IgG antibody tests to detect A. fumigatus antibodies. A smaller number of less well used and often usually incompletely validated tests are available for other species ofAspergillus including A. flavus, A. terreus, A. niger, A. versicolor and A. clavatus. All testing is on serum.
The sensitivity and performance of Aspergillus IgG antibody testing for diagnosis is summarised here:
- The best IgG assays have a 90-95% sensitivity for chronic pulmonary aspergillosis and aspergilloma caused by A. fumigatus, much more sensitive than culture. Requires compatible radiology and relatively non-immunocompromised patient for the diagnosis to be made. The antibody titre varies widely.
- 30-50% of patients with ABPA have detectable A. fumigatus antibodies, usually at low titre. High titres suggest the complication of chronic pulmonary aspergillosis, which needs radiological correlation.
- Fungal sinusitis is more often caused by A. flavus than A. fumigatus. A. flavus IgG antibody is useful confirmatory evidence of infection, especially if cultures are negative but histology or microscopy positive.
- Patients with Aspergillus bronchitis often have positive IgG antibodies (and negative IgE antibodies) but A. fumigatus is slightly less common as the cause, so non-fumigatusIgG antibodies (precipitins) may be required.
- The optimum cut-off for positive IgG titres varies and may be higher than some manufacturers recommend, because healthy control sera (which almost always have very low antibody titres) may have a low titre in which is too low in specific groups. An example is cystic fibrosis in which the baseline Aspergillus IgG antibody titres are higher than normal controls.
IgE antibody testing against A. fumigatus is useful to detect Aspergillus sensitisation. The skin prick test against A. fumigatus is more sensitive than blood testing. Either is required for the diagnosis of ABPA (usually both are positive) and are usually positive at a much lower level in patients with severe asthma with fungal sensitisation (SAFS). This test could be a useful screening test in asthmatic patients to detect ABPA or SAFS, if skin testing not done.
Some patients with other Aspergillus diseases, notably chronic pulmonary aspergillosis, have positive Aspergillus IgE antibody titres. Occasionally this test is positive when the IgG antibody test is negative, which is helpful.
Monitoring of treatment response
Falling IgG antibody titres is useful as a measure of therapeutic response in chronic pulmonary aspergillosis and Aspergillus rhinosinusitis. The rate of fall is slow and takes weeks or months even in patients doing well. Lack of fall is suspicious of treatment failure.
Agar gel double diffusion for Aspergillus precipitins
Counter immunoelectrophoresis equipment for precipitin tests.