There are 4 antigen tests available with excellent to reasonable clinical performance. Antigen testing is directed against a fungus-synthesised and released carbohydrate into body fluids. These antigens are generally poorly immunogenic and so not usually heavily complexed with specific antibody, with the exception of the Candida mannan antigen.
Aspergillus antigen testing
Galactomannan is secreted by all pathogenic Aspergillus species, although some isolates are poor producers. Only one commercial assay is available; Platelia Aspergillus EIA BioRad, using an ELISA format. A similar metabolite is also produced by several other fungal species including Penicilliumspp. Histoplasma capsulatum and other fungi.
Sample type and cutoff
The assay was originally designed for serum, but has also been used on bronchoalveolar lavage fluid (BAL), sputum, CSF, pleural fluid, pericardial fluid and tissue. The generally accepted cutoff in serum for positives is 0.5. The higher the OD the greater the likelihood of invasive aspergillosis. No cutoff has been defined for any other specimen type, but in BAL fluids different authors have recommended cutoffs from 1.0 to 2.0. This variation partly reflects different patient populations and different dilution factors during bronchoalveolar lavage.
Specificity and false positive results
Overall specificity of Aspergillus antigen detection using the galactomannan EIA is approximately 80%. In addition to a lack of specificity with respect to some rarer fungi (in which case probably the patient will receive antifungal therapy), several antibiotics and other patient factors may yield false positives. Several beta-lactam antibiotics are manufactured using fungal fermentation as s first step. This often results in carryover of galactomannan into the antibiotic preparation and ‘false positive’ galactomannan results that can persist for some days. The most common antibiotics to be implicated are piperacillin/tazobactam (Tazocin) and amoxicillin/sulbactam (Augmentin), but some cephalosporins and meropenom have also been implicated. False positive results have not been observed with glycopeptides, quinolones or aminoglycosides. In addition to this, many foods contain galactomannan including pasta and yoghurt (View false positive data). In patients with permeable small bowel, such as those with mucositis after chemotherapy, false positive galactomannan tests may occur, and perhaps slightly more commonly in children.
Sensitivity and false negative tests
The sensitivity of Aspergillus antigen detection using the galactomannan EIA in serum in neutropenic patients not receiving itraconazole or posaconazole prophylaxis (and possibly other agents) is approximately 80%. Itraconazole and posaconazole antifungal prophylaxis reduce the sensitivity to 0-20%. In addition, some isolates of Aspergillus appear to be poor producers of galactomannan. While galactomannan usually is detectable up to a week before a CT scan or other standard diagnostic tests are positive, in some cases, it is produced late in infection.
In non-neutropenic patients, reduced circulating and possibly antigen complexing reduces the serum sensitivity to 0-25%. However, respiratory samples may still be positive. For ventilated intensive care patients with invasive aspergillosis, galactomannan is detectable in ~85% of BAL samples and is the best means currently of establishing a probable diagnosis. Conversely, positive BAL galactomannan tests may be converted to negatives within 3 days by antifungal therapy, so false negative results should be expected soon after anti-Aspergillus therapy is started.
Response to therapy
In haematology patients (profound neutropenia and haematopoietic stem cell transplantation), failure of therapy is associated with persistently elevated galactomannan in serum and conversely a fall in OD by 3 weeks is associated with good outcomes.