galactomannan test

Galactomannan is a component of the cell wall of the mold Aspergillus and is released during growth. Detection of galactomannan in blood is used to diagnose invasive aspergillosis infections in humans.

Galactomannan is a polysaccharide on the Aspergillus cell wall, and antigen testing can be detected in blood and BAL fluid. Sensitivity may be higher in the BAL, and current recommendations are to obtain a BAL sample for galactomannan testing, when either the serum antigen is negative or there are confounding factors for false-positives in high-risk patients like this one.1

Aspergillus infection is associated with a positive serum and BAL GM enzyme immunoassay (EIA) (choice D is incorrect). GM is a polysaccharide contained in the Aspergillus cell walls. An optical density (OD) index of 0.5 or greater is usually used as positive for both serum and BAL, although some guidelines use a threshold OD index of 1.0 or greater in serum. It is 81% specific and 82% to 88% sensitive for invasive aspergillosis (IA) in patients who are at high risk for IA, those with hematologic malignancy, or those who have received stem cell transplants. A cutoff of 1.0 results in a sensitivity of 78% and a specificity of 93% in some studies. The GM EIA at BAL adds additional sensitivity compared with results with culture. The test is not as good in recipients of solid organ transplant, and other immunocompromised populations, and in those with disease limited to the airways, such as in recipients of lung transplants with Aspergillus tracheobronchitis, in whom false-positive BAL results can occur. False-positive GM EIA serum results were formerly described with piperacillin-tazobactam because of the presence of a cross-reactive antigen, but this is no longer the case. Outside the United States, false-positive GM results have also been reported with formulations of IV amoxicillin-clavulanate. Other causes of false-positive results of the Aspergillus GM EIA include infections with other fungi that share cross-reacting antigens such as Fusarium species, Penicillium species, Histoplasma, blastomycosis, Cryptococcus, contamination of foods with Aspergillus or closely related fungi, transfusions of blood products that were collected in specific bags (from a single manufacturer), and IV immunoglobulin. The sensitivity of detecting GM in serum is decreased by concurrent administration of antifungal therapy. The Aspergillus GM EIA result is notably negative with mucormycosis.

Cryptococcus generally has a negative BDG result, although it may have a positive GM result (choice B is incorrect). Mucormycosis generally has negative results for both assays (choice C is incorrect). Candida may have a positive BDG result but is unlikely to cause invasive respiratory infection.2

Footnotes

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