Galactomannan (GM) is a polysaccharide constituent of the cell walls of Aspergillus. It is a carbohydrate molecule with a mannose backbone. Galactomannan is released from the cell wall of Aspergillus hyphae during growth in vivo. In person with invasive aspergillosis, there may be high titers of galactomannan antigen in serum and bronchoalveolar lavage (BAL) fluid. Although the sensitivity and specificity of the galactomannan detection test vary based on settings and nature of test kits used, this test is a major milestone for the early detection of aspergillosis in patients at risk of developing the disseminated disease.
GM index is also used as a prognostic marker. Monitoring galactomannan level in the blood of a patient at risk allows the clinician to follow trends and take appropriate actions.
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Test Methods
Galactomannan is the main antigen used as a diagnostic marker in commercially available kits for the diagnosis of invasive aspergillosis (IA). Both lateral flow assay (LFA) and enzyme immunoassay (EIA) are available for the detection of this antigen in the serum or BAL sample of the patient.
Platelia Aspergillus Galactomannan Enzyme Immunoassay
Platelia Aspergillus GM EIA (Bio-Rad, Marne La Coquette, France) is a double sandwich enzyme immunoassay that utilizes a rat monoclonal antibody EBA-2 directed against the galactofuranoside side chain of the GM antigen.
The monoclonal antibodies are used
- To coat the wells of the microplate and bind the antigen, and
- To detect the antigen bound to the sensitized microplate (conjugate reagent: peroxidase-linked monoclonal antibodies)
In the presence of galactomannan antigen, a monoclonal antibody-galactomannan-monoclonal antibody/peroxidase complex is formed. Chromogen TMB solution is added. This in turn will react with the complexes bound to the well to form a blue color reaction. The enzyme reaction is stopped by the addition of acid, which changes the blue color to yellow. The absorbance of specimens and controls is determined with a spectrophotometer set at 450 and 620/630 nm wavelength.
Find more information from the manufacturer here.
IMMY sōna Aspergillus GM Lateral Flow Assay
The sōna Aspergillus galactomannan lateral flow assay is an immunochromatographic test system for the qualitative detection of Aspergillus galactomannan in serum and BAL samples. It is a sandwich immunochromatographic test system.
Serum and BAL specimens require heat pre-treatment prior to testing. After pre-treatment, the sample is pipetted into a clean tube, and a drop of Running Buffer (REF AFLFRB) is added, followed by an LFA strip (REF LFAF50). The test is run for 30 minutes and the results can be read within 10 minutes of completing the test.
Aspergillus Galactomannan specific antibodies is present in the LFA conjugated to colloidal gold. This antibodies then binds to any galactomannan that may be present in the specimen that moves upward. If any binding occurs, the antibody-antigen complex will migrate up the strip by capillary flow until it is captured by the Aspergillus Galactomannan specific antibodies in the test line. This results in the formation of a visible test line. Additionally, control antibodies conjugated to gold are present that move along with the specimen. The particle will be captured by the control antibodies present on the control line, regardless of positive or negative test results. Positive test results create two lines (test and control lines) and negative results form only one line (control line). If a control line fails to develop, the test is not valid.
Find more information from the manufacturer here.
Aspergillus Lateral-Flow Device
The Aspergillus LFD (AspLFD) is a rapid immunochromatography test developed by OLM Diagnostic. This is useful for the qualitative detection of Aspergillus diagnostic antigen in the human serum and BAL in patients with suspected invasive pulmonary Aspergillosis.
AspLFD uses a monoclonal Ab conjugated to nitrocellulose beads (NCB) to detect Aspergillus diagnostic antigen. The antibody-NCB conjugate binds specifically to Aspergillus diagnostic antigen in the patient sample to form a complex. The complex migrates along the strip until it is captured. These are then concentrated on the test zone (T), where the same antibody has been bound. This causes a red line to appear on the strip. If antigen concentrations are below detectable levels, no visible test line will be produced. Uncaptured NCB conjugate continues to flow towards the end of the strip where it is bound on the control (C) zone. The formation of a red C line indicates the test has been performed correctly.
Find more information from the manufacturer here.
Limitations
- Although GM is a useful marker for IA, there are many causes of false positivity. Piperacillin/tazobactam or amoxicillin with clavulanic acid, PlasmaLyte, and certain foodstuffs, including pasta, vegetables, and milk are all known to cause false positives.
- Several other fungal genera such as Penicillum, Alternaria, Rhodotorula, Paecilomyces, Cryptococcus, Blastomyces, and Histoplasma are known to be reactive and give false-positive results. For example, cryptococcal galactoxylomannan contains an epitope that cross-reacts with the galactomannan antigen of Aspergillus spp., which is the antigen target of the Platelia Aspergillus assay. Please refer to manufacturer guidelines for the cross-reactivity of their process.
- Detection of GM may be reduced in patients with chronic granulomatous disease and patients treated with mold-active antifungals.
