The Galactomannan Antigen Test is a diagnostic assay used to detect galactomannan, a polysaccharide component of the cell wall of Aspergillus species, in patient samples such as serum, plasma, or bronchoalveolar lavage (BAL) fluid. Detection of galactomannan serves as an early marker for invasive aspergillosis (IA), particularly in immunocompromised patients, including those with hematologic malignancies, transplant recipients, or prolonged neutropenia.