Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1354 page)

FUNGAL ANTIGEN, GALACTOMANNAN

   Definition

   Galactomannan is an
Aspergillus
antigen that may be detected in the serum of patients with invasive aspergillosis (AI). The most sensitive galactomannan detection uses monoclonal specific antibodies in an EIA format. Detection of
Aspergillus
galactomannan has been shown to provide good sensitivity and specificity for AI. Galactomannan testing may improve patient management of patients at risk for AI because culture and histopathologic methods have limited sensitivity for specific detection, and
Aspergillus
culture isolates may represent contamination or patient colonization.

   Special Collection and Transport Instructions

   Serum is most commonly collected for testing; standard laboratory procedures for collection and transport are used. Other specimens may be acceptable and are collected and transported according to instructions of the assay manufacturer.

   Use

   Specimens may be collected from patients at risk for invasive aspergillosis. Sequential testing may improve detection in patients with emerging AI. Testing is not recommended for patients receiving antifungal prophylaxis because this treatment markedly decreases sensitivity of the assay.

   Interpretation

   
Expected result:
Not detected.

   
Positive results:
Positive results support a diagnosis of invasive aspergillosis, but two consecutive positive results are required for a positive patient evaluation. A second specimen should be collected, before initiation of antifungal treatment, from patients to confirm initially positive galactomannan results. False-positive results may be caused by cross-reactive antigens from nonaspergillus species. Positive reactions, even persistently positive reactions, should be interpreted in the context of clinical presentation and other signs and symptoms.

   
Negative results:
The probability of invasive
Aspergillus
is reduced, but AI is not excluded. Repeat testing in high-risk patients may improve detection of early AI. False-negative reactions may be caused by collection of specimens after initiation of antifungal treatment, low fungal load in serum (e.g., localized infection), high galactomannan antibody titers, or other factors.

   Limitations

   Sensitivity of assays may be limited early in active infection. False positive results may be seen in up 18% of patients without AI. Attempts to confirm positive results by culture or histopathology should be attempted to minimize the possibility of false-positive diagnosis. Evaluation of patients at risk for invasive aspergillosis should not be evaluated with galactomannan as the sole diagnostic testing. Fungal culture, wet mount, imaging studies, histopathology and other relevant diagnostic evaluations should be performed as relevant.