Antigen Detection and Serological Testing

Antigen detection and serology diagnose fungal infection by immunological signals rather than by recovering the organism. Antigen assays detect fungal molecules such as galactomannan or cryptococcal capsular antigen directly in body fluids, while serology measures the host antibody response, providing rapid, often non-invasive evidence of infection.

Definition

Antigen detection and serological testing in mycology are immunological methods that diagnose fungal infection by detecting fungal antigens shed into body fluids or by measuring the host's antibody response, typically using enzyme immunoassays, latex agglutination, or lateral flow assays.

Scope

This topic covers immunoassays used in medical mycology: galactomannan for aspergillosis, cryptococcal antigen testing including lateral flow formats, and antibody-based serological tests, together with how their sensitivity and specificity shape interpretation. It is descriptive reference material on the methods and does not provide testing or treatment protocols.

Core questions

• Which fungal antigen is most informative for the infection in question?
• When does antibody serology add value over antigen detection?
• How do sensitivity and specificity, and the choice of specimen, affect interpretation of a result?
• What causes false-positive and false-negative antigen results?

Key concepts

• Galactomannan antigen (Aspergillus)
• Cryptococcal capsular antigen
• Enzyme immunoassay (EIA)
• Lateral flow assay and lateral flow device
• Latex agglutination
• Antibody serology and the host response
• Sensitivity, specificity, and cross-reactivity

Mechanisms

Antigen assays use antibodies to capture fungal molecules released during infection. The galactomannan enzyme immunoassay detects a polysaccharide of the Aspergillus cell wall in serum or bronchoalveolar lavage fluid, and lateral flow formats give rapid point-of-care readouts. Cryptococcal antigen testing detects the capsular polysaccharide of Cryptococcus in serum or cerebrospinal fluid by latex agglutination, enzyme immunoassay, or a highly sensitive lateral flow assay. Serology instead measures antibodies the host raises against fungal antigens, which can indicate exposure or infection but may lag behind, or be blunted in, immunocompromised patients. Because antigens can cross-react and antibody responses vary, each assay carries characteristic false-positive and false-negative patterns, so results are interpreted with knowledge of the assay's sensitivity and specificity and alongside culture, microscopy, and molecular findings.

Clinical relevance

Antigen and antibody tests provide some of the fastest and least invasive evidence used to recognise fungal infections, and results such as galactomannan contribute to consensus categories of probable invasive fungal disease. This entry explains how the assays work and what limits their interpretation, as reference material rather than guidance for ordering tests or treating patients.

Evidence & guidelines

A meta-analysis of the galactomannan assay quantified its diagnostic performance for invasive aspergillosis and showed how performance varies with the patient population, and the EORTC/MSGERC consensus definitions include specified antigen results among mycological criteria. Studies of the cryptococcal antigen lateral flow assay document its high accuracy and point-of-care utility, and best-practice recommendations situate antigen and antibody testing within the diagnostic pathway.

History

Serological methods for fungal disease, including antibody tests and early latex agglutination assays for cryptococcal antigen, were established in the mid-to-late twentieth century. Enzyme immunoassays for galactomannan brought antigen detection into routine use for invasive aspergillosis, and the subsequent development of sensitive lateral flow assays, notably for cryptococcal antigen, extended rapid antigen testing to point-of-care settings.

Seminal works

• pfeiffer-2006
• kabanda-2014

Frequently asked questions

What is the difference between antigen detection and serology?

Antigen detection looks for molecules of the fungus itself in body fluids, indicating the organism is present, whereas serology measures the host's antibodies against the fungus, reflecting the immune response to exposure or infection.

Why can antigen tests give false-positive results?

Fungal antigens can cross-react with structurally similar molecules, including some from other microbes, foods, or medical products, so a positive result is interpreted in clinical context and alongside other diagnostic methods.