What is a fungal diagnostic test?

Fungal Diagnostic Tests

Fungal diagnostic tests are laboratory methods used to detect fungal infections through various techniques including antigen testing, serological testing, PCR-based assays, direct visualization, and culture methods, with selection of specific tests depending on the suspected fungal pathogen and patient immune status. 1

Types of Fungal Diagnostic Tests

Direct Visualization and Culture

• Direct microscopic examination of clinical samples (using KOH preparations, stains)
• Culture of specimens from normally sterile body sites
• Important note: Tissue homogenization should be avoided when processing samples for filamentous fungi, as it significantly reduces organism recovery rates and detection of fungal elements - instead, tissue should be chopped into small portions (dicing) 2

Antigen Testing

• Galactomannan (GM) testing:

  • Recommended for serum testing in severely immunocompromised patients with suspected invasive pulmonary aspergillosis (IPA) 1
  • Recommended for bronchoalveolar lavage (BAL) testing in patients with suspected invasive fungal diseases 1

• (1→3)-β-D-glucan (BDG) assays:

  • Used for invasive candidiasis detection
  • Caution: Should not be relied upon solely for diagnostic decision-making in critically ill patients 1

• Specific antigen tests for endemic mycoses:

  • Histoplasma antigen testing (urine and serum)
  • Blastomyces antigen testing (urine)
  • Coccidioides antigen testing (urine and serum) 1

Serological Testing (Antibody Detection)

• Complement fixation (CF) test - highest sensitivity for coccidioidal meningitis 3
• Immunodiffusion (ID) testing
• Enzyme immunoassay (EIA) panels - offer local availability with rapid turnaround and higher sensitivity, though less specific 1

Molecular Testing

• PCR-based assays for detection of fungal DNA
• Loop-mediated isothermal amplification (LAMP) assays for certain fungi like Histoplasma 4

Diagnostic Approach by Fungal Infection Type

Invasive Pulmonary Aspergillosis

• For severely immunocompromised patients with suspected IPA:
  • Serum GM testing is strongly recommended 1
  • BAL testing with GM is recommended, especially when serum GM is negative but risk factors are strong 1

Invasive Candidiasis

• Multiple testing modalities recommended:
  • Culture remains the cornerstone for identification and susceptibility testing
  • BDG testing can be helpful but should not be used alone for diagnostic decisions 1

Endemic Mycoses (Blastomycosis, Histoplasmosis, Coccidioidomycosis)

• Multi-test approach recommended for all endemic mycoses:
  • Direct visualization and culture
  • Serum antibody testing
  • Urine and serum antigen testing 1

Coccidioidomycosis

• For suspected coccidioidomycosis:
  • Use more than one diagnostic test (direct visualization, culture, antigen testing, serology)
  • For coccidioidal meningitis specifically, CSF complement fixation test is the best diagnostic test 3
  • Typical CSF findings include elevated opening pressure, elevated WBC with lymphocytic predominance, markedly elevated protein, and very low glucose 3

Blastomycosis

• For suspected blastomycosis:
  • Serum antibody testing against BAD-1 antigen is recommended alongside clinical and epidemiological data 1
  • Urinary antigen testing is particularly useful in immunocompromised patients 1
  • Caution: Antigen assays have substantial cross-reactivity with other mycoses, including histoplasmosis 1

Common Pitfalls and Considerations

• Relying on a single test can lead to missed diagnoses - use multiple diagnostic modalities 1, 3
• Geographic exposure history is critical for suspecting endemic mycoses 3
• Inadequate sample volume can lead to false negatives, particularly for CSF testing 3
• Antifungal therapy can reduce sensitivity of conventional diagnostic techniques - consider this in patients already receiving antifungals 5
• Serological testing for coccidioidomycosis can wane over time, potentially becoming negative after successful treatment 1
• Cross-reactivity between fungal pathogens can occur with antigen and antibody testing 1

Emerging Technologies

• Point-of-care tests are being developed but are not yet widely available for fungal infections 4, 6
• Promising developments include:
  • Aspergillus-specific lateral-flow device tests
  • Cryptococcal lateral-flow assays
  • Loop-mediated isothermal amplification assays for histoplasmosis 4

Early and accurate diagnosis is critical as fungal infections cause significant morbidity and mortality if left untreated, requiring prompt initiation of appropriate antifungal therapy 3.