In an immunocompromised patient (e.g., with hematologic malignancy, stem‑cell or solid‑organ transplant, prolonged neutropenia, or high‑dose steroids), which serum tests are most useful for diagnosing a suspected mold infection?

Serum Testing for Mold Infections in Immunocompromised Patients

In immunocompromised patients with suspected mold infection, serum galactomannan testing performed at least twice weekly is the first-line diagnostic test, with serum β-D-glucan serving as a complementary but non-specific adjunct that should never be used alone for diagnosis. 1

Primary Serum Test: Galactomannan

Serum galactomannan is the recommended first-line serum test for suspected invasive aspergillosis in high-risk immunocompromised patients. 1

Testing Protocol

• Perform serum galactomannan testing at least twice weekly due to transient circulation of the antigen 1
• Use an optical density index cutoff of ≥0.5 on two consecutive samples to define positivity 1
• Serial monitoring enables preemptive antifungal therapy initiation before clinical symptoms develop 1

Performance Characteristics

• Sensitivity: 58-71% in patients with hematologic malignancies or hematopoietic stem cell transplant 1, 2
• Specificity: 65-95% depending on the patient population 1
• A single negative test does NOT rule out invasive mold infection—serial testing is mandatory 1, 2

Critical Limitations and False Results

False-Positive Results: 1, 3

• Piperacillin-tazobactam and other β-lactam/β-lactamase combinations
• Chemotherapy or mucositis (cross-reactive epitopes from gut bacteria/fungi penetrate intestinal mucosa)
• Certain batches of β-lactam antibiotics

False-Negative Results: 3, 2

• Concurrent mold-active antifungal prophylaxis or therapy (significantly reduces sensitivity)
• Aspergillus fumigatus infections (lower sensitivity of 13% compared to 49% for non-fumigatus species) 4

Pathogen Coverage

• Detects: Aspergillus species and Penicillium species only 1
• Does NOT detect: Zygomycetes (Mucorales), Fusarium, Scedosporium, Cryptococcus, or Candida 1

Secondary Serum Test: β-D-Glucan

Serum β-D-glucan should be used as a complementary test when galactomannan is negative but clinical suspicion remains high, but never as the sole diagnostic test. 1, 5

When to Order

• Galactomannan negative but strong clinical suspicion persists 2
• Suspected non-Aspergillus mold infection (Fusarium, Scedosporium) 1
• To increase overall sensitivity when combined with galactomannan 2

Performance Characteristics

• Sensitivity: 50-85% depending on patient population 5
• Specificity: 36-99% (highly variable, lower in ICU patients) 5
• Higher sensitivity than galactomannan for detecting Aspergillus fumigatus and other molds 4

Pathogen Coverage

• Detects: Candida, Aspergillus, Pneumocystis, Fusarium, Acremonium, Trichosporon 1
• Does NOT detect: Zygomycetes (Mucorales) or Cryptococcus 1

Critical Limitations

• Cannot differentiate between fungal species (e.g., cannot distinguish Aspergillus from Candida) 1
• High rate of false-positives from: hemodialysis, hemolysis, hyperlipidemia, bacteremia, blood products (immunoglobulin, albumin), gauze exposure 1
• Should not be relied upon solely for diagnostic decision-making 1, 5

Molecular Testing: Aspergillus PCR

Blood or serum Aspergillus PCR should be used in combination with galactomannan testing to improve diagnostic accuracy, but never as a standalone test. 2

When to Order

• Galactomannan negative but high clinical suspicion 2
• To confirm positive galactomannan and reduce false-positive rates 2
• Serial negative results (≥2) provide >90% negative predictive value 2

Performance Characteristics

• Single positive PCR: ~79% sensitivity, ~80% specificity 2
• A single negative PCR does NOT rule out invasive aspergillosis 2
• Combination of galactomannan + PCR yields superior diagnostic accuracy compared to either alone 2

Important Caveats

• Not FDA-approved or standardized in the United States 2
• Antifungal therapy markedly reduces sensitivity 2
• Does not differentiate Aspergillus species (important for antifungal susceptibility) 2

Diagnostic Algorithm for Suspected Mold Infection

Step 1: Initial Serum Testing

• Order serum galactomannan twice weekly in all high-risk patients 1, 2
• Order serum β-D-glucan if non-Aspergillus mold suspected or as complementary test 2, 5

Step 2: If Serum Tests Negative but Suspicion High

• Proceed to bronchoalveolar lavage (BAL) with galactomannan testing (sensitivity 84%, specificity 88%) 2
• BAL galactomannan has superior performance to serum galactomannan 1
• Consider Aspergillus PCR on blood to increase sensitivity 2

Step 3: Combination Testing for Optimal Accuracy

• BAL galactomannan + serum β-D-glucan: sensitivity 92%, specificity 93% 2
• BAL galactomannan + BAL PCR: sensitivity 85%, specificity 97% (highest diagnostic accuracy) 2

Tests NOT Recommended

Do not order the following tests in immunocompromised patients: 1

• Aspergillus antibody testing (antibodies often absent in immunocompromised hosts)
• Candida antibody or antigen testing
• Single serum tests to rule out infection (serial testing required)

Special Consideration: Mucormycosis (Zygomycetes)

In patients with negative galactomannan and β-D-glucan but persistent suspicion for mold infection, consider mucormycosis. 1

• Galactomannan is typically negative in mucormycosis, which paradoxically increases suspicion for this pathogen 1
• β-D-glucan is negative in mucormycosis (low cell wall glucan content) 1
• Diagnosis requires tissue biopsy with histopathology and culture 1
• Mucorales-specific PCR on tissue specimens (not serum) may be helpful but is not standardized 1