When is the β‑D‑glucan assay indicated and how should its results be interpreted in a patient with suspected invasive fungal infection?

β-D-Glucan Testing: Indications and Interpretation

When to Order β-D-Glucan Testing

Serial β-D-glucan testing (twice weekly) should be reserved for high-risk immunocompromised patients, specifically those with hematologic malignancies, allogeneic stem cell transplant recipients, or prolonged profound neutropenia. 1

Specific High-Risk Populations

• Acute myeloid leukemia or myelodysplastic syndrome patients undergoing intensive chemotherapy 1
• Allogeneic hematopoietic stem cell transplant recipients during neutropenic periods 1
• Persistently febrile neutropenic patients despite 4+ days of broad-spectrum antibiotics 2, 1
• Neonates with suspected invasive candidiasis when blood cultures are negative or delayed 1
• ICU patients with multiple risk factors (central venous catheter, broad-spectrum antibiotics, parenteral nutrition, dialysis, recent surgery, pancreatitis) and clinical suspicion for invasive fungal infection 2, 3

When NOT to Order

• Do not order in low-risk patients without significant immunosuppression 1
• Do not use for routine screening in general ICU populations due to poor specificity 2
• Not recommended to guide pediatric clinical decision-making (mean levels higher in uninfected children, optimal thresholds not established) 3
• Avoid routine surveillance in lung transplant recipients (specificity as low as 9%, positive predictive value only 14%) 3

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Interpreting Positive Results (>80 pg/mL)

A single positive β-D-glucan result should NOT trigger immediate antifungal therapy; instead, obtain repeat testing within 3-5 days combined with comprehensive evaluation for confounding factors and additional diagnostic workup. 3, 1

What a Positive Result Means

• Indicates probable invasive fungal infection but is NOT species-specific 2, 1
• Detects Candida, Aspergillus, Pneumocystis jirovecii, and Fusarium species 2, 1
• Does NOT detect Mucormycosis (zygomycetes) or most Cryptococcus species 3, 1
• Sensitivity: 75-80% for invasive candidiasis, approximately 80% for aspergillosis 2, 4
• Specificity: approximately 80%, but significantly lower in ICU patients 2

Critical False-Positive Causes (Must Exclude Before Treatment)

• Beta-lactam antibiotics: piperacillin-tazobactam, amoxicillin-clavulanate, cefepime, ceftriaxone, carbapenems, ampicillin-sulbactam 3, 1, 5
• Hemodialysis (especially with cellulose membranes) 2, 3, 1
• Albumin or intravenous immunoglobulin administration 2, 3, 1
• Gram-positive or gram-negative bacteremia 3, 5
• Exposure to surgical gauze or glucan-containing materials 3, 1
• Gastrointestinal mucositis or mucosal disruption 3
• Fungal colonization without invasive disease 3

Immediate Diagnostic Actions

• Obtain at least two sets of blood cultures from different sites 3, 1
• Repeat β-D-glucan testing within 3-5 days (consecutive positive results significantly improve specificity to >96%) 2, 3, 6
• Perform serum galactomannan if aspergillosis suspected 3
• Obtain high-resolution chest CT if pulmonary symptoms present 3, 1
• Culture site-specific specimens from normally sterile sites when clinically indicated 3

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Interpreting Negative Results (<80 pg/mL)

A negative β-D-glucan has high negative predictive value (>90%) and effectively rules out invasive fungal infection in appropriate clinical contexts. 2, 1

Important Caveats for Negative Results

• Does not exclude invasive aspergillosis in patients already receiving mold-active antifungal prophylaxis 1
• Sensitivity may be reduced by concurrent antifungal therapy 2, 3
• Serial negative results in high-risk patients support withholding empiric antifungal therapy 1

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Treatment Decision Algorithm

DO NOT Start Antifungals Based On:

• Single positive β-D-glucan result without supporting clinical evidence 3, 5
• Positive result in presence of known false-positive causes (beta-lactams, hemodialysis, etc.) 3, 5
• Respiratory secretions or colonization alone 1

START Empiric Antifungal Therapy When:

Two consecutive positive β-D-glucan results (>80 pg/mL) PLUS high-risk clinical features PLUS signs of sepsis unresponsive to antibiotics. 3, 5

Specific Treatment Triggers

• Positive blood culture for Candida species (even single positive culture requires treatment) 5
• Radiographic findings suggestive of invasive fungal infection (halo sign, nodular lesions) plus positive β-D-glucan 3, 5
• Critically ill patient with septic shock and high clinical suspicion for invasive candidiasis 3
• Neutropenic patient with persistent fever despite broad-spectrum antibiotics and high-risk features 3
• Clinical deterioration with imaging findings suggestive of invasive fungal infection 3

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First-Line Antifungal Regimens

For Suspected Invasive Candidiasis

Non-neutropenic critically ill patients: Echinocandin (preferred) 1, 5

• Caspofungin: 70 mg loading dose, then 50 mg daily 3, 5
• Micafungin: 100 mg daily 3, 5
• Anidulafungin: 200 mg loading dose, then 100 mg daily 3, 5

Neutropenic patients: Echinocandin OR liposomal amphotericin B (3-5 mg/kg daily) 1, 5

For Suspected Invasive Aspergillosis (with concurrent positive galactomannan)

• Voriconazole or isavuconazole (first-line) 1
• Liposomal amphotericin B (alternative) 1

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Treatment Duration and Monitoring

• Documented candidemia: Minimum 14 days after first negative blood culture AND resolution of clinical symptoms 1, 5
• Mandatory ophthalmologic examination to rule out endophthalmitis in all candidemia cases 1, 5
• Source control is crucial: remove infected catheters, drain abscesses 1
• Decreasing β-D-glucan levels may correlate with response to antifungal therapy 2

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Critical Pitfalls to Avoid

• Do not ignore beta-lactam antibiotics as a cause of false positivity - this is the most common overlooked confounding factor in ICU patients 3, 5
• Do not use β-D-glucan results from respiratory secretions - only serum testing is validated 1
• Do not rely on single positive result - require consecutive positivity for treatment decisions unless patient is critically ill 3, 6
• Remember the test cannot differentiate between fungal pathogens - clinical context and additional testing (galactomannan, cultures) are essential 2, 1
• Do not order in lung transplant recipients for routine surveillance - 90% have at least one positive result, making it unreliable 3