When should serum (1→3)-β-D-glucan be ordered, what is the diagnostic cutoff, how are positive and negative results interpreted, and how should the results guide antifungal therapy?

Serum (1→3)-β-D-Glucan Testing: Clinical Indications and Interpretation

When to Order the Test

Order serial serum (1→3)-β-D-glucan testing twice weekly in high-risk patients with hematologic malignancies, allogeneic hematopoietic stem cell transplant recipients, or those with prolonged profound neutropenia who are at elevated risk for invasive fungal infections. 1

Specific Clinical Scenarios for Testing:

• Patients with acute myeloid leukemia or myelodysplastic syndrome undergoing intensive chemotherapy 1
• Allogeneic HSCT recipients during neutropenic periods 1
• Persistently febrile neutropenic patients despite broad-spectrum antibiotics 1
• Neonates with suspected invasive candidiasis when blood cultures are negative or delayed 1

Do NOT Order in:

• Low-risk patients without significant immunosuppression 1
• As a standalone screening test in general ICU populations 2
• For diagnosis of invasive aspergillosis as the primary diagnostic tool (galactomannan is preferred) 1, 3

Diagnostic Cutoff Values

The FDA-cleared Fungitell assay uses a cutoff of >80 pg/mL to define a positive result, though some studies have evaluated 60 pg/mL. 1, 4

• At 80 pg/mL cutoff: sensitivity 64-77%, specificity 92%, positive predictive value 89% 4
• At 60 pg/mL cutoff: sensitivity 70-81%, specificity 87%, positive predictive value 84% 4
• Require two consecutive positive results rather than a single positive test to improve specificity and reduce false-positive interpretation 5

Interpretation of Results

Positive Result (>80 pg/mL):

A positive β-D-glucan indicates probable invasive fungal infection but is NOT species-specific—it cannot distinguish between Candida, Aspergillus, Fusarium, or Pneumocystis jirovecii. 1, 5, 3

Immediate Actions:

• Obtain blood cultures and site-specific cultures immediately 2
• Perform chest CT imaging if pulmonary symptoms present 1
• Consider additional biomarkers: galactomannan for aspergillosis, mannan/anti-mannan for candidiasis 2
• Assess for source control needs (remove central lines, drain abscesses) 2

Organisms Detected:

• Positive for: Candida species (81% sensitivity), Aspergillus (80% sensitivity), Fusarium, Pneumocystis jirovecii 1, 4
• Negative for: Cryptococcus neoformans, Mucor, Rhizopus (zygomycetes lack β-D-glucan) 1, 6, 4

Negative Result (<80 pg/mL):

A negative β-D-glucan has high negative predictive value (73-75%) and can help rule out invasive fungal infection in the appropriate clinical context. 2, 4

• Does NOT exclude invasive aspergillosis, especially in patients already receiving mold-active antifungal therapy 1
• Serial negative results in high-risk patients support withholding empiric antifungal therapy 1

Critical False-Positive Causes (Must Exclude Before Acting)

False-positive results are extremely common and must be systematically excluded before initiating antifungal therapy: 1, 5, 2

• Hemodialysis with cellulose membranes 1, 6
• Blood products: albumin, IVIG, immunoglobulin infusions 1, 5, 2
• Antibiotics: β-lactam/β-lactamase combinations (piperacillin-tazobactam, amoxicillin-clavulanate), some cephalosporins, carbapenems, ampicillin-sulbactam 1
• Gram-negative bacteremia: 59% false-positive rate, particularly with Gram-negative bacilli bloodstream infections 7
• Gauze exposure to surgical sites or glucan-contaminated materials 1
• Mucosal barrier injury: severe mucositis, gastrointestinal disruption 5

Guiding Antifungal Therapy

Initiation Criteria:

Start antifungal therapy when positive β-D-glucan is accompanied by:

1. Clinical signs of infection (fever, hemodynamic instability, organ dysfunction) AND
2. High-risk factors (neutropenia, recent abdominal surgery, ICU admission) AND
3. Supporting evidence (positive cultures, suggestive imaging, or second consecutive positive β-D-glucan) 2

Empiric Therapy Selection:

For suspected invasive candidiasis:

• Non-neutropenic patients: Echinocandin (caspofungin, micafungin, anidulafungin) as first-line 2
• Neutropenic patients: Echinocandin or liposomal amphotericin B 2
• Fluconazole is an alternative only in hemodynamically stable, non-neutropenic patients 2

For suspected invasive aspergillosis (if galactomannan also positive):

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

Duration and Monitoring:

• Treat candidemia for ≥14 days after first negative blood culture and symptom resolution 2
• Perform ophthalmologic examination to exclude endophthalmitis in all candidemia cases 2
• Serial β-D-glucan levels can monitor therapeutic response—declining levels suggest treatment efficacy 8

Key Pitfalls to Avoid

• Never diagnose or treat based solely on a single positive β-D-glucan without clinical correlation 2, 3
• Do not treat Candida isolated from respiratory secretions—this represents colonization, not infection 2
• Remember β-D-glucan has poor specificity for aspergillosis; use galactomannan as the primary biomarker 1, 3
• In neonates, blood culture sensitivity is only 21-71%; negative cultures do not exclude invasive candidiasis 1
• Antifungal prophylaxis or treatment reduces test sensitivity—interpret cautiously in patients already on antifungals 1, 4