Is a fungal culture of value in patients who have received multiple lines of systemic antifungal treatment, and what is the best protocol to confirm diagnosis in such cases?

Fungal Culture Value in Patients with Prior Antifungal Exposure

Fungal culture remains valuable even after multiple lines of systemic antifungal therapy, as it provides critical information for species identification and antifungal susceptibility testing, which are essential for guiding therapy in treatment-refractory cases. 1

Why Culture Remains Important Despite Prior Treatment

Species Identification and Resistance Detection

• Antifungal susceptibility testing (AST) must be performed for all Candida strains isolated from blood and deep sites, particularly in patients exposed to antifungal agents or with clinical failure. 1
• Prior azole exposure increases the likelihood of fluconazole-resistant species such as Candida krusei and Candida glabrata, making species identification critical for appropriate therapy selection. 1
• Strains from patients with relapsing infections or those belonging to rare and emerging species require AST to guide management. 1

Limitations of Non-Culture Diagnostics After Treatment

• The sensitivity of fungal antigen tests (1,3-β-D-glucan or galactomannan) is low particularly in patients receiving antifungal agents. 1
• Blood cultures are positive in less than 50% of hematogenously disseminated candidiasis cases, but when positive, they provide definitive diagnosis and allow for susceptibility testing. 1, 2
• Non-culture diagnostics may yield false-positive results in patients on antifungal therapy, potentially leading to inappropriate treatment decisions. 2

Best Protocol to Confirm Diagnosis in Previously Treated Cases

Comprehensive Diagnostic Approach

1. Obtain Tissue Samples When Feasible

• If clinically feasible, biopsy specimens should be taken from suspected areas (skin, organ lesions) and examined by both microscopy and culture. 1
• Tissue samples provide higher diagnostic yield than blood cultures alone and allow for histopathological confirmation. 1
• Recovery of fungi from aspiration or biopsy of skin or deep soft tissues warrants aggressive systemic antifungal therapy. 1

2. Multiple Culture Sites and Repeated Sampling

• All clinical samples from patients at high risk for invasive fungal disease must be cultured for fungi. 1
• Fungal cultures should be taken intra-operatively if surgical intervention occurs, especially if the patient has had prior antifungal therapy. 1
• In cases with high clinical suspicion but initial negative results, consecutive laboratory fungal tests will eventually produce positive results in the majority of cases (94% in one study). 3

3. Combine Culture with Non-Culture Methods

• A combination of various methods with regular screenings is mandatory for early diagnosis of invasive fungal disease and for monitoring response to antifungal treatment. 1
• Use 1,3-β-D-glucan testing in combination with clinical assessment, recognizing its high negative predictive value for discontinuing empirical therapy. 1
• Molecular diagnostic tools (e.g., Aspergillus PCR) display high sensitivity and specificity and should be used in combination with other non-cultural tests such as antigen detection. 1

4. Perform Antifungal Susceptibility Testing

• Reference procedures or validated commercial techniques should be used for AST, with cautious interpretation requiring expertise. 1
• AST is particularly useful for strains from patients exposed to antifungal agents, isolates from patients with clinical failure, and species known to be resistant or less susceptible to antifungal drugs. 1
• For superficial isolates, AST is recommended for patient management in cases who failed to respond to antifungal agents or have relapsing infection. 1

Common Pitfalls to Avoid

Culture Interpretation Challenges

• Yeasts found in sputum or bronchoalveolar lavage fluid should be regarded as contamination or colonization until invasive disease is proven. 1
• However, recovery of moulds from sputum in patients with clinical signs suggestive for invasive fungal disease and prolonged granulocytopenia should be regarded as a possible indicator of fungal pneumonia. 1
• Any fungi cultured from urine of a severely granulocytopenic patient without a urinary catheter may be interpreted as an indication of fungal infection. 1

Timing and Sample Quality

• Collect samples for microbiological analysis before adjusting antibiotic or antifungal therapy whenever possible. 1
• Semi-quantitative techniques using fungal selective agar are recommended for optimal culture results. 1
• All fungi recovered from sterile sites should be identified down to the species level. 1

Risk Stratification for Empiric Therapy Decisions

While awaiting culture results in previously treated patients:

• Positive peritoneal fungal culture is a significant risk factor for adverse outcomes, but empiric antifungal therapy should be reserved for critically ill or severely immunocompromised patients. 1
• Clinical prediction rules such as the Candida Score (surgery, multifocal colonization, total parenteral nutrition, severe sepsis) can help identify high-risk patients, though these have high specificity but low sensitivity. 1
• The highest level of evidence for the presence of invasive fungal disease should be obtained before initiating systemic antifungal therapy. 1