What is the appropriate test to diagnose a suspected urinary fungal infection (candiduria)?

Diagnostic Testing for Urinary Fungal Infection

For suspected urinary fungal infection (candiduria), obtain a freshly voided urine sample for microscopy and culture, which remain the cornerstone diagnostic tests, with species identification mandatory for all positive cultures. 1, 2

Primary Diagnostic Approach

Urine Microscopy and Culture

• Collect freshly obtained urine from the bladder and process it with both microscopy and culture on selective fungal media. 1, 3
• Microscopy can identify both yeasts and pseudohyphae, which serve as clinical signs of fungal UTI, though this finding alone does not distinguish infection from colonization. 1, 3
• Culture on selective media is essential to avoid bacterial overgrowth and to confirm viable fungal organisms. 1, 4
• No validated colony count threshold exists for fungal UTI - the traditional bacterial cutoff of 10^5 CFU/mL does not apply to Candida, and concentrations >10^3/mm^3 are typically considered significant in the appropriate clinical context. 5, 6

Species Identification is Mandatory

• All positive cultures must undergo species identification, as different Candida species have vastly different antifungal susceptibility patterns. 1, 2, 7
• C. glabrata exhibits inherently reduced azole susceptibility, making species identification critical for treatment decisions. 7, 4
• Antifungal susceptibility testing is recommended for all isolates from symptomatic patients, especially in recurrent or complicated cases and those with prior azole exposure. 1, 7

Special Considerations for Catheterized Patients

Critical Diagnostic Limitations

• In patients with indwelling urinary catheters, standard urine culture gives >50% false-negative results for biofilm infections. 1
• Freshly obtained bladder urine should still be processed as in non-catheterized patients, but clinicians must recognize the high false-negative rate. 1
• Examination of removed catheters or stents is necessary for definitive detection of biofilm infection, though routine examination of all removed catheters is not recommended. 1

Adjunctive Diagnostic Tests

Beta-D-Glucan (BDG) Testing

• BDG testing is recommended when invasive candidiasis is suspected, as it can detect infection days to weeks before positive cultures. 2, 7
• The test has 75-80% sensitivity and 80% specificity for invasive candidiasis. 7
• BDG is particularly useful in critically ill patients where candiduria may be a marker for disseminated infection. 2, 6

Imaging Studies

• Renal ultrasonography is the preferred initial imaging study in symptomatic or critically ill patients with candiduria to evaluate for pyelonephritis, abscess, or obstruction. 6
• CT imaging is superior to ultrasound for detecting pyelonephritis or perinephric abscess. 1, 6
• Imaging is not routinely indicated for simple cystitis but should be obtained if symptoms persist beyond 72 hours or if upper tract involvement is suspected. 1, 6

Tests NOT Recommended

Molecular and Novel Diagnostics

• PCR-based and molecular techniques cannot distinguish true infection from asymptomatic bacteriuria and lack clinical validation for urinary fungal infections. 1
• These tests cannot determine bacterial viability or quantitation, which are crucial for differentiating colonization from infection. 1
• Molecular diagnostics may lead to overtreatment by detecting clinically insignificant organisms. 1

Echinocandins and Non-Fluconazole Azoles

• Echinocandins achieve negligible urinary concentrations and should not be used for lower urinary tract Candida infections. 2, 4, 8

Common Diagnostic Pitfalls

Distinguishing Infection from Colonization

• Most candiduria represents asymptomatic colonization rather than true infection - treatment decisions must be based on clinical context, not culture results alone. 2, 4, 6
• Pyuria is nonspecific and does not reliably distinguish infection from colonization. 6
• Simply removing an indwelling catheter eliminates candiduria in 40-50% of asymptomatic patients without antifungal therapy. 2, 8

When Discrepant Results Occur

• Microscopy may be positive when culture is negative because microscopy detects both viable and non-viable cells. 9
• Negative cultures do not exclude infection, as some Candida species require 5-14 days to grow. 9
• Consider repeat testing with longer incubation times if clinical suspicion remains high despite negative initial cultures. 9