How do you test for a yeast infection?

Testing for Vaginal Yeast Infections

The most reliable approach to diagnose a vaginal yeast infection is microscopic examination of vaginal secretions using a 10% potassium hydroxide (KOH) preparation to visualize pseudohyphae and budding yeast, combined with pH testing showing pH <4.5. 1, 2

Primary Diagnostic Testing

Specimen Collection

• Collect vaginal secretions directly from the vaginal walls or pooled discharge using a swab 1, 2
• Spread the secretions directly onto a microscopy slide immediately after collection 1
• Avoid contaminating the specimen with cervical mucus, as this reduces test accuracy 2

Essential Microscopic Examination

• Prepare a 10% KOH wet mount by adding KOH solution to the vaginal secretions on the slide 1, 2
• The KOH dissolves cellular debris and red blood cells while preserving fungal elements, making visualization easier 2
• Look for characteristic pseudohyphae (branching filaments) and budding yeast cells under the microscope 1, 2
• A saline wet mount can also be prepared to visualize yeast cells, though KOH provides superior visualization 1
• The specimen must be examined within 30 minutes to 2 hours for optimal results 2

pH Testing

• Measure vaginal pH using pH paper 1, 2
• **Yeast infections typically have pH <4.5**, which helps differentiate from bacterial vaginosis or trichomoniasis (pH >4.5) 1, 2

Whiff Test

• Perform the "whiff test" by adding KOH to vaginal secretions 1
• Absence of a fishy odor (negative whiff test) supports yeast infection and helps rule out bacterial vaginosis 1

When to Order Additional Testing

Vaginal Culture

• Order a vaginal yeast culture when microscopy is negative but symptoms persist 1, 2
• Culture is essential for recurrent or complicated infections to identify the specific Candida species 1
• Some species like C. glabrata do not form pseudohyphae, so only yeast cells may be visible on microscopy 1
• Fungal selective media should be used, and cultures may take 5-14 days to grow 3

Species Identification and Susceptibility Testing

• Perform species identification and antifungal susceptibility testing for:
  • Recurrent infections 1
  • Complicated cases 1
  • Patients previously exposed to azole antifungals 1
  • Treatment failures 1
• Non-albicans Candida species may have reduced susceptibility to azoles, leading to treatment failure if not identified 1

Molecular Testing (NAAT)

• Multiplex NAAT panels (e.g., BD Max Vaginal Panel) can detect Candida species with superior sensitivity and specificity compared to wet prep 2
• NAAT is particularly useful when initial testing is equivocal or negative despite persistent symptoms 2
• These panels can simultaneously detect other causes of vaginitis including bacterial vaginosis and trichomoniasis 2

Critical Interpretation Points

Positive Results

• The presence of pseudohyphae and/or budding yeast on microscopy strongly suggests active yeast infection 1
• However, Candida can be present as a commensal in normal vagina, so correlation with symptoms is important 4
• Yeast isolation from vaginal specimens requires clinical correlation, unlike isolation from normally sterile sites 3

Negative Results

• Negative microscopy does not exclude yeast infection, as sensitivity is imperfect 3
• If clinical suspicion remains high, proceed to culture 1, 2
• Consider alternative diagnoses including bacterial vaginosis, trichomoniasis, or mixed infections 2

Common Diagnostic Pitfalls

• Self-diagnosis by patients is unreliable; microscopic confirmation is required to avoid unnecessary treatments 1
• Not all Candida infections present with classic white "cottage cheese" discharge; some present with only erythema or minimal discharge 1
• Blood contamination does not interfere with KOH preparation, as KOH dissolves red blood cells while preserving fungal elements 2
• Gram stain showing yeast is highly accurate but more resource-intensive than KOH preparation 5
• Standard clinical criteria (pruritus, discharge, erythema) are relatively unreliable without microscopic confirmation 5