What is the differential diagnosis between bacterial vaginosis (BV) and yeast infection?

Differentiating Bacterial Vaginosis from Yeast Infection

The key to distinguishing BV from yeast infection lies in vaginal pH, discharge characteristics, microscopy findings, and the whiff test—BV presents with elevated pH (>4.5), thin homogeneous discharge, clue cells, and fishy odor, while yeast infection shows normal pH (3.8-4.2), thick white "curdled" discharge, and hyphae/budding yeast on microscopy. 1, 2, 3

Clinical Presentation Differences

Bacterial Vaginosis Characteristics:

• Homogeneous, thin, white noninflammatory discharge that smoothly coats the vaginal walls 1, 2
• Fishy odor, particularly noticeable after intercourse or with alkaline exposure 1, 3
• Minimal to no vulvar irritation or inflammation 1
• Up to 50% of women may be completely asymptomatic 1

Yeast Infection (Candidiasis) Characteristics:

• Thick, "curdled" white discharge with cottage cheese-like appearance 3
• Intense vulvar pruritus (itching) as the predominant symptom 3
• Hyperemic vagina with erythematous and/or excoriated vulva 3
• No characteristic odor 3, 4

Diagnostic Algorithm

Step 1: Measure Vaginal pH (Before Any Other Testing)

• pH >4.5: Suggests BV or trichomoniasis 1, 2, 5
• pH 3.8-4.2 (normal range): Strongly suggests candidiasis 3, 4

Critical pitfall: Test pH before applying KOH or performing wet mount, as subsequent testing may alter results 2

Step 2: Perform Whiff Test (10% KOH Application)

• Positive (fishy odor): Indicates BV 1, 2, 3
• Negative: More consistent with candidiasis 3, 4

Step 3: Microscopic Examination

For BV diagnosis (wet mount or Gram stain):

• Clue cells present (>20% of epithelial cells with bacteria obscuring borders) 1, 2, 3
• Few white blood cells (noninflammatory) 3, 4
• Decreased or absent Lactobacillus morphotypes on Gram stain 2
• Mobiluncus is pathognomonic but found in only 20% of cases 3

For candidiasis diagnosis (KOH preparation or wet mount):

• Hyphae or budding yeast visible in 50-70% of cases 3, 4
• Increased white blood cells may be present 4

Step 4: Apply Amsel Criteria for BV Confirmation

BV is diagnosed when 3 of 4 criteria are met (sensitivity 66.67%, specificity 94.74%): 1, 2

1. Homogeneous white discharge
2. Vaginal pH >4.5
3. Positive whiff test
4. Clue cells on microscopy

Key Distinguishing Features Summary

Feature	Bacterial Vaginosis	Yeast Infection
pH	>4.5 [1,2]	3.8-4.2 (normal) [3]
Discharge	Thin, homogeneous, white [1]	Thick, white, "curdled" [3]
Odor	Fishy (especially with KOH) [1]	None [3]
Pruritus	Minimal [1]	Prominent [3]
Inflammation	Minimal [1,3]	Marked vulvar erythema [3]
Microscopy	Clue cells [1,2]	Hyphae/budding yeast [3]

Common Diagnostic Pitfalls to Avoid

• Do not culture for Gardnerella vaginalis as it is not specific for BV diagnosis and can be present in normal vaginal flora 2
• Microscopic proficiency is essential as sensitivity for detecting clue cells varies widely (40-70%) 2
• Women often self-misdiagnose as yeast infection when they actually have BV, as awareness of BV is low 6
• Both conditions can coexist simultaneously in the same patient, requiring appropriate testing for all common causes 4
• Approximately 15% of Candida albicans organisms are resistant to clotrimazole and miconazole, and non-albicans species (C. tropicalis, C. glabrata) are increasingly prevalent 3

Clinical Significance

While both conditions cause discomfort, BV carries more serious long-term consequences including endometritis, pelvic inflammatory disease, adverse pregnancy outcomes (preterm birth, premature rupture of membranes), and increased susceptibility to sexually transmitted infections 1, 7. Candidiasis rarely leads to serious sequelae but significantly impacts quality of life 7. This makes accurate differentiation clinically important, as BV requires treatment even in asymptomatic pregnant women or before invasive gynecological procedures 1, 2.