What is the differential diagnosis and treatment for dark brown vaginal discharge?

Differential Diagnosis of Dark Brown Vaginal Discharge

Dark brown vaginal discharge most commonly represents old blood from menstruation, but the differential diagnosis must systematically exclude bacterial vaginosis, trichomoniasis, vulvovaginal candidiasis, cervicitis from sexually transmitted infections, and less commonly, endometrial pathology or cervical lesions. 1, 2

Diagnostic Approach

Initial Assessment

• Measure vaginal pH immediately using narrow-range pH paper applied directly to vaginal secretions 1, 3:

  • pH >4.5 indicates bacterial vaginosis or trichomoniasis
  • pH ≤4.5 suggests vulvovaginal candidiasis or physiologic discharge

• Perform saline wet mount microscopy to identify 1, 3:

  • Motile trichomonads (trichomoniasis)
  • Clue cells (bacterial vaginosis)

• Perform KOH preparation to identify 1, 3:

  • Yeast or pseudohyphae (candidiasis)
  • Positive whiff test (fishy odor = bacterial vaginosis or trichomoniasis)

Key Clinical Features by Diagnosis

Bacterial Vaginosis 3:

• Homogeneous white-gray discharge (not typically brown, but can mix with blood)
• Fishy odor, especially after intercourse
• pH >4.5
• Clue cells on microscopy
• Most prevalent cause of vaginal discharge overall

Trichomoniasis 3, 1:

• Yellow-green, frothy discharge (can appear brown with blood)
• Malodorous
• Vulvar irritation and dysuria
• pH >4.5
• Motile trichomonads on wet mount

Vulvovaginal Candidiasis 3, 1:

• White, cottage cheese-like discharge (rarely brown)
• Intense pruritus and vulvar erythema
• pH ≤4.5
• Yeast/pseudohyphae on KOH prep

Cervicitis (Chlamydia/Gonorrhea) 2:

• Mucopurulent discharge
• Requires pelvic exam to assess cervical friability
• Can present with brown discharge if bleeding present

Treatment by Diagnosis

Bacterial Vaginosis

Metronidazole 500 mg orally twice daily for 7 days is the preferred regimen with 95% cure rate 1. Alternative: clindamycin cream intravaginally 1. Do not treat male partners—this does not prevent recurrence 3, 1.

Trichomoniasis

Metronidazole 2 grams orally as a single dose achieves 90-95% cure rate 3, 1, 4. Sexual partners must be treated simultaneously to prevent reinfection 1, 4. Instruct patients to avoid sexual contact until both partners complete treatment and are asymptomatic 3.

Vulvovaginal Candidiasis

Fluconazole 150 mg orally as a single dose is the standard treatment with 55% therapeutic cure rate 1. For recurrent cases (≥4 episodes/year), use fluconazole 150 mg weekly for 6 months as maintenance therapy 1, 5. Do not treat partners 1.

Cervicitis

Treat empirically for both chlamydia and gonorrhea if cervicitis is identified on exam, as these are sexually transmitted and require partner notification 2.

Critical Pitfalls to Avoid

• Never treat asymptomatic Candida colonization—10-20% of women harbor Candida without symptoms and do not require treatment 6, 3
• Avoid vaginal douching—this disrupts normal lactobacilli flora and increases infection risk 6, 3
• Do not allow self-medication except in women previously diagnosed with candidiasis experiencing identical symptoms 5, 6
• Culture is more sensitive than microscopy for trichomoniasis—negative wet mount does not exclude infection 3

Special Populations

Pregnant women 1, 5:

• Use only 7-day topical treatments for bacterial vaginosis and candidiasis
• Oral metronidazole is acceptable for trichomoniasis
• Follow-up at one month post-treatment to verify cure due to risk of preterm labor and premature rupture of membranes

HIV-infected women 1, 5:

• Receive identical treatment regimens as non-HIV-infected women for all three conditions

Follow-Up

Return only if symptoms persist or recur within 2 months—routine test-of-cure is unnecessary if symptoms resolve 1, 5. Recurrence of bacterial vaginosis is common (50-80% within one year) but does not change initial management 1.