What are the differential diagnoses for a foul-smelling vaginal discharge?

Differential Diagnoses of Foul-Smelling Vaginal Discharge

The most common causes of foul-smelling vaginal discharge are bacterial vaginosis and trichomoniasis, which together account for the vast majority of malodorous vaginal complaints. 1, 2

Primary Differential Diagnoses

Bacterial Vaginosis (Most Common)

• BV is the single most prevalent cause of vaginal discharge and malodor, affecting 10-35% of women in gynecologic practice. 3
• Results from replacement of normal hydrogen peroxide-producing Lactobacillus species with anaerobic bacteria including Gardnerella vaginalis, Prevotella, Mobiluncus, and Mycoplasma hominis. 2
• Presents with homogeneous, thin, white-to-gray discharge that smoothly coats vaginal walls, accompanied by a characteristic fishy odor that intensifies after intercourse or menses. 1, 3
• Up to 50% of women meeting diagnostic criteria are asymptomatic, making objective testing essential rather than relying on symptoms alone. 1, 3

Trichomoniasis (Second Most Common)

• Caused by the sexually transmitted protozoan Trichomonas vaginalis, presenting with copious, yellow-green, frothy discharge and prominent foul or fishy odor. 1, 2
• Associated findings include vulvar irritation, punctate cervical hemorrhages ("strawberry cervix" in 25% of cases), and vaginal pH >4.5. 2, 4
• Carries significant health implications including increased HIV transmission risk and adverse pregnancy outcomes (preterm delivery, premature rupture of membranes). 2

Mucopurulent Cervicitis

• Caused by Neisseria gonorrhoeae or Chlamydia trachomatis, producing visible yellow endocervical discharge that may be perceived as vaginal discharge. 1
• Most infected women lack overt mucopurulent cervicitis signs on examination, with up to 50% of gonorrhea or chlamydia infections presenting without visible cervical discharge. 1
• Characterized by cervical friability, hyperemia, and easily induced bleeding on examination. 3

Retained Foreign Body

• Forgotten tampons, contraceptive devices, or other foreign objects produce profoundly malodorous discharge due to bacterial overgrowth. 5
• In approximately one-third of women presenting with genital malodor, no infectious cause is identified, necessitating consideration of non-infectious etiologies. 5

Diagnostic Algorithm

Step 1: Vaginal pH Testing

• pH >4.5 strongly suggests bacterial vaginosis or trichomoniasis; pH ≤4.5 indicates candidiasis or physiologic discharge. 1, 3
• This single test immediately narrows your differential before microscopy. 3

Step 2: Whiff Test

• Add 10% potassium hydroxide to vaginal discharge; an immediate fishy amine odor is pathognomonic for bacterial vaginosis or trichomoniasis. 3
• A positive whiff test combined with elevated pH makes candidiasis extremely unlikely. 3

Step 3: Microscopic Examination

• Saline wet mount: Look for clue cells (epithelial cells densely coated with bacteria, >20% of cells) confirming BV, or motile flagellated trichomonads indicating trichomoniasis. 1, 3
• KOH preparation: Reveals yeast cells or pseudohyphae if candidiasis is present (though candidiasis rarely causes foul odor). 1

Step 4: Nucleic Acid Amplification Testing (NAAT)

• NAAT is required for definitive diagnosis because wet-mount microscopy detects only 40-80% of T. vaginalis infections. 3
• Order NAAT for T. vaginalis, N. gonorrhoeae, and C. trachomatis when clinical presentation suggests infection but microscopy is negative or equivocal. 1, 3

Amsel Criteria for Bacterial Vaginosis (3 of 4 Required)

• Homogeneous white or yellow discharge coating vaginal walls 1
• Clue cells on microscopy (>20% of epithelial cells) 2
• Vaginal pH >4.5 1
• Positive whiff test (fishy odor with KOH) 1

Treatment Based on Diagnosis

Bacterial Vaginosis

• Oral metronidazole 500 mg twice daily for 7 days is first-line therapy. 1, 2
• Do not treat male partners—partner therapy does not prevent BV recurrence because it represents dysbiosis rather than simple sexual transmission. 2, 3

Trichomoniasis

• Metronidazole 2 g orally as a single dose achieves microbiologic cure. 2
• Simultaneous treatment of all sexual partners is mandatory to prevent reinfection. 2
• Patients must abstain from sexual intercourse until both partners complete therapy and are asymptomatic. 2

Mucopurulent Cervicitis

• Treat according to NAAT results, or in high-prevalence settings, provide empiric therapy covering both gonorrhea and chlamydia while awaiting results. 1

Critical Pitfalls to Avoid

• Never diagnose based on discharge appearance alone—clinical characteristics are unreliable for distinguishing between causes, and symptoms-only diagnosis leads to missed infections. 1, 3
• Do not rely solely on wet mount for trichomoniasis—its 40-80% sensitivity means negative microscopy does not exclude infection; NAAT is the gold standard. 3
• Do not assume negative culture rules out trichomoniasis—culture sensitivity is only 40-80% compared to NAAT. 3
• Avoid culturing Gardnerella vaginalis for BV diagnosis—it is isolated from 50% of asymptomatic women and lacks diagnostic specificity. 3
• Consider mixed infections—multiple pathogens may coexist and require combined therapeutic approaches. 3
• Perform thorough cervical inspection—up to 50% of women with gonorrhea or chlamydia lack visible mucopurulent cervicitis, so test based on risk factors, not just examination findings. 1