Differentiating STIs by Vaginal Discharge and Odor
You cannot reliably differentiate between STIs based solely on discharge and odor characteristics—microscopy, pH testing, and nucleic acid amplification testing (NAAT) are essential for accurate diagnosis. 1, 2, 3
Key Clinical Characteristics
Bacterial Vaginosis (Not an STI, but most common cause)
- Discharge: Homogeneous, thin, white-gray discharge that smoothly coats vaginal walls 1, 2, 4
- Odor: Fishy odor, especially after addition of 10% KOH (positive whiff test) 1, 2, 4
- pH: Elevated (>4.5) 1, 2
- Microscopy: Clue cells (vaginal epithelial cells with stippled appearance from adherent bacteria) on saline wet mount 1, 2
- Key feature: Minimal to no vulvar inflammation or irritation 1, 4
Trichomoniasis (STI)
- Discharge: May be copious, yellow-green, frothy discharge 1
- Odor: Fishy or foul odor possible (amine odor with KOH) 1
- pH: Elevated (>4.5) 1
- Microscopy: Motile trichomonads on saline wet mount, though sensitivity is only 40-80% 2, 3
- Cervical findings: Red punctate lesions ("strawberry cervix") may be visible on speculum exam 1
- Key feature: Often associated with vulvar inflammation and cervical friability 1
- Critical caveat: The CDC recommends NAAT for diagnosis due to poor wet mount sensitivity 2, 3
Vulvovaginal Candidiasis (Not typically an STI)
- Discharge: Thick, white, "cottage cheese-like" discharge, though discharge may be minimal or absent 5, 3
- Odor: No odor 1, 5
- pH: Normal (<4.5) 5, 3
- Microscopy: Yeasts or pseudohyphae on KOH preparation 5, 3
- Key feature: Prominent pruritus, vulvar erythema, and burning are the hallmark symptoms, not discharge 5
Cervicitis from Gonorrhea/Chlamydia (STIs)
- Discharge: Mucopurulent cervical discharge (yellow-green from cervical os) 1
- Odor: Variable, not distinctive 1
- Cervical findings: Cervical friability, hyperemia, easily induced bleeding 1
- Key feature: Discharge originates from cervix, not vaginal walls 1
- Critical point: These STIs uncommonly cause vaginal discharge alone 1
Diagnostic Algorithm
Step 1: Assess pH (using narrow-range pH paper)
Step 2: Perform Whiff Test (10% KOH)
- Positive fishy odor: Suggests bacterial vaginosis or trichomoniasis 1, 2
- Negative: More consistent with candidiasis 1, 5
Step 3: Microscopy
- Saline wet mount: Look for clue cells (BV) or motile trichomonads (trichomoniasis) 1, 2
- KOH preparation: Look for yeast/pseudohyphae (candidiasis) 1, 5
Step 4: When Microscopy is Negative or Equivocal
- Order NAAT for Trichomonas vaginalis (wet mount misses 20-60% of cases) 2, 3
- Consider Gram stain (Nugent criteria) for definitive BV diagnosis 2, 3
- Order culture for Candida if clinical suspicion high but microscopy negative 5, 3
- Test for gonorrhea and chlamydia if cervicitis present or patient at risk 1
Critical Pitfalls to Avoid
Never diagnose based on appearance alone—clinical appearance of discharge is unreliable for distinguishing between causes. 5, 3 Women frequently self-misdiagnose yeast infections when bacterial vaginosis or other conditions are actually present. 6
Do not diagnose BV without clue cells unless confirmed by Gram stain, as this leads to treating the wrong condition. 2
Do not rely solely on wet mount for trichomoniasis—its sensitivity is only 40-80%, and NAAT is the CDC-recommended diagnostic method. 2, 3
Examine the cervix during speculum exam—mucopurulent cervical discharge, friability, or hyperemia suggests gonorrhea or chlamydia rather than vaginitis. 1
Check for genital ulcers—HSV and syphilis are the most common causes of genital ulcers in the United States and require viral culture and serologic testing respectively. 1
When Multiple Conditions Coexist
Approximately one-third of women with vaginal complaints have no identifiable cause even after thorough evaluation. 7 In these cases, consider non-infectious causes including atrophic vaginitis, chemical irritation, or retained foreign bodies. 3, 8 Always perform a thorough physical examination including visualization of the cervix and perianal area. 1, 8