Vaginal Discharge Differential Diagnosis
Vaginal discharge is a classic Step 1/CK differential built on a small number of discriminating features: the character of the discharge, odor, vaginal pH, wet mount/KOH findings, and the correct treatment. Master the four core entities—bacterial vaginosis, trichomoniasis, vulvovaginal candidiasis, and gonococcal/chlamydial cervicitis—and you can distinguish them from a single vignette. The two most powerful splitters are pH (elevated >4.5 in BV and trich versus normal <4.5 in candidiasis) and the wet mount finding (clue cells, motile trichomonads, or budding yeast/pseudohyphae).
Bacterial Vaginosis
BV is a polymicrobial condition of altered vaginal flora—decreased lactobacilli with overgrowth of anaerobes such as Gardnerella and Prevotella. It is technically not an STI but is associated with sexual activity; risk factors include multiple sexual partners, a new sexual partner, female sex partners, and douching. Classic presentation is a thin, gray-white, fishy-smelling discharge with NO inflammation (it is not a true "vaginitis"), and it is often asymptomatic. Diagnosis uses the Amsel criteria (3 of 4): thin homogeneous discharge, vaginal pH >4.5, positive whiff test (fishy odor with KOH), and clue cells on wet mount. Treatment is metronidazole (500 mg BID × 7 days, or intravaginal gel × 5 days) or intravaginal clindamycin cream × 7 days. Partner treatment is not routinely recommended.
Trichomoniasis
Caused by Trichomonas vaginalis, a flagellated protozoan. In women it produces a frothy, malodorous, yellow-green discharge with vulvar irritation, pruritus, and dysuria; the classic "strawberry cervix" (cervical petechiae) is characteristic but uncommon, and infection may be asymptomatic. In men it is usually asymptomatic but may cause urethritis or epididymitis. Vaginal pH is elevated (>4.5). Wet mount shows motile trichomonads with flagella (sensitivity ~60%); NAAT is more sensitive. Treatment is metronidazole (2 g PO × 1 dose, or 500 mg BID × 7 days), and unlike BV, partners must be treated. Warn patients to avoid alcohol with metronidazole due to a disulfiram-like reaction.
Vulvovaginal Candidiasis
Most often caused by Candida albicans. Risk factors include antibiotic use, diabetes, immunosuppression, pregnancy, and high-dose estrogen. Presentation is a thick, white, "cottage cheese" discharge that is odorless, with prominent vulvar pruritus, burning, erythema, excoriation, external dysuria, and dyspareunia. This is the key entity with a NORMAL vaginal pH (<4.5). KOH prep reveals budding yeast and pseudohyphae; diagnosis can be clinical when the presentation is classic. Treatment is fluconazole or a topical azole.
Gonorrhea and Chlamydia (Cervicitis)
Neisseria gonorrhoeae and Chlamydia trachomatis cause cervicitis in women, which is often asymptomatic but can present with mucopurulent cervical discharge and cervical friability, sometimes with dysuria or vaginal discharge. Odor is minimal and pH is variable. Diagnosis is by NAAT (not wet mount). Because gonorrhea and chlamydia frequently coexist, treat empirically for both: ceftriaxone (for gonorrhea) plus doxycycline (for chlamydia). Fluoroquinolones are not recommended for gonorrhea due to rising resistance, and ceftriaxone is now the only reliable option. Untreated infection can ascend to cause pelvic inflammatory disease, with cervical motion tenderness, adnexal tenderness, and complications of tubo-ovarian abscess, infertility, ectopic pregnancy, and chronic pelvic pain.
High-yield
- pH >4.5: bacterial vaginosis and trichomoniasis; pH <4.5 (normal): candidiasis
- Clue cells = BV; motile trichomonads = trich; budding yeast/pseudohyphae = candidiasis
- Fishy odor with KOH (positive whiff test) = bacterial vaginosis
- Frothy yellow-green discharge + strawberry cervix = trichomoniasis
- Thick, odorless "cottage cheese" discharge + itching = candidiasis
- Both BV and trichomoniasis are treated with metronidazole
- Gonorrhea/chlamydia coinfection is common—treat empirically for both with ceftriaxone + doxycycline
- NAAT (not wet mount) is used to diagnose gonorrhea and chlamydia
- Avoid alcohol with metronidazole (disulfiram-like reaction)
- Amsel criteria (3 of 4) diagnose BV: thin discharge, pH >4.5, positive whiff test, clue cells
Pitfalls
- Confusing pH: candidiasis has NORMAL pH (<4.5), whereas BV and trichomoniasis both elevate pH >4.5
- Forgetting partner treatment for trichomoniasis while correctly withholding it for BV
- Treating gonorrhea alone (ceftriaxone) or chlamydia alone (doxycycline)—dual coinfection requires both
- Using a fluoroquinolone for gonorrhea—no longer recommended due to resistance; ceftriaxone is the reliable agent
- Calling BV a "vaginitis"—it causes no true inflammation
- Relying on wet mount alone for trichomonas (sensitivity only ~60%) or attempting wet mount for gonorrhea/chlamydia rather than NAAT
- Assuming symptoms are always present—BV, trichomoniasis, and gonococcal/chlamydial cervicitis are frequently asymptomatic
Clinical pearls
- A fishy odor released when KOH is added (whiff test) instantly points to bacterial vaginosis.
- Cottage cheese discharge with a normal pH and no odor is candidiasis until proven otherwise.
- Strawberry cervix is classic for trichomoniasis but is seen in the minority of cases.
- When you diagnose one of gonorrhea or chlamydia, treat for both.
- Metronidazole covers both BV and trich—but only trich requires treating the partner.
Frequently asked
How does vaginal pH help distinguish the causes of discharge?
An elevated pH (>4.5) suggests bacterial vaginosis or trichomoniasis, while a normal pH (<4.5) points to vulvovaginal candidiasis. pH is one of the fastest discriminators on exam vignettes.
What wet mount finding is specific to each condition?
Clue cells indicate bacterial vaginosis, motile flagellated trichomonads indicate trichomoniasis, and budding yeast with pseudohyphae (on KOH prep) indicates candidiasis. Gonorrhea and chlamydia are diagnosed by NAAT, not wet mount.
Why are both metronidazole regimens used for BV and trichomoniasis, and does treatment differ?
Both are metronidazole-responsive. BV is treated with metronidazole 500 mg BID × 7 days (or intravaginal gel), and partners are not routinely treated. Trichomoniasis is treated with metronidazole 2 g PO × 1 (or 500 mg BID × 7 days), and partners must be treated.
Why treat empirically for both gonorrhea and chlamydia?
Because coinfection is common. Ceftriaxone covers gonorrhea and doxycycline covers chlamydia; giving only one leaves the other untreated. Treat both unless one can be confidently excluded.
What is the whiff test and which condition does it identify?
Adding KOH to the discharge releases a fishy amine odor—a positive whiff test—which is characteristic of bacterial vaginosis and is one of the four Amsel criteria.
Which discharge condition classically causes NO inflammation?
Bacterial vaginosis. It reflects altered flora rather than true tissue inflammation, so it is not considered a 'vaginitis' and often causes no irritation despite a fishy discharge.
What counseling point is critical when prescribing metronidazole?
Advise the patient to avoid alcohol, because the combination can cause a disulfiram-like reaction.
What are the consequences of untreated gonococcal or chlamydial cervicitis?
Ascending infection causes pelvic inflammatory disease with cervical motion tenderness and adnexal tenderness, and can lead to tubo-ovarian abscess, infertility, ectopic pregnancy, and chronic pelvic pain.
Turn this into reasoning you can use on exam day — practice Vaginal Discharge Differential Diagnosis on branching cases where your decisions shape the patient.