What is the most likely diagnosis and appropriate management for a woman presenting with yellow, foul‑smelling vaginal discharge and urinary‑tract infection symptoms?

Yellow Foul-Smelling Vaginal Discharge with UTI Symptoms

The most likely diagnosis is trichomoniasis with possible concurrent bacterial vaginosis, and you should initiate empiric metronidazole 500 mg orally twice daily for 7 days while awaiting nucleic acid amplification testing (NAAT) results for Trichomonas vaginalis, Neisseria gonorrhoeae, and Chlamydia trachomatis. 1

Differential Diagnosis Priority

The combination of yellow, foul-smelling discharge strongly suggests:

• Trichomoniasis presents with yellow-green, frothy, malodorous discharge and is caused by Trichomonas vaginalis; this is the most likely diagnosis given the color and odor characteristics. 1
• Bacterial vaginosis produces homogeneous white-to-yellow discharge with a fishy odor and commonly coexists with other infections. 1
• Mucopurulent cervicitis from gonorrhea or chlamydia produces visible yellow endocervical discharge, though most infected women lack overt signs. 1

Critical point: Yellow discharge with foul odor is pathognomonic for trichomoniasis or bacterial vaginosis, not a typical UTI presentation. 1 Concurrent STI and bacterial UTI is unlikely—the "UTI symptoms" may actually represent urethritis from trichomoniasis. 2

Immediate Diagnostic Workup

Point-of-Care Testing (Perform During Visit)

• Measure vaginal pH with narrow-range paper: pH > 4.5 strongly suggests bacterial vaginosis or trichomoniasis, whereas pH ≤ 4.5 suggests candidiasis. 1
• Perform the whiff test by adding 10% KOH to discharge: a fishy amine odor is pathognomonic for bacterial vaginosis or trichomoniasis. 1, 3
• Saline wet mount microscopy to identify motile T. vaginalis (trichomoniasis) or clue cells (bacterial vaginosis). 1
• KOH preparation to rule out candidiasis by detecting yeast cells or pseudohyphae. 1

Send-Out Laboratory Testing (Required)

• NAAT for T. vaginalis, N. gonorrhoeae, and C. trachomatis from vaginal swab is mandatory because wet-mount microscopy detects only 40–80% of trichomoniasis infections. 1, 4
• Do not rely on wet mount alone—NAAT is the gold standard for trichomoniasis diagnosis. 1

Important caveat: Urine culture results are irrelevant for diagnosing vaginal infections; vaginal pH must be measured from vaginal discharge, not urine. 1 Testing positive for trichomoniasis, gonorrhea, or chlamydia is not associated with bacteriuria ≥10,000 CFU/mL. 2

Empiric Treatment Strategy

First-Line Therapy

Metronidazole 500 mg orally twice daily for 7 days is the recommended empiric regimen because it treats both bacterial vaginosis and trichomoniasis simultaneously. 1, 3

• This multi-day regimen achieves approximately 95% cure rates for bacterial vaginosis and 88–95% cure for trichomoniasis. 1
• The 7-day course is superior to single-dose metronidazole 2 g for bacterial vaginosis, though single-dose therapy may be used for trichomoniasis if adherence to multi-day therapy is uncertain. 1, 3

Alternative Regimen

• Tinidazole 2 g orally once daily for 2 days is FDA-approved for both bacterial vaginosis and trichomoniasis, with cure rates of 92–100% for trichomoniasis and 22–27% therapeutic cure for bacterial vaginosis. 5
• Tinidazole may be preferred if metronidazole intolerance exists, though it is more expensive. 5

Partner Management

• Treat sexual partners simultaneously with metronidazole 2 g single dose to prevent reinfection of trichomoniasis. 1, 3
• Do not treat male partners for bacterial vaginosis—partner therapy does not prevent recurrence. 1

Adjust Treatment Based on NAAT Results

If Trichomoniasis Confirmed

• Continue or complete metronidazole 500 mg twice daily for 7 days (or metronidazole 2 g single dose if not yet given). 1
• Ensure partner receives metronidazole 2 g single dose. 3

If Gonorrhea or Chlamydia Detected

• Add dual therapy per current CDC guidelines for gonorrhea and chlamydia because clinical presentation cannot differentiate these pathogens. 1
• Mucopurulent cervicitis requires treatment even if no visible discharge is present—up to 50% of women with gonorrhea or chlamydia lack overt cervicitis. 1

If All NAAT Negative but Symptoms Persist

• Consider metronidazole-resistant T. vaginalis and refer for culture or higher-dose metronidazole (500 mg twice daily for 7 days if single-dose was used initially). 1
• Re-evaluate for non-infectious causes: mechanical, chemical, or allergic vulvovaginal irritation from soaps, detergents, douches, or latex condoms. 1

Critical Pitfalls to Avoid

• Never diagnose based on discharge appearance alone—clinical characteristics are unreliable for distinguishing causes. 4
• Never assume negative wet mount rules out trichomoniasis—sensitivity is only 40–80%; NAAT is required. 1, 4
• Never treat "UTI" empirically without vaginal examination—dysuria in women with vaginal discharge usually reflects vaginitis or cervicitis, not cystitis. 6, 2
• Never rely on urine pH to diagnose vaginal infections—vaginal pH must be measured from vaginal discharge. 1
• Never treat asymptomatic bacterial vaginosis unless the patient is undergoing invasive gynecologic procedures (abortion, hysterectomy) to reduce postoperative pelvic inflammatory disease risk. 1

Follow-Up

• Re-evaluate only if symptoms persist or recur within 2 months. 1
• Confirm both patient and partner completed trichomoniasis therapy. 1
• Perform test-of-cure for cervicitis if gonorrhea or chlamydia was detected. 1