In a sexually active woman of reproductive age presenting with green, frothy vaginal discharge, what is the most likely diagnosis, recommended diagnostic work‑up, and appropriate treatment?

Green Vaginal Discharge: Trichomoniasis Until Proven Otherwise

In a sexually active woman presenting with green vaginal discharge, trichomoniasis is the most likely diagnosis and should be treated empirically with metronidazole 2 g orally as a single dose while awaiting confirmatory testing. 1, 2

Most Likely Diagnosis

• Trichomoniasis is the classic cause of green, frothy vaginal discharge accompanied by vulvar irritation and a foul odor. 1
• The CDC defines the hallmark presentation as a diffuse, malodorous, yellow-green discharge with prominent itching. 1
• On speculum examination, look for a "strawberry cervix" (punctate hemorrhagic lesions), which strongly supports trichomoniasis but is present in only about 25% of cases. 1, 3

Critical Diagnostic Work-Up

Point-of-care testing:

• Vaginal pH testing – Trichomoniasis elevates pH above 4.5, distinguishing it from candidiasis (pH < 4.5). 1, 2
• Whiff test – Adding 10% KOH may produce a fishy odor in trichomoniasis or bacterial vaginosis, helping differentiate from candidiasis. 1, 2
• Saline wet mount – Look for motile, flagellated trichomonads; however, this test detects only 40–80% of infections, so a negative result does not rule out trichomoniasis. 1, 2

Confirmatory testing:

• Nucleic acid amplification testing (NAAT) for Trichomonas vaginalis is the gold standard and should be ordered in all cases of green discharge, as microscopy misses 20–50% of infections. 2
• NAAT for Neisseria gonorrhoeae and Chlamydia trachomatis should be obtained if mucopurulent cervical discharge or cervical friability is present, as cervicitis can mimic vaginal trichomoniasis. 1, 2

Recommended Treatment

First-line therapy:

• Metronidazole 2 g orally as a single dose is the CDC-recommended first-line treatment, achieving microbiologic cure in the majority of patients. 1, 4, 5
• An alternative is metronidazole 500 mg orally twice daily for 7 days, which may be preferred in complicated cases or treatment failures. 1

Partner management:

• All sexual partners must receive the same metronidazole 2 g single-dose regimen concurrently to prevent reinfection, as treatment failure is most commonly due to untreated partners. 1, 4, 3
• Advise abstinence from sexual intercourse until both the patient and partner have completed therapy and are asymptomatic. 1

Patient counseling:

• Instruct patients to avoid alcohol during metronidazole therapy and for 24 hours after the last dose to prevent a disulfiram-like reaction. 6

Differential Diagnosis to Consider

Bacterial vaginosis:

• Presents with a thin, homogeneous white-gray discharge (not green) and a fishy odor, but lacks the frothy appearance and prominent vulvar irritation of trichomoniasis. 1, 2
• Diagnosed by Amsel criteria: pH > 4.5, positive whiff test, clue cells on wet mount, and homogeneous discharge. 6

Cervicitis (Neisseria gonorrhoeae or Chlamydia trachomatis):

• Mucopurulent cervical discharge can appear greenish and may be mistaken for vaginal discharge. 1
• Look for cervical friability, hyperemia, and easily induced bleeding on speculum examination. 2

Candidiasis:

• Presents with thick, white "cottage cheese-like" discharge, intense pruritus, and normal pH (< 4.5), making it easily distinguishable from trichomoniasis. 1, 2

Critical Pitfalls to Avoid

• Never rely solely on wet mount microscopy – Its sensitivity for trichomoniasis is only 40–80%, so always order NAAT when trichomoniasis is suspected. 2
• Do not diagnose based on discharge appearance alone – Clinical characteristics are unreliable for distinguishing between causes; always perform pH testing, whiff test, and microscopy. 2
• Do not forget to treat sexual partners – Failure to treat partners is the most common cause of treatment failure and recurrence. 1, 3
• Consider mixed infections – Multiple pathogens can coexist; if symptoms persist after treating trichomoniasis, reassess for bacterial vaginosis or candidiasis. 2

Clinical Significance and Complications

• Trichomoniasis increases the risk of HIV acquisition and transmission, making prompt diagnosis and treatment critical. 1
• In pregnancy, trichomoniasis is associated with preterm delivery, premature rupture of membranes, and low birth weight. 1, 7
• Untreated trichomoniasis increases the risk of postabortal infection, postcesarean infection, and other reproductive tract complications. 7

When to Treat Empirically

• If the discharge is green, copious, and frothy, initiate empiric metronidazole 2 g orally while awaiting NAAT results, especially in high-risk or symptomatic patients. 2
• Empiric therapy is justified because delayed treatment increases the risk of complications and ongoing transmission. 2