What is the cause of green vaginal discharge without foul odor in a patient with negative STI (Sexually Transmitted Infection) tests and no evidence of vaginal infection?

Green Vaginal Discharge with Negative Testing: Likely Missed Bacterial Vaginosis or Aerobic Vaginitis

The most likely cause is missed bacterial vaginosis (BV), which standard clinical testing fails to detect in 20-30% of cases, and you should retest using Gram stain with Nugent criteria or DNA probe testing. 1, 2

Primary Diagnostic Consideration: Missed Bacterial Vaginosis

• BV remains the most common cause of vaginal discharge (40-50% of identified cases) and is frequently missed on initial testing. 1, 2
• Up to 50% of women with BV meeting diagnostic criteria are asymptomatic or have atypical presentations, meaning the infection can be present even without classic fishy odor. 1, 2
• Gram stain has 90% sensitivity and is the most specific diagnostic method for BV, superior to clinical criteria alone. 1
• The green color of discharge does not rule out BV—while classically described as gray or white, BV can produce various discharge colors. 2

Secondary Consideration: Aerobic Vaginitis

• Aerobic vaginitis (AV) presents with yellow-to-green thick mucoid discharge, vaginal inflammation, and elevated pH, but is often undiagnosed because it was not formally recognized until 2002. 3
• AV differs from BV by having significant inflammation (red, edematous vagina), presence of leucocytes and immature epithelial cells on microscopy, and sometimes a foul, rotten smell rather than fishy odor. 3
• AV prevalence ranges between 7-12% and can co-occur with other conditions or be missed entirely on standard testing. 3
• Diagnosis requires wet mount microscopy with phase contrast to calculate an AV score based on lactobacillary grade, inflammation, toxic leucocytes, microflora characteristics, and immature epithelial cells. 3

Recommended Diagnostic Algorithm

Step 1: Retest for Bacterial Vaginosis

• Obtain Gram stain with Nugent criteria (90% sensitivity) or DNA probe testing for G. vaginalis. 1, 2
• Measure vaginal pH: elevated pH >4.5 suggests BV or trichomoniasis (though trichomonas was already negative); pH <4.5 suggests candidiasis or non-infectious causes. 2

Step 2: If BV Testing Remains Negative, Evaluate for Aerobic Vaginitis

• Perform wet mount microscopy with phase contrast looking for: leucocytes, parabasal/immature epithelial cells, absence of lactobacilli, and aerobic bacteria. 3
• Note presence of vaginal inflammation (redness, edema) on examination, which is absent in BV but characteristic of AV. 3

Step 3: Consider Non-Infectious Causes Only After Infections Truly Ruled Out

• Assess for chemical/mechanical irritation from soaps, douches, or hygiene products—these can cause external vulvar inflammation with minimal discharge and symptoms mimicking infection. 4
• Evaluate for retained foreign body through visual inspection. 4
• Consider cervical ectropion, which can cause significant vaginal discharge and is a normal developmental finding in adolescents. 5

Treatment Based on Findings

If BV Confirmed on Retesting:

• Metronidazole 500 mg orally twice daily for 7 days (cure rate 80-90%, higher than single-dose regimens). 1, 2
• Counsel patient that BV has a 50-80% recurrence rate within one year, which may explain persistent symptoms. 1, 2

If Aerobic Vaginitis Confirmed:

• Treatment with ampicillin or amoxicillin-clavulanate targeting aerobic bacteria. 1
• Consider local estrogen therapy, corticosteroids, or probiotics depending on microscopic findings. 3

If All Testing Negative:

• Discontinue irritants, avoid douching, and use only water or mild soap for external cleansing. 4
• Do not treat asymptomatic patients with incidental bacterial findings and no clue cells, as this leads to inappropriate antibiotic use. 1

Critical Pitfalls to Avoid

• Do not assume negative initial testing rules out infection—laboratory testing fails to identify the cause of vaginitis in a substantial minority of women. 1
• Do not dismiss green discharge as requiring positive trichomonas testing—both BV and AV can present with green discharge. 3
• The pelvic pain described (cramping, pressure, 7/10 severity) warrants consideration of pelvic inflammatory disease, though cervical motion tenderness and other PID criteria should be assessed. 5
• Regular soap can cause mechanical and chemical irritation presenting with discharge, itching, and burning without actual pathogens present. 4