High Vaginal Swab

The high vaginal swab (HVS) is a sample of vaginal discharge from the posterior fornix, used to diagnose the causes of vaginitis and vaginal discharge: bacterial vaginosis (BV), vulvovaginal candidiasis, trichomoniasis, aerobic vaginitis, and less often group A streptococcal or staphylococcal infection.

Tests are a Gram stain (Nugent score or Hay-Ison grade), wet mount with saline and potassium hydroxide, vaginal pH, whiff test, culture on blood agar, chocolate agar and Sabouraud or chromogenic Candida agar, and nucleic acid amplification for Trichomonas vaginalis, Candida species and BV-associated organisms.

Culture alone does not diagnose BV, because the organisms are part of a disturbed flora and not a single pathogen.

Vaginal discharge accounts for about 10% of gynaecological consultations. BV is the commonest cause of discharge in women of reproductive age (prevalence 20 to 30% worldwide, up to 50% in some African regions), vulvovaginal candidiasis affects 75% of women at least once, and trichomoniasis is the commonest curable non-viral sexually transmitted infection (about 156 million new cases yearly).

BV is linked to preterm birth, postpartum endometritis, pelvic inflammatory disease and increased acquisition of HIV and other sexually transmitted infections.

Indications. Abnormal vaginal discharge, odour, itching, soreness, dyspareunia or dysuria. Recurrent vaginitis. Pregnancy with symptoms, or with previous preterm birth and suspected BV. Before gynaecological procedures in symptomatic women. Postmenopausal bleeding or discharge. Suspected group A streptococcal vaginitis in a child. Foreign body or toxic shock syndrome. Not for asymptomatic screening in non-pregnant women.

Specimen. Speculum examination, then a swab from the posterior fornix and lateral vaginal walls, avoiding the cervix. Separate swabs for culture (Amies transport medium), a slide for Gram stain, and a swab for NAAT. Vaginal pH measured on the lateral wall with indicator paper. Wet mount examined at once at the bedside.

For trichomonas culture, an inoculated pouch or Diamond medium. Self-collected vaginal swabs perform as well as clinician collected swabs for NAAT. Collect before any vaginal preparation or douching, and avoid lubricants and speculum creams.

Reference values. Vaginal pH 3.8 to 4.5. Gram stain: predominance of Gram positive rods (Lactobacillus), Nugent score 0 to 3. Wet mount: few leucocytes, epithelial cells without clue cells, no yeast forms or motile trichomonads. Whiff test negative. Culture: normal flora with lactobacilli, with Candida in 10 to 20% of healthy asymptomatic women (not treated).

Interpretation.

• Bacterial vaginosis: Nugent score 7 to 10 (0 to 3 normal, 4 to 6 intermediate), or three of four clinical criteria (thin homogeneous grey discharge, pH over 4.5, positive whiff test, clue cells over 20% of epithelial cells). Loss of lactobacilli with overgrowth of Gardnerella, Prevotella, Atopobium and Mobiluncus.
• Vulvovaginal candidiasis: curdy white discharge, pH normal (4.0 to 4.5), pseudohyphae or budding yeast on wet mount or Gram stain, and growth of Candida on culture. Candida albicans accounts for 80 to 90%, and non-albicans species (Candida glabrata) are more resistant to fluconazole.
• Trichomoniasis: frothy yellow green discharge, pH over 5.0, motile flagellated trichomonads on wet mount (sensitivity 50 to 60%), and NAAT (sensitivity over 95%). Always treated with the partner.
• Aerobic vaginitis: purulent discharge, pH over 5, many leucocytes, parabasal cells and growth of Escherichia coli, group B streptococcus, Staphylococcus aureus or enterococci with loss of lactobacilli.
• Group B streptococcus: colonises 10 to 30% of women, not treated outside pregnancy (a different screening swab is used in late pregnancy).
• Atrophic vaginitis, desquamative inflammatory vaginitis, lichen planus, allergic and irritant vulvovaginitis: pH raised and many parabasal cells, with negative cultures for pathogens.
• Mixed infections occur in 20% of cases.

Critical values and findings needing immediate action.

• Fever, lower abdominal pain, cervical motion tenderness or adnexal tenderness with discharge: pelvic inflammatory disease, empirical antibiotics without waiting for results, and chlamydia and gonorrhoea NAAT.
• Retained tampon or foreign body with fever, rash and hypotension: toxic shock syndrome, urgent resuscitation, removal and antibiotics.
• Pregnancy with BV and a history of preterm birth, or with trichomoniasis: treatment in pregnancy.
• Offensive discharge with blood in a postmenopausal woman: exclusion of malignancy and urgent referral.
• Prepubertal child with purulent discharge: group A streptococcus, foreign body, threadworm or abuse, with appropriate safeguarding assessment.

Interferences and pitfalls.

• Culture of Candida in an asymptomatic woman represents colonisation, and treatment is for symptomatic disease only.
• Isolation of Gardnerella vaginalis by culture is not diagnostic of BV, because it is found in 30 to 50% of healthy women.
• Recent douching, spermicides, lubricants, antibiotics, antifungals, menstrual blood, semen and sexual intercourse within 24 hours alter pH and flora.
• Wet mount must be examined within 10 to 15 minutes to detect motile trichomonads, and chilled or delayed samples lose motility.
• Cervical mucus, blood and semen raise pH and give false indications for BV or trichomoniasis.
• Chlamydia and gonorrhoea are not detected on an HVS culture, and need a separate NAAT.
• Recurrent BV (three or more episodes in a year) and recurrent candidiasis (four or more) need confirmation by culture or microscopy, species identification, and review of risk factors (diabetes, immunosuppression, antibiotics, contraceptives, sexual partners).
• Treatment of asymptomatic partners of women with BV does not reduce recurrence, whereas partners in trichomoniasis must be treated.

Special groups.

• Pregnancy: BV and trichomoniasis are associated with preterm birth, and group B streptococcus is screened separately at 35 to 37 weeks.
• Postmenopausal women: atrophic vaginitis, and swabs for bleeding or discharge need exclusion of malignancy.
• Children: vulvovaginitis from poor hygiene, group A streptococcus, Shigella, threadworm and foreign body.
• Immunosuppressed, diabetic and HIV-positive women: frequent and severe candidiasis, and non-albicans species.
• Regions: BV prevalence is highest in sub-Saharan Africa and the Caribbean, and syndromic management (without tests) is used where laboratory services are limited, with poorer specificity.

Follow up tests. Chlamydia, gonorrhoea, Mycoplasma genitalium and trichomonas NAAT, HIV, syphilis and hepatitis B serology, pregnancy test, endocervical swab, urine culture, group B streptococcus swab in late pregnancy, speciation and antifungal susceptibility for recurrent candidiasis, HbA1c or fasting glucose in recurrent candidiasis, cervical screening, pelvic ultrasound, and gynaecological referral for persistent symptoms or atrophic or dermatological causes.