NuSwab^SM VG: A new diagnostic approach to vaginitis

A nucleic acid amplification, single-specimen assay for detection of bacterial vaginosis, trichomoniasis, and vulvovaginal candidiasis

FACULTY Jeff Waldman, MD
Associate Clinical Professor, Department of Obstetrics and Gynecology, University of California, San Francisco, and Medical Director, Planned Parenthood Shasta Pacifica, Concord, Calif.

DISCLOSURE Dr. Waldman reports that he is a consultant to Graceway Pharmaceuticals. Dr. White has nothing to disclose.

This supplement is sponsored by Laboratory Corporation of America^® Holdings. Laboratory Corporation of America^® Holdings assumes no liability for the material published herein.

NuSwab^SM is a service mark of Laboratory Corporation of America^® Holdings.

It’s likely that ObGyns don’t need to see the statistics to know that vulvovaginitis is a major health problem. In fact, you probably devote a significant percentage of your clinical practice to the diagnosis and treatment of vulvovaginal infections—the reason for at least 10 million gynecologic office visits each year in the United States.¹ The three most common infections identified at these visits? Bacterial vaginosis (BV), trichomoniasis, and vulvovaginal candidiasis (VVC).¹ Conditions that once were discussed in low voices behind closed doors are now the subject of national media attention, thanks to their broad prevalence and the desire of patients to keep informed about their health.

Just how prevalent are these infections? According to an analysis from the National Health and Nutrition Examination Survey (NHANES), BV affects approximately 29.2% of women.² And the American Congress of Obstetricians and Gynecologists (ACOG) estimates the prevalence of trichomoniasis at 4% to 35%, and VVC at 17% to 39%.³

Many infected women lack symptoms, however, or their symptoms overlap those of other vaginal complaints. As a result, as many as 72% of women who have vaginitis remain undiagnosed or misdiagnosed.³ This is an important consideration, as effective treatment depends on accurate diagnosis.

IDENTIFYING CHARACTERISTICS

Bacterial vaginosis represents disruption of the vaginal flora, with overgrowth of anaerobic and facultative organisms such as Gardnerella vaginalis, Mycoplasma hominis, Atopobium vaginae, and other species. BV is the most common cause of abnormal vaginal discharge in women of reproductive age—but not all women who have BV exhibit abnormal discharge.⁴ In fact, most women who have BV are asymptomatic. BV is associated with an increased risk of acquisition of HIV and herpes simplex virus-2 (HSV-2), as well as postoperative infection, preterm delivery (and other complications of pregnancy), and pelvic inflammatory disease.³

The precise cause, or causes, of BV remain to be elucidated. The role of sexual activity in its pathogenesis is unknown.⁵

Trichomoniasis develops as a consequence of sexually transmitted infection with a protozoan parasite, Trichomonas vaginalis. In the United States, approximately 7.4 million cases of trichomoniasis are diagnosed each year, but only about 30% of patients develop symptoms or signs.^6,7 Because trichomoniasis is not reportable to public health agencies or included in routine screening for sexually transmitted diseases, its prevalence is likely to be underestimated and may be as high as 32% in some populations.⁷ When trichomoniasis is present, a person may be more likely to acquire or transmit other sexually transmitted diseases, such as HIV and Neisseria gonorrhoeae.⁶ Trichomoniasis is also associated with an increased risk of preterm delivery.³

As its name suggests, candidiasis is caused by species of the yeast genus Candida—usually C albicans, the species identified in most cases.⁸ In recent years, however, other species have emerged in women who have VVC, including C glabrata, C tropicalis, and C krusei. Approximately 75% of women are affected by candidiasis during their lifetime—nearly 50% of them on more than one occasion.⁸ Candidiasis is common in pregnancy.

THE CHALLENGE OF DIAGNOSING VAGINITIS

Bacterial vaginosis. Traditional clinical diagnosis of BV involves the use of 1) Amsel’s criteria or 2) the Nugent score, based on analysis of a Gram stain.

A diagnosis of BV based on Amsel’s criteria requires the presence of at least three of the following:

• abnormal vaginal discharge that is homogeneous, thin, and gray in color

• vaginal pH level above 4.5

• a positive “whiff” or amine test, i.e., a fishy odor before or after addition of 10% potassium hydroxide to a sample of vaginal secretions

• a wet mount revealing that at least 20% of epithelial cells are clue cells.⁴

The Nugent score, used primarily in research, is the gold standard diagnostic test. In determining the score, a value is assigned to the various bacterial morphotypes found on a Gram stain of vaginal secretions:

• A score of 0 to 3 is negative for BV

• A score of 7 or higher is positive

• A score of 4 to 6 signals intermediate risk

Compared with the Nugent score, Amsel’s criteria exhibit sensitivity of 92% and specificity of 77%.⁹

Cultures are of no value in the diagnosis of BV because some organisms associated with the infection, such as G vaginalis, can also be found in normal vaginal flora. Moreover, cultures do not recover all organisms associated with BV.

Although the Nugent score and Amsel’s criteria are reliable tests, they aren’t always practical for clinical use. Obtaining the Nugent score, for example, can be time-consuming and requires the presence of specially trained staff. Even diagnosis based on Amsel’s criteria requires the use of microscopy and has the potential for variability in interpretation of specimens.

Trichomoniasis. Traditional diagnosis of trichomoniasis involves a culture or examination of a wet mount for trichomonads. However, these methods have low sensitivity, ranging from 43.0% to 83.3%.⁷ In addition, examination of the wet mount for trichomonads must be performed within 10 to 20 minutes after taking the vaginal sample—or the organisms lose their viability.⁶

Moreover, as mentioned above, many women who have trichomoniasis are asymptomatic. Other diagnostic challenges include a risk that trichomoniasis may be misdiagnosed as BV, or may be part of a mixed infectious process.⁷

Candidiasis. VVC is traditionally identified by microscopic visualization of yeast-like cells or isolation of Candida species by culture.³ However, microscopy lacks sensitivity and fails to identify the species, whereas cultures may have a long turn-around time. Also, because 10% to 20% of women harbor Candida species and other yeasts in the vagina with no adverse effects, a finding of Candida by itself (by means of culture) is not an indication for treatment unless symptoms are also present.⁸

Overall, the diagnosis of vaginitis using traditional methods tends to be subjective, time-consuming, or low in sensitivity (see the Table).

Diagnosing vaginitis: How 4 approaches stack up

BREAKTHROUGHS IN DIAGNOSTIC TESTING

DNA-based diagnostic tests, such as polymerase chain reaction (PCR), involve the amplification of a small fragment of DNA by several orders of magnitude, making it a more objective tool to detect and identify infectious organisms. Nucleic acid amplification (NAA) by PCR not only identifies BV-associated bacteria, but some PCR methods can also quantify their numbers. Information derived from PCR-based NAA has added to our understanding of the complexity of the microflora colonizing the vagina and aided in the development of more informative diagnostic tests.

Three markers of BV

An NAA test for BV was evaluated in a clinical trial involving 396 women.¹⁰ In the trial, sponsored by LabCorp and conducted in association with Jane Schwebke, MD, of the University of Alabama at Birmingham, all vaginal specimens were assessed using Amsel criteria and the Nugent Gram stain. In addition, vaginal samples from the same 396 women were tested by quantitative PCR to detect the presence of five potential markers of BV. Analysis of the five markers and combinations of multiple markers led to development of an NAA test for BV based on PCR measurement of the three organisms found to be most predictive:

• A vaginae

• bacterial vaginosis–associated bacterium (BVAB)-2

• megasphaera-1.¹⁰

Somewhat surprisingly, although G vaginalis is commonly associated with BV, its presence in vaginal specimens did not add to the predictive value of the three-marker profile; nor was G vaginalis alone as informative as the three-marker profile. Lactobacillus crispatus was omitted from the final assay for the same reason. Moreover, the presence of L crispatus in normal vaginal microflora varies considerably, depending on the race and ethnicity of the individual; its inclusion in an assay for BV might, therefore, confound test results.

Sensitivity and specificity of the NAA test for BV were 96.2% and 92.1%, respectively, compared with Amsel diagnosis and Nugent Gram stain.¹¹ The NAA test, which evolved to become the BV component of NuSwab^SM VG, had a positive predictive value of 94.0% and a negative predictive value of 95.0%.¹¹

One clear advantage of the NAA test is the quick availability of test results. PCR NAA tests also omit the need for invasive collection of test specimens and can be performed on a patient-collected vaginal swab. Although the cost of an NAA test is slightly higher than traditional diagnostic methods, NAA tests might be preferred by patients for the ease of sampling and quick results.^12-14 In a clinical setting, they are easy to collect and perform and offer impressive sensitivity.

NAA for trichomoniasis and VVC

When NAA was applied to the diagnosis of trichomoniasis, sensitivity improved considerably, compared with traditional diagnostic methods. An NAA test (the automated APTIMA^®Trichomonas vaginalis assay [Gen-Probe Incorporated]) was tested in 1,025 asymptomatic and symptomatic women using vaginal swabs and other collection methods. Clinical sensitivity and specificity of the APTIMA^® assay were 100.0% and 99.0%, respectively, for the vaginal swab. The assay performed similarly in asymptomatic and symptomatic women.¹⁵

An NAA assay also performed well in the diagnosis of VVC. When candidiasis was identified by NAA assay, results were concordant with those of culture in 89.8% of specimens tested (230 of 256 specimens).¹⁶ (C glabrata is the predominant non-albicans Candida species associated with VVC in the United States, Europe, and Australia.¹⁷ According to the findings of two large studies in the United States, C albicans and C glabrata constitute approximately 93% to 97% of all Candida species.^18,19 Other species are comparatively rare.)

How NAA testing can guide treatment decisions

The ability of the NAA assay to identify the Candida species as C albicans versus C glabrata is an important distinction. C albicans infection typically responds to standard azole antifungal therapy. Women who have infection with a non-albicans species require more aggressive treatment, however. Among the treatment options for women who have “complicated” VVC (i.e., non-albicans infection) is a standard course of topical imidazole, which may be effective in as many as 50% of cases. When this approach fails to eliminate infection, vaginal boric acid for a minimum of 14 days is an option. Refractory cases should be referred to a specialist.³

NAA tests are also useful when microscopy or clinical findings, or both, are inconclusive, or when infection involves more than one pathogen. For example, a patient who has both BV and VVC may require individualized azole therapy and, in some cases, extended antibiotic treatment as well.³

In addition, NAA tests eliminate the long turnaround time (which can delay treatment) associated with cultures and are particularly useful when there is a likelihood that the patient would be lost to follow-up during the wait for culture results.

THE BENEFITS OF MULTIPLE NAATs ON A SINGLE SWAB

The trials mentioned above led to development of the NuSwab^SM VG assay (LabCorp), which identifies BV, trichomoniasis, and candidiasis (two species: C albicans and C glabrata) in a single test with high sensitivity and specificity. Tests to identify Chlamydia trachomatis, N gonorrhea, HSV-1, and/or HSV-2 with high sensitivity are also available on the same collection device.

This quick test helps clinicians determine whether the cause of vaginitis is bacterial, parasitic, or fungal, and enables them to tailor treatment accordingly. It also simplifies management of test supplies and can reduce errors associated with use of the “wrong swab.”

See “NuSwab^SM VG test in practice,” below, for discussion of its use in a clinical setting.

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