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Obstetrics & Gynecology 2000;96:504-506
© 2000 by The American College of Obstetricians and Gynecologists
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ORIGINAL RESEARCH

Wet Smear Compared With Gram Stain Diagnosis of Bacterial Vaginosis in Asymptomatic Pregnant Women

JOAN M. MASTROBATTISTA, MD, KAREN D. BISHOP and EDWARD R. NEWTON, MD

From the Department of Obstetrics, Gynecology, and Reproductive Sciences, University of Texas Houston Medical School, Houston, Texas; and Department of Obstetrics, Gynecology, and Reproductive Sciences, East Carolina State University Medical School, Greenville, North Carolina

Address reprint requests to: Joan M. Mastrobattista, MD University of Texas, Houston Division of Maternal-Fetal Medicine Department of Obstetrics, Gynecology and Reproductive Sciences 6431 Fannin, Suite 3.604 Houston, TX 77030 E-mail: joan.m.mastrobattista{at}uth.tmc.edu


   Abstract
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Abstract
Materials and Methods
 Results
 Discussion
 References
 
Objective: To compare wet smear and Gram stain diagnoses of bacterial vaginosis among asymptomatic pregnant women.

Methods: Between November 1, 1996 and December 31, 1997, asymptomatic women who initiated prenatal care in our obstetric clinics were invited to participate. Exclusion criteria included antimicrobial use within 2 weeks, cervical cerclage, vaginal bleeding, placenta previa, spermicide use, douching, or intercourse within 8 hours. Clinical diagnosis that required two of three positive criteria for bacterial vaginosis (vaginal pH, whiff test, and clue cells on wet smear) was compared with Gram stain diagnosis (Bacterial vaginosis score 7–10 by Nugent criteria).

Results: Population characteristics (n = 69) included an average (± standard deviation [SD]) maternal age of 27.3 ± 6.6 years, 26 nulliparas (38%), 28 black women (41%), 23 white women (38%), 15 Hispanic women (22%), and three Asian women (4%). The mean (±SD) gestational age at entry was 15.6 ± 7.6 weeks. Twenty-seven percent (18 of 67) of the study population was diagnosed with bacterial vaginosis by definitive Gram stain. Two slides were lost or were of poor quality and not included. Using Gram stain diagnosis of bacterial vaginosis as the standard, clinical diagnosis had sensitivity of 56% (95% confidence interval [CI] 32%, 78%), a specificity of 96% (95% CI 90%, 100%), a positive predictive value of 83%, and a negative predictive value of 85%.

Conclusion: In asymptomatic pregnant women, bacterial vaginosis can be diagnosed reliably by Gram stain.

Preterm birth is a major cause of perinatal morbidity and mortality. In the United States, the rate of preterm birth is about 10% and has remained relatively constant.1 No effective method of preventing preterm labor and delivery has been identified.2 Bacterial vaginosis is a common vaginal infection that affects 12–22% of pregnant women.3 It is associated with a group of genital microorganisms and is characterized microbiologically by a shift in the vaginal flora from normal Lactobacillus-dominant species to a mixed flora including Gardnerella vaginalis, Bacteroides species, Mobiluncus species, and Mycoplasma hominis.4 Bacterial vaginosis has been associated with many adverse pregnancy outcomes, such as preterm birth, premature rupture of membranes, infection of the chorion and amnion, histologic chorioamnionitis, and infection of amniotic fluid.3

Hillier et al3 found that women with bacterial vaginosis in the second trimester were 40% more likely to have preterm delivery of low-birth-weight infants than women without it. Hauth et al2 reported that treatment of bacterial vaginosis with metronidazole and erythromycin reduced rates of preterm births in women with bacterial vaginosis and in those with increased risk of preterm delivery. Many pregnant women can have shifts in their vaginal flora consistent with bacterial vaginosis, yet remain asymptomatic. Two well-described diagnostic methods for bacterial vaginosis are clinical or wet-smear diagnosis (Amsel criteria)5 and Gram stain diagnosis (Nugent criteria).4 Clinical diagnosis depends on the acuity of the clinician doing the test and the equipment available.4 A Gram stain from vaginal fluid is a reproducible and reliable test.4 The purpose of this study was to compare clinical and Gram stain diagnoses of bacterial vaginosis among asymptomatic pregnant women.


   Materials and Methods
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Abstract
 Materials and Methods
Results
 Discussion
 References
 
Asymptomatic pregnant women who initiated prenatal care in our obstetric clinics were invited to participate. Sixty-nine women were enrolled between November 1, 1996 and December 31, 1997. Exclusion criteria included antimicrobial use within 2 weeks, cervical cerclage, vaginal bleeding, placenta previa, spermicide use, recent douching, or sexual intercourse within 8 hours. Subjects had prenatal assessments including thorough histories and physical examinations. Cultures of the vagina and cervix were collected. The protocol was reviewed and approved by our institutional review board, and each subject gave written informed consent.

Routine pelvic examination was done with a clean, unlubricated speculum to inspect vaginal tissues. Routine cervical swabs were collected to test for Chlamydia trachomatis and Neisseria gonorrhoeae. Sterile cotton swabs were used to collect material from the posterior vagina and sidewalls. Vaginal pH was measured with Hydrion pH papers (Micro Essential Laboratory, Brooklyn, NY). The whiff test was positive if a fishy odor was noted after a drop of 10% potassium hydroxide was added to vaginal discharge. That slide was then evaluated for budding yeast, hyphae, and pseudohyphae under bright-field microscopy at x400 magnification. Additional vaginal discharge was suspended in a drop of saline and examined under bright-field microscopy for clue cells and trichomonads. With another swab, discharge from the posterior fornix and vaginal sidewalls was collected for a smear that was air-dried and sent to the laboratory for Gram staining and interpretation by a microbiologist (KDB).

Clinical diagnosis of bacterial vaginosis was considered positive if two of the following three criteria were met: vaginal pH exceeded 4.5, whiff test was positive, and clue cells were present on wet smear preparation. The character of vaginal secretion was not used as in the Amsel criteria5 because discharge in pregnant women is less easily characterized than in nonpregnant women. Gram stain diagnosis was based on a criteria score described by Nugent et al4 and considered positive if the score was 7–10. The Nugent criteria score vaginal flora as normal (0–3), intermediate (4–6), and bacterial vaginosis (7–10).4

Population characteristics were evaluated using means and standard deviations (SDs). Standard calculations were used for the measurement of sensitivity, specificity, positive predicative value, and negative predictive value.


   Results
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Abstract
 Materials and Methods
 Results
Discussion
 References
 
There were 69 subjects whose average (± SD) age was 27.3 ± 6.6 years, including 26 nulliparas (38%), 28 black women (41%), 23 white women (38%), 15 Hispanic women (22%), and three Asian women (4%). The mean (± SD) gestational age at entry was 15.6 ± 7.6 weeks. Eighteen of 67 women had bacterial vaginosis by definitive Gram stain. Two slides were lost or of poor quality and not included. Using Gram stain diagnosis of bacterial vaginosis as the standard, efficacy of clinical diagnosis had a sensitivity of 56% (95% confidence interval [CI] 32%, 78%), specificity of 96% (95% CI 90%, 100%) positive predictive value of 83%, and negative predicative value of 85%. Those statistical measures were applied to individual components of the clinical diagnosis and are shown in Table 1Go. False-positive rates of individual components of the clinical versus Gram stain–based diagnosis were pH over 4.5 (48%), positive whiff test (29%), clue cells present on wet smear (25%), and clinical diagnosis positive (17%).


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  		Table 1. Efficacy of Clinical (Wet Smear) Diagnosis*
 

   Discussion
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Abstract
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Bacterial vaginosis is often misdiagnosed using clinical criteria because the components are subjective and dependent on the acuity of the clinician and available equipment. The results of the study by Nugent et al4 indicated that criteria for diagnosis of bacterial vaginosis using Gram stain can be reproduced reliably by different centers and microbiologists. Summary scores can assess the degree of alteration in vaginal flora4 and allow for standardized interpretation based on bacterial morphotypes identified.6 In a multicenter study by Schwebke and colleagues,7 vaginal Gram stain (Nugent criteria) was more sensitive than Amsel criteria for diagnosis of bacterial vaginosis. Lower sensitivity of clinical diagnosis compared with Gram stain–based diagnosis and false-positive rates of individual clinical components in our study might be explained by the subjective nature of the clinical test. The reliability of diagnostic methods is particularly important when evaluating an asymptomatic population. Gram stain–based diagnosis is reliable and reproducible, so we recommend it for diagnosis of bacterial vaginosis in asymptomatic pregnant women.


   Footnotes
 
PII S0029-7844(00)00994-7

Received December 23, 1999. Received in revised form May 10, 2000. Accepted June 15, 2000.


   References
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Abstract
 Materials and Methods
 Results
 Discussion
 References
 
1. Novy MJ, McGregor JA, Iams JD. New perspectives on the prevention of extreme prematurity. Clin Obstet Gynecol 1995;38:790–808.[Medline]

2. Hauth JC, Goldenberg RL, Andrews WW, DuBard MB, Copper RL. Reduced incidence of preterm delivery with metronidazole and erythromycin in women with bacterial vaginosis. N Engl J Med 1995;333:1732–6.[Abstract/Free Full Text]

3. Hillier SL, Nugent RP, Eschenbach DA, Krohn MA, Gibbs RS, Martin DH, et al. Association between bacterial vaginosis and preterm delivery of a low-birth-weight infant. N Engl J Med 1995;333:1737–42.[Abstract/Free Full Text]

4. Nugent RP, Krohn MA, Hillier S. Reliability of diagnosing bacterial vaginosis is improved by a standardized method of gram stain interpretation. J Clin Microbiol 1991;29:297–301.[Abstract/Free Full Text]

5. Amsel R, Totten PA, Spiegel CA, Chen KCS, Eschenbach DA, Holmes KK. Non-specific vaginitis: Diagnostic criteria and microbial and epidemiological associations. Am J Med 1983;74:14–22.[Medline]

6. Hillier SL, Krohn MK, Nugent RP, Gibbs RS. Characteristics of three vaginal flora patterns assessed by Gram stain among pregnant women. Am J Obstet Gynecol 1992;166:938–44.[Medline]

7. Schwebke JR, Hillier SL, Sobel JD, McGregor JA, Sweet RL. Validity of the vaginal Gram stain for the diagnosis of bacterial vaginosis. Obstet Gynecol 1996;88:573–6.[Abstract]




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