Bacterial vaginosis laboratory findings

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Bacterial vaginosis can be diagnosed by the use of clinical criteria (Amsel’s Diagnostic Criteria) or Gram stain. A Gram stain is considered the gold standard laboratory method for diagnosing bacteria vaginosis.[1]

Laboratory Findings

In clinical practice, the diagnosis of bacterial vaginosis is based on the presence of at least three Amsel criteria on laboratory studies.[1] Vaginal swabs following speculum examination will be tested for:

  • Whiff test: A small amount of an alkali is added to a microscope slide that has been swabbed with the discharge—a "fishy" odor is a positive result for bacterial vaginosis.
  • Loss of acidity: A swab of the discharge is put onto litmus paper to check the acidity. A positive result for bacterial vaginosis would be a pH>4.5 (normally slightly acidic with a pH of 3.8–4.2).
  • Clue cells: epithelial cells coated with bacteria (under microscopic examination of the discharge)

In research studies, the use of Nugent or Hay/Ison criteria to evaluate a Gram-stained smear of vaginal discharge is the diagnostic standard.

Gram Stain

A Gram stain is considered the gold standard laboratory method for diagnosing bacteria vaginosis. It is used to determine the relative concentration of lactobacilli (Gram-positive rods), Gram-negative and Gram-variable rods and cocci (Gardnerella vaginalis, Prevotella, Porphyromonas, and peptostreptococci), and curved Gram- negative rods (Mobiluncus) which are characteristic of bacterial vaginosis.Gram staining is usually reserved for research studies and can be used with the Hay/Ison criteria or the Nugent criteria.[1][2]

Hay/Ison Criteria

The Hay/Ison criteria are defined as follows: [3][4]

Grading Definition
Grade 0 No bacteria present
Grade 1 Lactobacillus morphotypes predominate
Garde 2 Mixed flora with (some lactobacilli, gardnerella,or mobiluncus morphotypes present)
Garde 3 Predominantly gardnerella and/or mobiluncus morphotypes (few or absent lactobacilli)
Garde 4 Gram-positive cocci predominate
  • Grade 3 is diagnostic

Nugent Criteria

The standard for research are the Nugent Criteria.[5]

Average per high powered (1000× oil immersion) field. View multiple fields
Score 0 1 2 3 4
Lactobacillus morphotypes >30 15-30 <14 <1 (this is an average) 0
Gardnerella/Bacteroides morphotypes 0 <1 (this is an average) 1-4 5-30 >30
Curved Gram variable rods 0 <5 >5

In this scale a score of 0-10 is generated from combining three other scores. It is time consuming and requires trained staff but is has high interobserver reliability:

  • 0–3 is considered negative for bacterial vaginosis
  • 4–6 is considered intermediate
  • 7+ is considered indicative of bacterial vaginosis


The Papanicolaou smear is not reliable for diagnosis of bacterial vaginosis.[6]


Vaginal culture has no role in diagnosis of bacterial vaginosis. [1][7]


  1. 1.0 1.1 1.2 1.3 Center for Disease Control and prevention. Mortality and morbidity weekly reports. Sexually transmitted disease treatment guideline. (2015) Accessed on October 20, 2016
  2. Nugent RP, Krohn MA, Hillier SL (1991). "Reliability of diagnosing bacterial vaginosis is improved by a standardized method of gram stain interpretation". J Clin Microbiol. 29 (2): 297–301. PMC 269757. PMID 1706728.
  3. Guideline Clearing House. "2002 national guideline for the management of bacterial vaginosis". Unknown parameter | ignored (help)
  4. Ison, CA; Hay, PE (2002), "Validation of a simplified grading of Gram stained vaginal smears for use in genitourinary medicine clinics", Sex Transm Infect, 78: 413–415
  5. Nugent, R. P., M. A. Krohn, and S. L. Hillier (1991). "Reliability of diagnosing bacterial vaginosis is improved by a standardized method of Gram stain interpretation". J. Clin. Microbiol. 29: 297&ndash, 301.
  6. Greene JF, Kuehl TJ, Allen SR (2000). "The papanicolaou smear: inadequate screening test for bacterial vaginosis during pregnancy". Am J Obstet Gynecol. 182 (5): 1048–9. PMID 10819823.
  7. Stockdale CK (2016). "A Positive Culture Result for Gardnerella Is Not Diagnostic of Bacterial Vaginosis". J Low Genit Tract Dis. 20 (4): 281–2. doi:10.1097/LGT.0000000000000237. PMID 27428605.

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