Diagnostic concordance between Amsel’s criteria and the Nugent scoring method in the assessment of bacterial vaginosis

Keywords: Amsel’s criteria, bacterial vaginosis, diagnostics, Nugent scoring method, reproductive health, sexual and reproductive health, vaginal dysbiosis, vaginal ecology, vaginal microbiota, women.

Bacterial vaginosis (BV) is a clinical syndrome associated with altered vaginal ecology and a spectrum of vulvovaginal symptoms, including malodorous discharge and elevated pH. It is common among pre-menopausal women and has been implicated in adverse sexual and reproductive outcomes.^1–3 Traditionally, BV has been diagnosed by Amsel’s clinical criteria or laboratory-based Nugent scoring method. However, due to realistic constraints in clinical practice, either syndromic case management is favoured or modified clinical criteria are used, leading to misdiagnosis and unfavourable health outcomes. We therefore sought to evaluate the concordance between the standard diagnostic tests – Amsel’s criteria and the Nugent scoring method. Additionally, we evaluated the accuracy of individual and combinations of two Amsel’s criteria for BV diagnosis.

A prospective, longitudinal study was conducted from August 2018 to February 2021 in the Sexually Transmitted Diseases (STD) and Obstetrics and Gynaecology out-patient departments at the All India Institute of Medical Sciences, New Delhi, India. Women who were pregnant, lactating, menstruating, or had received oral antibiotics in the preceding 4 weeks were excluded. The study was approved by the Institute Ethics Committee, All India Institute of Medical Sciences, New Delhi and informed consent was obtained from all participants.

A total of 293 participants were enrolled in the study and vaginal swabs were collected. Additionally, BV-positive women were treated and repeat swabs were collected at week 1, week 4 and thereafter quarterly till relapse. Finally, 404 vaginal samples were evaluated using the Nugent scoring method and a score of 0–3 was considered negative, 4–6 as intermediate, and 7–10 as BV-positive. Clinical diagnosis was based on the following signs: (1) thin homogenous vaginal discharge; (2) vaginal pH > 4.5; (3) positive whiff test; and (4) presence of clue cells. A positive diagnosis using standard Amsel’s criteria was made when any three of the four signs were present and for simplified criteria, the presence of any two of the signs were considered. Using the Nugent score as the gold standard, sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), accuracy, and kappa statistic (κ) for standard, simplified, and individual Amsel’s criteria were calculated. All statistical tests were performed using STATA ver. 12 (StataCorp LLC).

The Nugent score diagnosed 110 (27.2%), Amsel’s criteria detected 108 (26.7%), and simplified Amsel’s criteria identified 161 (39.9%) women as BV-positive (Table 1). The overall sensitivity, specificity, PPV, NPV, and accuracy for standard Amsel’s criteria were 71.8% (95% CI = 62.4–80.0), 90.1% (95% CI = 62.4–86.1), 73.1%, 89.5%, and 85.1%, whereas for simplified Amsel’s criteria, the parameters were 88.2% (95% CI = 80.6–93.6), 78.2% (95% CI = 73.1–82.8), 60.2%, 94.7%, and 80.9% respectively (Table 1). The overall concordance between standard Amsel’s criteria and the Nugent scoring method was substantial (κ = 0.62; 95% CI = 0.53–0.72), but decreased on using the simplified Amsel’s criteria (κ = 0.58; 95% CI = 0.49–0.67) (Table 1). Clue cells were the single most valuable marker of BV with highest concordance (κ = 0.70; 95%,CI = 0.60–0.80) and accuracy (87.9%). Although vaginal pH had the highest sensitivity (81.8%; 95% CI = 73.3–88.5), a noteworthy observation is its fair concordance (κ = 0.26; 95% CI = 0.17–0.34) when the Nugent scoring method is used. This finding holds relevance in busy clinical settings where pH is often used as a standalone tool for BV diagnosis.⁴ As vaginal pH is subject to fluctuations in the presence of semen or blood, un-aided by other criteria, it may lead to over-diagnosis and subsequently over-treatment. In our study, vaginal pH > 4.5 (specificity 52.4%; 95% CI = 46.5–58.2) and abnormal vaginal discharge (sensitivity 64.5%; 95% CI = 54.9–65.2) were the least reliable predictors of BV. Vaginal pH in combination with clue cells, however, had the highest concordance (κ = 0.64; 95% CI = 0.54–0.74) when using the Nugent scoring method (Table 1).

Table 1.  Diagnostic concordance between standard, individual and simplified Amsel’s criteria and the Nugent scoring method in Indian women.

Amsel’s criteria may be simplified using a combination of vaginal pH and clue cells with high diagnostic accuracy. A combination of vaginal pH and a whiff test may be useful when microscopic facilities are unavailable, however, with a considerable loss to sensitivity.

The data that supports this study may be shared upon reasonable request to the corresponding author.

Somesh Gupta is a Joint Editor of Sexual Health but was blinded from the peer-review process for this paper.

The work was supported by grants from Intramural Collaborative Research Award (Grant number: 8-06/AC-06/2018-RS) by the All India Institute of Medical Sciences, New Delhi, India.