For the infections we most commonly encounter in obstetric practice, I review in this article the selection of specific antibiotics. I focus on the key pathogens that cause these infections, the most useful diagnostic tests, and the most cost-effective antibiotic therapy. Relative cost estimates (high vs low) for drugs are based on information published on the GoodRx website (https://www.goodrx.com/). Actual charges to patients, of course, may vary widely depending on contractual relationships between hospitals, insurance companies, and wholesale vendors. The infections are listed in alphabetical order, not in order of frequency or severity.
1. Bacterial vaginosis
Bacterial vaginosis (BV) is a polymicrobial infection that results from perturbation of the normal vaginal flora due to conditions such as pregnancy, hormonal therapy, and changes in the menstrual cycle. It is characterized by a decrease in the vaginal concentration of Lactobacillus crispatus, followed by an increase in Prevotella bivia, Gardnerella vaginalis, Mobiluncus species, Atopobium vaginae, and Megasphaera type 1.1,2
BV is characterized by a thin, white-gray malodorous (fishlike smell) discharge. The vaginal pH is >4.5. Clue cells are apparent on saline microscopy, and the whiff (amine) test is positive when potassium hydroxide is added to a drop of vaginal secretions. Diagnostic accuracy can be improved using one of the new vaginal panel assays such as BD MAX Vaginal Panel (Becton, Dickinson and Company).3
Antibiotic selection
Antibiotic treatment of BV is directed primarily at the anaerobic component of the infection. The preferred treatment is oral metronidazole 500 mg twice daily for 7 days. If the patient cannot tolerate metronidazole, oral clindamycin 300 mg twice daily for 7 days, can be used, although it is more expensive than metronidazole. Topical metronidazole vaginal gel (0.75%), 1 applicatorful daily for 5 days, is effective in treating the local vaginal infection, but it is not effective in preventing systemic complications such as preterm labor, chorioamnionitis, and puerperal endometritis.2 It also is significantly more expensive than the oral formulation of metronidazole. Topical clindamycin cream, 1 applicatorful daily for 5 days, is even more expensive.
Tinidazole 2 g orally daily for 2 days is an effective alternative to oral metronidazole. Single-dose therapy with oral secnidazole (2 g), a 5-nitroimidazole with a longer half-life than metronidazole, has been effective in small studies, but experience with this drug in the United States is limited. Secnidazole is also very expensive.4
2. Candidiasis
Vulvovaginal candidiasis usually is caused by Candida albicans. Other less common species include C tropicalis, C glabrata, C auris, C lusitaniae, and C krusei. The most common clinical findings are vulvovaginal pruritus in association with a curdlike white vaginal discharge. The diagnosis can be established by confirmation of a normal vaginal pH and identification of budding yeast and hyphae on a potassium hydroxide preparation. As noted above for BV, the vaginal panel assay improves the accuracy of clinical diagnosis.3 Culture usually is indicated only in patients with infections that are refractory to therapy.
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