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Subject: Genital Culture
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Genital cultures should be collected from patients with signs and symptoms of localized genital tract infection or sexually transmitted disease, including discharge, dysuria, or lower abdominal pain.
This culture is used to detect common bacterial pathogens from genital specimens. Target pathogens typically include N. gonorrhoeae, yeast, group A and B β-hemolytic streptococci, Staphylococcus aureus, and Listeria monocytogenes. Gardnerella vaginalis should be reported if predominant and isolated in moderate to heavy growth. Invasively collected specimens should be cultured for isolation of these, as well as a broad range of other bacterial pathogens.
A Gram stain should be prepared from specimens submitted for genital culture. In male patients, the presence of many intracellular gram-negative diplococci is consistent with a diagnosis of gonorrhea. In female patients, a vaginal Gram stain may be used to identify “clue cells”; the absence of lactobacilli may be a marker of disruption of the normal vaginal flora, as with bacterial vaginosis.
Specimens are plated onto selective and nonselective media that support growth of fastidious pathogens. Examples include
Blood and chocolate agar
CNA and MacConkey agar, or comparable selective agar for gram-positive and gram-negative isolation
Selective agar for N. gonorrhoeae, such as Thayer-Martin, Martin-Lewis, NYC, or comparable media
Turnaround time: Routine genital cultures are incubated for up to 72 hours. Additional time is required in positive cultures for isolation, final identification, and further testing.
Male: An urethral swab should be collected. It may be possible to collect discharge expressed from the penile urethra. Collection of urethral discharge after prostatic massage may improve detection in patients with symptoms of prostatitis.
Urethral swabs or swabs from the cervical os are recommended. The cervix is visualized using a speculum lubricated only with water. Prior to collection of cervical specimens, mucus from the exocervix should be removed by use of a cleaning swab.
Vaginal specimens are not recommended for routine genital cultures. Vaginal specimens may be useful for diagnosis of vaginal candidiasis, Trichomonas vaginalis infection, or S. aureus superinfection.
Other specimens usually require more invasive sampling techniques, such as endometrial curettage, Bartholin gland aspiration, and culdocentesis.
Expected results: Cultures should yield only endogenous flora for the specimen submitted.
Positive: The interpretation of positive cultures may depend on the organism isolated and the quantity. N. gonorrhoeae is never normal flora and indicates gonorrhea.
Negative: A single negative culture does not rule out infection with N. gonorrhoeae or other genital pathogen. Sampling several sites, like the cervix and urethra, and serial sampling may improve detection.
The symptoms related to genital infections may overlap with those of UTI, so urine cultures are recommended for most patients for whom genital cultures are submitted. Routine genital cultures are most often submitted for diagnosis of an STD caused by N. gonorrhoeae. A number of STDs will not be detected by routine bacterial genital culture, including C. trachomatis, Treponema pallidum, Haemophilus ducreyi, Ureaplasma urealyticum, T. vaginalis, HSV, and HPV. Special cultures or procedures are needed for detection of infections with these pathogens. See Group B Streptococcus Rectovaginal Culture Screen for detection of group B beta-hemolytic Streptococcus carriage during pregnancy.
Special cultures are required to detect N. gonorrhoeae infections of nongenital sites, such as the rectum or throat.
Molecular diagnostic techniques provide improved sensitivity for diagnosis of genital infections caused by N. gonorrhoeae and C. trachomatis.
Isolation of a sexually transmitted pathogen from a child must be investigated as a sign of possible abuse.