Group B Streptococcus (GBS) Infections Workup

Updated: Apr 21, 2021
  • Author: Christian J Woods, MD, FACP, FCCP, FIDSA; Chief Editor: Michael Stuart Bronze, MD  more...
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Laboratory Studies

For pregnant women, both vaginal and rectal swabs should be obtained at 36 0/7 and 37 6/7 weeks of gestation. [3, 4]  A single swab is used for culture; first a specimen is obtained from the lower vagina, then a specimen is obtained from the rectum. This method increases the culture yield significantly when compared with either sampling from the cervix or rectum alone or a single vaginal swab without a rectal swab. [15] Samples can be collected by the patients themselves, and studies have shown that self-collected samples are as accurate as those collected by physicians. [16]

Gram staining of an appropriate specimen is a useful first test because it can reveal early streptococcal infection.

Isolation of group B streptococci from blood, cerebrospinal fluid, and/or a site of local suppuration is the only method for diagnosing invasive GBS infection.

GBS antigen may be detected in blood, cerebrospinal fluid, and/or urine and may aid in diagnosis in certain circumstances.


Imaging Studies

Consider pneumonia in the elderly bedridden patient with fever and other relevant symptoms and neurologic deficits. Radiographs may show infiltrate or effusion in such patients.

Consider meningitis in a patient who has undergone a neurosurgical procedure and has  fever and other relevant symptoms. CT scanning of the head may reveal an abscess or contiguous infection. Rule out increased intracranial pressure so lumbar puncture can be performed safely.

Consider bacteremia, endocarditis, and line-related sepsis in a patient with a fever. Echocardiography may reveal vegetations or evidence of valve destruction.

In a patient who is elderly, bedridden, or has diabetes with fever and relevant symptoms, consider soft-tissue infection, osteomyelitis, diskitis, epidural abscess, wound infection, necrotizing fasciitis, and decubitus ulcer. Radiography of the involved area may reveal evidence of gas or bone destruction. CT scans or MRI of the involved area may reveal phlegmon, abscess, or osteomyelitis.

In a postpartum woman or older man or woman with fever and relevant symptoms, consider urinary tract infection and pelvic abscess. Ultrasonography of the genitourinary tract or pelvis may reveal evidence of genitourinary obstruction or abscess. CT scans and MRI may show evidence of obstruction or abscess.


Other Tests

Although rapid tests are available for assessment of maternal GBS colonization at the onset of labor, a study of 2 rapid tests by Daniels et al revealed that neither was sufficiently accurate to recommend their routine use in clinical practice. Of the 2 tests, polymerase chain reaction proved significantly more accurate than optical immunoassay; nevertheless, when combined vaginal and rectal swabs were obtained, polymerase chain reaction had a sensitivity of 84% and a specificity of 87%. The authors concluded that screening using a rapid test was not cost effective based on its current sensitivity, specificity, and cost, and that intravenous antibiotic prophylaxis directed by screening with enriched culture at 35 to 37 weeks' gestation (timing base on old American College of Obstetricians and Gynecologists guidelines) is likely to be the most acceptable cost-effective strategy. [17]

Another study revealed that although intrapartum antibiotic prophylaxis was effective in preventing mother-to-newborn transmission of group B streptococci, prenatal cultures obtained by health care providers during routine care did not accurately predict the occurrence of infection found during labor. [18]

Schwope et al advise that, when collecting samples for GBS screening during a pelvic examination, practitioners should obtain the sample before using a bacteriostatic surgical lubricant. In a prospective cohort study of 168 patients, 20 patients had positive GBS test result before the pelvic examination, but only 10 of those had a positive GBS test result after a pelvic examination was performed with a chlorhexidine-based surgical lubricant. [19]




A patient with GBS pneumonia may require diagnostic and therapeutic thoracentesis if a pleural effusion is present; empyema requires drainage by thoracentesis, chest tube, or surgery.

A patient with GBS bacteremia, endocarditis, or line-related sepsis may require valve replacement because of destructive endocarditis.

A patient with a GBS soft-tissue infection, arthritis, osteomyelitis, diskitis, or epidural abscess may require diagnostic aspiration and curative surgery. Necrotizing fasciitis and septic arthritis are surgical emergencies. A patient with an epidural abscess may require emergency surgery.

A patient with a GBS urinary tract infection or pelvic abscess may require aspiration with ultrasonography or CT scan guidance for a diagnostic tap to isolate the organism, relieve obstruction, or drain an abscess.