Accuracy of an Accelerated, Culture-Based Assay for Detection of Group B Streptococcus

Objective. To determine the validity of a novel Group B Streptococcus (GBS) diagnostic assay for the detection of GBS in antepartum patients. Study Design. Women were screened for GBS colonization at 35 to 37 weeks of gestation. Three vaginal-rectal swabs were collected per patient; two were processed by traditional culture (commercial laboratory versus in-house culture), and the third was processed by an immunoblot-based test, in which a sample is placed over an antibody-coated nitrocellulose membrane, and after a six-hour culture, bound GBS is detected with a secondary antibody. Results. 356 patients were evaluated. Commercial processing revealed a GBS prevalence rate of 85/356 (23.6%). In-house culture provided a prevalence rate of 105/356 (29.5%). When the accelerated GBS test result was compared to the in-house GBS culture, it demonstrated a sensitivity of 97.1% and a specificity of 88.4%. Interobserver reliability for the novel GBS test was 88.2%. Conclusions. The accelerated GBS test provides a high level of validity for the detection of GBS colonization in antepartum patients within 6.5 hours and demonstrates a substantial agreement between observers. 1. Introduction Group B Streptococcus (GBS) has been shown to cause significant neonatal morbidity, and current national guidelines recommend antenatal screening be performed on all pregnant women between 35 to 37 weeks gestation [1]. Current culture methods for GBS require 48 hours for identification, and if antibiotic sensitivity is necessary, an additional 24 hours may be needed [2]. Culture of GBS is routinely performed on agar supplemented with sheep’s blood, and colonies are detected by the breakdown of red blood cells, producing a characteristic zone of hemolysis, termed beta hemolysis [3]. The presence of GBS may be confirmed by performing Gram stain, CAMP testing, or typing via an agglutinin reaction on selected beta-hemolytic colonies [1]. This is often labor intensive, and in cases in which the patient’s sample contains many different species, beta hemolysis may be missed [4–6]. There is the risk that samples may be overgrown with competing organisms, such as Proteus or Enterococcus, and false negatives are not uncommon. When hemolysis is identified, follow-up testing is often required to prevent a false positive from being reported [7]. While culture continues to be regarded as the gold standard, it is labor intensive and is not without shortcomings. Several options are available which attempt to address the deficiencies inherent with traditional culture techniques. The most notable of