Results of the test accuracy study

Parts of this chapter have been reproduced from Daniels et al.1 This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY 4.0) license, which permits others to distribute, remix, adapt and build upon this work, for commercial use, provided the original work is properly cited. See: https://creativecommons.org/licenses/by/4.0/. The text below includes minor additions and formatting changes to the original text.

This chapter reports the test accuracy parameters of the Xpert GBS test on the GeneXpert testing platform compared with selective enrichment culture as reference standard.

The secondary objectives were to (1) establish a standard operating procedure for the rapid test, with turnaround times compatible with provision of a suitable duration of antibiotic exposure to test positive mothers, and (2) determine the time to availability of test results in practice and the time remaining before birth to give an adequate antibiotic exposure for effective prevention of mother-to-baby transmission.

Development of a standard operating procedure and training

With the assistance of the field application specialist from Cepheid, a simple protocol for using the GeneXpert testing platform was developed. The field application specialist and trial manager trained research midwives and clinical midwives who were designated as GBS2 trial champions at each maternity unit at a face-to-face training session. The trained midwives were then responsible for cascading the training to all clinical midwives until there were sufficient numbers of midwives trained to be able to offer the test at all times and have someone capable of using the GeneXpert machine. The average time between the initial training of midwives on the GeneXpert machine and the first women accrued to the study was 12 weeks (range 3.5–20.8 weeks).

Test data completeness

Overall, 557 of the 722 (77%) women in the rapid test group contributed to positive or negative results with the rapid intrapartum test (whether first or subsequent tests), 619 of the 722 (86%) women contributed to selective enrichment culture results and 534 of the 722 (74%) women contributed information on both tests. A flow chart to illustrate the availability of data and diagnosis by selective enrichment culture is shown in Figure 8.

FIGURE 8 Flow chart of test data from rapid test and selective enrichment culture. (Figure not reproduced here; see the original article.)

Although a vaginal/rectal swab was required by the protocol, this was not obtained in all cases, as women declined the rectal swab. In 90 women (i.e. 14% of all 667 women swabbed), only a vaginal swab was taken. There were also 253 (45%, 253/557) women with a test result who had vaginal cleansing with an antimicrobial solution that could have interfered with the viability of the swab sample and the test result. The reference standard data were not available for 4% (23/557) of women with test results and 15% (15/100) of those with failed rapid test. An ear canal swab was taken from 445 babies and a test result obtained by selective enrichment culture. A corresponding maternal colonisation status was known for 443 of these babies.

Prevalence of maternal and neonatal group B Streptococcus colonisation

The prevalence of GBS colonisation was similar for each test method (Table 20) and removing data from those women who provided a vaginal swab only did not have an impact on the prevalence.

The neonatal colonisation rate was 11% (95% CI 8% to 14%) among 445 babies who had a result from selective enrichment culture of a neonatal ear swab in the rapid test units. The association between maternal and neonatal GBS colonisation is shown in Table 21. The concordance between maternal and neonatal colonisation status is 20%, irrespective of the method of determination of maternal status.

Diagnostic test accuracy of the rapid intrapartum test for maternal group B Streptococcus colonisation

The test result data for the 534 women providing results for both tests using vaginal/rectal or vaginal swab data are shown in Table 22. The sensitivity of the rapid test was 86% (95% CI 81% to 91%) and the specificity was 89% (95% CI 85% to 92%). The sensitivity of the rapid test was not statistically different from the expected sensitivity (p = 0.052) and specificity of 90% (p = 0.34).

The results of the sensitivity analyses excluding the 87 women who provided a vaginal swab only, and the 245 women who had vaginal cleansing or lubrication with an antimicrobial solution, are provided in Tables 23 and 24, respectively. The sensitivity of the test increased slightly to 88% and 89%, respectively.