Background: The aim of this study was to assess the performance of
GeneXpert
Group B streptococcus (GBS) is the most frequent cause of early-onset GBS
infection (EOGBS), which is associated with significant morbidity and mortality
among newborns. The incidence rate of EOGBS ranges from 0.5 to 3.0 per 1000 live
births, with 4 to 10% mortality [1, 2, 3, 4]. In Denmark, where the risk-based strategy
is recommended, the incidence ranges from 0.1 to 0.3 per 1000 live births [5].
Intrapartum antibiotic prophylaxis (IAP) is administered to women at risk. The
five risk factors for EOGBS are GBS during current pregnancy, prior infant with
EOGBS, temperature
Rapid nucleic acid amplification tests are increasingly performed during labor
[9, 10]. The sensitivity and specificity of GeneXpert
The aim of this study was to assess the performance of
GeneXpert
Three hundred and sixty-six (366) women were included in the study, which was
conducted between December 2018 and July 2019. Participants with one or more of
the following risk factors for GBS carriage were tested at the labor ward of
Lillebaelt University Hospital, Denmark: GBS during current pregnancy, prior
infant with EOGBS, temperature
The study was approved by the Danish Data Protection Agency (j.nr. 2012 58-0018). According to Danish legislation, quality assessment studies do not require approval from an ethics committee.
Intrapartum rectovaginal swab samples (ESwab, Copan diagnostics, Brescia, Italy)
were taken during labor, collected by a midwife, tested by
GeneXpert
The test was designed for use at the point of care in a labor ward and is run on
the GeneXpert
Training was given by the manufacturer during installation, which included training in sample collection, preparing the cartridge(s), and analyzing results. Training took about 30 minutes and additional training materials were provided to staff.
After sampling, the swab was transferred to the designated chamber of the
GeneXpert
The GeneXpert
All swabs were sent to the Department of Clinical Microbiology at Lillebaelt
University Hospital, Denmark. The samples were cultured as soon as possible,
otherwise they were kept at 4
Fifty microliters from the ESwab were cultured directly on the Granada agar and examined after incubation under anaerobic atmosphere for 24 and 48 hours.
Another 200
All GBS-like colonies were routinely confirmed as Streptococcus agalactiae using MALDI-TOF (Bruker Daltonik, Bremen, Germany). They were identified by the orange color on Granada plates and from the enrichment broth.
We determined the sensitivity, specificity, positive predictive value (PPV), and
negative predictive value (NPV) for the GeneXpert
Among the 366 women in labor tested, 99 were positive by culture and 95 were
positive by GeneXpert
Number | GeneXpert |
Culture |
88 | + | + |
8 | – | + |
3 | Error | + |
7 | + | – |
12 | Error | – |
248 | – | – |
+, positive result; -, negative result; Error, inconclusive result due to technical error. |
Among the GBS culture-positive samples, 88.9% (88/99) were positive using
GeneXpert
Among the 366 GeneXpert
GeneXpert | ||
% (n/N) | (95% CI) | |
Sensitivity | 91.7 (88/96) | 84.2–96.3 |
Specificity | 97.3 (248/255) | 94.4–98.9 |
PPV | 92.6 (88/95) | 85.8–96.3 |
NPV | 96.9 (248/256) | 94.1–98.4 |
The risk factors and corresponding GBS PCR results are shown in Table 3. The PCR test was positive in 26% (95/366) of all cases with one or more risk factors for EOGBS. Among women with GBS bacteriuria during their current pregnancy, 67% were positive. Among women with ROM for more than 18 hours, only 16% were positive (Table 3).
Intrapartum PCR GBS test | ||||
Risk factors | Negative N (%) | Positive N (%) | Missed test N (%) | Total N |
GBS in urine | 17 (28%) | 41 (67%) | 3 (5%) | 61 |
Preterm delivery |
72 (86%) | 10 (12%) | 2 (2%) | 84 |
ROM |
135 (80%) | 26 (16%) | 7 (4%) | 168 |
Temperature |
28 (62%) | 15 (33%) | 2 (5%) | 45 |
Previous EOGBS | 4 (50%) | 3 (38%) | 1 (12%) | 8 |
Total | 256 (70%) | 95 (26%) | 15 (4%) | 366 |
There was very little inconsistency between cultures with and without enrichment broth. Three percent (3/99) of swab samples were only positive in the enrichment broth, indicating that this broth pre-enrichment step is of limited value in a clinical setting. The two culture methods were therefore not evaluated separately. There was 100% consistency between the results from MALDI-TOF performed on direct culture and those from culture with the enrichment broth.
Total hands-on time required from the midwife who performed the testing (i.e., time from inserting the swab into the chamber S Insert Cartridge to starting the assay) was less than 1 minute.
The study was designed to assess the
performance of GeneXpert
One strength of our study is that the testing was conducted on fresh specimens in order to avoid cycles with freeze-thawing on a stored material. Another strength of our study is that the rectovaginal swabs were prospectively collected from women who fulfilled one or more of the risk factor criteria. A third strength is that the PCR and the two versions of GBS culture (with and without broth pre-enrichment) were tested on the same set of samples.
It could be considered a weakness that midwives performed the PCR analysis, and the fact that they are typically less experienced in this than trained lab technicians could be considered a bias. The PCR assay was performed in the delivery ward by midwives, who had received a brief introduction to the equipment by the local distributor in Denmark. Based on the methods described, the swab is placed directly in the machine and a result is delivered in 50 min. Thus, there is minimal user interference and the machine is suitable for midwives to perform. The user interface is very simple, even for a non-technical person. There were no issues encountered with midwives using the technology; they were generally positive towards the rapid testing. Total hands-on time (i.e., time from inserting the swab into the chamber S Insert Cartridge to starting the assay) was less than 1 min. With adequate training, this simple procedure can be incorporated as a clinical routine.
The results were reported by the GeneXpert
Helmig et al. [13] found that less than 1% of results were inconclusive; however, the PCR analysis of the swabs were not performed, as in our study, by midwives at a labor ward, but instead were performed by trained lab technicians. Mueller et al. [11] reported that 55.3% of their test results were initially inconclusive, but this reduced to 13.4% after the midwives were trained for two hours. Håkansson et al. and Helali et al. [8, 14], who also ran their PCR tests in labor wards, recorded about 15% and 9% invalid test results, respectively.
Invalid results are an important issue when assessing the feasibility of point-of-care technology. In a busy clinical setting, an inconclusive test result may very well result in IAP, since there is often no time to wait for a new test.
There was very little inconsistency between cultures with or without enrichment broth; only three samples that were positive only in the enrichment broth. Therefore, the two culture methods were not evaluated separately. The difference in detection rates between direct plating on the Granada medium and plating after prior Lim broth enrichment had been earlier found by El Aila NA et al. [15] to be 4%. Granada medium cannot detect non-hemolytic GBS, thereby potentially decreasing the sensitivity of this culture medium for GBS screening [2]. However, the frequency of non-hemolytic GBS isolates is only 1% among invasive GBS strains [16].
Our hypothesis for the inconsistencies between culture and PCR is primarily
informed by a failure to detect vaginal colonization with low numbers of GBS,
which may be of less risk to the newborn during birth [17]. False negative
results obtained by PCR were reported in samples with low colony growth, which
were probably below the detection limit of PCR techniques [18]. Also, Tickler
et al. [19] noted four types of chromosomal deletions in the region of
the cfb gene in GBS isolates that resulted in negative Xpert GBS tests, which
could possibly explain our culture-positive but
GeneXpert
El Helali et al. [14] performed a large cost-effectiveness study of
intrapartum PCR compared to antenatal cultures and concluded that the final costs
for both techniques were similar. They reported a significant decrease in the
prevalence of EOGBS in the intrapartum PCR negative group. They also found a
significant reduction in the use of IAP. The GeneXpert
We conclude from this study that the GeneXpert
EOGBS, early-onset onset GBS infection; GBS, group B streptococcus; IAP, intrapartum antibiotic prophylaxis; NPV, negative predictive value; PCR, polymerase chain reaction; PPV, positive predictive value; ROM, rupture of membranes.
MRK was in charge of the collection of samples; SYN was in charge of the culture of all samples. JKM supervised the project and was in charge of the data management. All authors participated in the writing process. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript.
The study was approved by the Danish Data Protection Agency (j.nr. 2012 58-0018). According to Danish legislation, quality assessment studies do not require approval from an ethics committee.
The authors would like to thank the midwives at the Department of Gynecology and Obstetrics, Kolding Hospital, University Hospital of Southern Denmark and the lab technicians at the Department of Clinical Microbiology, Vejle Hospital, University Hospital of Southern Denmark.
This research received no external funding.
The authors declare no conflict of interest.