Reproduced with kind permission of Group B Strep Support
Currently the best way of knowing which women carry GBS in labour is through testing for GBS at 35-37 weeks of pregnancy – using sensitive enriched culture medium (ECM) tests. As recommended by Group B Strep Support (GBSS.org.uk). If positive, then preventative measures can minimise the risk of transmission to the child. Intravenous antibiotics given in labour to women whose babies are at higher risk of developing GBS infection have been proven to dramatically reduce that risk.
Sadly, the NHS do not routinely test pregnant women for GBS.
Laboratory testing is the only way in which GBS carriage can be reliably identified.
Knowing the result of a test sensitive for GBS test is always good news. If it’s negative within 5 weeks of delivery, then it’s hugely unlikely the baby will develop GBS infection. If it’s positive, although it does mean that the baby is at a raised risk of developing GBS infection, it also means that – as the GBS carriage has been identified – simple, straightforward steps can be taken which have been proven to be extremely effective at minimising that risk.
In the UK, three different tests for GBS carriage may be used and it is important to know which is being offered.
- Standard Direct Plating method
- Enriched Culture Medium (ECM) method
- Polymerase Chain Reaction ( PCR) method
Standard Direct Plating (Conventional NHS Test)
This is the method generally used in the NHS when investigations are undertaken as a result of, say, vaginal symptoms. It was not specifically designed to detect group B Strep and is not a sensitive test for this purpose. Usually only a vaginal (and often a high vaginal) swab is taken and sent to the laboratory. There the cells from the swab are transferred onto a dish or ‘plate’ containing agar (a growth medium) and after 24 and 48 hours incubation, the plate is examined to see if GBS has grown.
A positive result using this test method is highly reliable – there are few falsely positive results.
A negative result using this method is not very reliable – it gives a high proportion of falsely negative results. GBS will be isolated in only around 50% of cases where the Mum is carrying GBS as the swab was taken. This test is unselective – other bacteria present on the swab may outgrow and ‘swamp’ the GBS sample.
Many health professionals and most pregnant women are unaware of just how high the false-negative rate is for these tests.
Enriched Culture Medium (ECM) Test
Designed specifically to detect group B Strep carriage, this test is available from an increasing number of NHS trusts and privately (see here). This method requires samples taken from the low vagina and rectum (either using two separate swabs or one combined vaginal then rectal swab) which are then sent off to the laboratory marked “for GBS culture in ECM.”
In the laboratory, the cells from the swab(s) are incubated in an enriched culture medium specifically designed to encourage the growth of GBS and so enhance its detection. After incubation, the specimen is sub-cultured onto an agar plate. The bacteria have to grow into a sizeable colony before they can be identified, so getting a result takes a minimum of 24 hours, and more usually 48-72 hours to establish whether GBS has grown.
The ECM test, recognised as the ‘gold standard’ for detecting GBS, is highly sensitive. Research has shown that, when the ECM test is properly performed within 5 weeks of delivery, a negative result was 96% predictive of not carrying GBS at delivery (4% of women acquired carriage between the test and giving birth) and a positive result was 87% predictive of carrying GBS at delivery (13% of women lost carriage between performing the test and giving birth). The test can be done earlier, but then isn’t as reliable at predicting colonisation status at delivery. It can be done later, but the chance of the baby arriving before the result increases.
ECM tests have been used for many in other countries and have been validated during more than a decade of use. The national standard method for testing for GBS carriage (Public Health England’s UK Standards for Microbiology Investigations B 58) describes this method in more detail and the document is available from Public Health England’s UK Standards for Microbiology Investigations or click here.
Polymerase Chain Reaction (PCR) Test
PCR is a commonly used technique in biology, for DNA testing. Only recently has its application been used in testing for GBS.
PCR Testing is rapid, and could potentially be used as a point-of-care test
Samples taken from the low vagina and rectum (either using two separate swabs or one combined vaginal then rectal swab). The samples are then places in the PCR machine – the PCR test amplifies the amount of DNA present and requires shorter incubation times than other tests.
This test method may be as sensitive as the ECM test. It’s biggest advantage however, is that the results can be obtained much more quickly, although expensive equipment is required to process the swabs. If the sample turnaround times could be reduced still further and the cost of the equipment lowered, this could in the future be used to determine carriage status upon admission to the labour ward (a point-of-care test).
This method is approved by the Federal Drug Administration (FDA) in the USA and Health Canada, and bears the CE Mark for the detection of GBS (having the CE mark means that the test is approved for use in any European country). However, this test method has not been validated for use in the UK.
The test kit that you will receive from us is an ECM sensitive test from The Doctors Laboratory (a fully CPA accredited laboratory). Testing for GBS is undertaken in the laboratory by using an enriched culture medium (ECM). Two swabs (lower vaginal and rectal) taken at 35-37 weeks’ gestation best predict colonisation with GBS around the time of delivery.