GBSS believes all pregnant women should be informed about group B Strep and that all low-risk women should be offered the opportunity to have a sensitive test for GBS carriage late in pregnancy.
Whatever the result of a GBS specific test, it is good news. If such a test detects no group B Strep, that is great. If such a test detects GBS, that is also good news – although it means the baby is at a raised risk of developing GBS infection, it also means the risk is known so steps can be considered that have been proven to be hugely effective at minimising that risk.
If you are or have recently been taking antibiotics, this may affect the result of a GBS test. Please discuss this with your health professionals.
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’ 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)
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 GBS 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 the situation 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)
PCR is a commonly used technique in biology, for DNA testing. Only recently has its application been used in testing for GBS. 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.
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. Its 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).
*Images taken from Wikimedia Commons