Which test for GBS?
GBSS believes all low-risk women should be offered the opportunity to have a sensitive test to detect GBS carriage late in pregnancy.
Whatever the result, it is good news. If you test and find you’re not a carrier that is great. If you test and find you are a carrier that is also good news – although it means your baby is at a raised risk of developing GBS infection, it also means you know about it so you can decide whether to put into place simple straightforward steps which have been proven to be hugely effective at minimising that risk.
However, there are three different tests for GBS carriage and it is important to know which you are being offered.
Which is the best method to test for GBS carriage – ECM, PCR or Direct Plating?
There are three types of tests for GBS carriage, described in more detail below
Standard Direct Plating method
Enriched Culture Medium (ECM) method
Polymerase Chain Reaction ( PCR) method
Standard Direct Plating (Conventional NHS Test)
This is the method the NHS usually offers, when testing is offered at all. Usually only a vaginal (and often a high vaginal) swab is taken. This is less effective then swabs taken from the lower vagina and rectum at detecting group B Strep. In the laboratory, 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 very few falsely positive results. But it is not a particularly sensitive test and gives a high proportion of falsely negative results – it will only detect GBS in around 50% of cases where it is there. (This is because 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)
This method usually requires both a low vaginal and rectal swab (either using two separate swabs or one combined vaginal then rectal swab) and is offered at a small number of enlightened NHS hospitals (we know of 4 offering it). This is also the method used by private laboratories listed here.
In the laboratory, the cells from the swab are incubated in an enriched culture medium specifically designed to encourage the growth of GBS specifically 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 reliable. Research has showed that, when the ECM test was 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.
Although only available in the UK since May 2003 and mostly only privately, ECM tests have been used for many years 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 the Public Health England's UK Standards for Microbiology Investigations or you can download a PDF copy here).
PCR (Polymerase Chain Reaction)
PCR is a commonly used technique in biology, for DNA testing. Only recently has it's application been used in testing for GBS.
PCR Testing is rapid, and could potentially be used as a point-of-care test
Swabs are taken in the same way (ideally from the lower vagina and rectum). The PCR test amplifies the amount of DNA present, and therefore does not require the same incubation times as the 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 quicker, as the sample incubation is not needed, however expensive equipment is required to process the swabs. If the sample turnaround times could be reduced 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 diagnostic).
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.
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*Images taken from Wikimedia Commons