Which is the best method to test for GBS carriage – ECM, PCR or HVS?

At GBSS we believe 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.

There are three types of tests for GBS carriage:

Standard High Vaginal Swab (HVS) method

Enriched Culture Medium (ECM) method

Polymerase Chain Reaction ( PCR) method

Standard (HVS)

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. In the laboratory, the cells from the swab are transferred onto a dish or ‘plate’ containing agar 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 – i t will only pick up GBS in around 50% of cases where it is there .

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 (ideally using two separate swabs, but sometimes one combined vaginal then rectal swab is used) and is offered at a small number of enlightened NHS hospitals (we have heard of three offering it). This is also the method used by two private laboratories, each of which offers a postal service – The Doctors Laboratory and Mullhaven Medical Laboratory.

In the laboratory, the cells from the swab 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 national standard method for testing for GBS carriage (Bacteriological Standard Operating Procedure 58) describes this method in more detail and the document is available from the Health Protection Agency Evaluation & Standards Laboratory (e-mail standards@hpa.org.uk or you can download a PDF copy atwww.hpa-standardmethods.org.uk/documents/bsop/pdf/bsop58.pdf ).

The ECM test, sometimes referred to as the ‘gold standard’ 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, ECM tests have been used for many years in many countries and have been validated during more than a decade of use.

PCR (Polymerase Chain Reaction)

These tests require a low vaginal swab PLUS a rectal swab (ideally using two separate swabs, but sometimes one combined vaginal/rectal swab is used).

The swab(s) are sent to the laboratory, where they use special equipment, which uses a rapid test to detect special molecules found only in 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.

This test method is at least as sensitive than the ECM test and can produce results much more quickly.

How reliable are the test results?

Any test result positive for GBS during pregnancy means that the pregnant woman should be offered intravenous antibiotics from the onset of labour or waters breaking and then 4-hourly until delivery.

A negative ECM or PCR test result means that the pregnant woman does not need to be offered intravenous antibiotics from the onset of labour or waters breaking against GBS infection in her baby.

Where no ECM test result is available OR the less reliable NHS test has returned a negative result, the pregnant woman should be offered intravenous antibiotics during labour against GBS infection in her baby if one or more risk factors is present.

The ECM test has only been available in the UK since 1st May 2003 so many health professionals may not yet be aware of its availability. You can find more information about this test which you can give to your health professional from The Doctors Laboratory and from Mullhaven Laboratory.