You may have heard that not all tests are equal in terms of how reliable the results are, and that is true.
Despite some of the testing methods being more accurate than others, it is unlikely that a test will give a false positive result (that is detecting GBS when GBS is not present). Therefore with all three of the test methods, a positive result is a strong indication that the pregnant woman was carrying GBS when the swabs were taken.
Any positive GBS test result during pregnancy means that the pregnant woman should be offered intravenous antibiotics from the start of labour or waters breaking and then usually 4-hourly until delivery. Your health professionals should discuss this with you.
The reliability of a negative GBS result, however, is dependent upon which test was used:
Standard Direct Plating (HVS) (more info here)
Whilst a positive HVS result is highly reliable, a negative one is not. Only around 50% of women who are carrying GBS when the swabs are taken will correctly be told they carry GBS – the other half will be incorrectly told they are not. So trust a positive direct plating result, but be wary of a negative one.
Enriched Culture Method (ECM) (more info here)
This test is recognised as optimal for detecting GBS carriage. It is becoming increasingly available within the NHS. This test has been specifically designed for the isolation of GBS carriage and is highly sensitive.
Research shows that, if the ECM test is performed within 5 weeks of delivery, a negative result is 96% predictive of not carrying GBS at delivery (so in that trial 4% of women acquired carriage between the test and giving birth) and a positive result is 87% predictive of carrying GBS at delivery (so 13% of women lost carriage between performing the test and giving birth). A negative ECM test result within 5 weeks of delivery means that the pregnant woman would not need to be offered intravenous antibiotics from the onset of labour or waters breaking against GBS infection in her baby unless known risk factors are present.
Polymerase Chain Reaction (PCR) Test (see more on this method here)
A PCR test is unlikely to give a falsely negative result. Its accuracy is comparable, if not better than the ECM test. However, it is not widely available in the UK at present.
If known risk factors for GBS infection developing in a newborn baby are present, and no ECM test result is available OR the less sensitive Standard Direct Plating test was negative, then the pregnant woman should be offered intravenous antibiotics during labour against GBS infection in her baby (click here for more information on risk factors).