What does my test result mean?
You may have heard that not all tests are as reliable as others, and that is true. Group B Strep carriage is much more likely to be detected from samples taken from the low vagina and the rectum (compared with only from the lower or higher vagina), which are then cultured using an extra enrichment step to facilitate the growth of the GBS bacteria.
Positive Test Results
It is very unlikely that a test will give a false positive result (ie finding GBS when it’s not present). So, even though some test methods are less sensitive than others, any positive test result is a strong indication that you were carrying GBS when the swabs were taken.
If you have received a positive test result, tell your health professional. If you received the result by text, you can screenshot that to keep until a written notification arrives from the laboratory.
Any positive GBS test result during pregnancy means that the pregnant woman should be offered intravenous antibiotics as soon as possible from the start of labour (and usually 4-hourly) until delivery. Your health professional should discuss this with you.
Negative Test Results
The reliability of a negative GBS test result depends both on where the samples were taken from (see above), and on which test is used.
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, you should be offered intravenous antibiotics during labour against GBS infection in your newborn baby.
Standard Direct Plating (more info here)
A negative test result from test cultured using Standard Direct Plating is not necessarily reliable (although a positive result is). This method will only find GBS up to half the times it should. So around half of those who are carrying GBS will be incorrectly told they are not. So a positive direct plating result can be trusted, but be wary of a negative one.
Enriched Culture Method (ECM) (more info here)
The ECM test is recognised as the ‘gold standard’ for detecting GBS carriage and is increasingly available within the NHS. It has been specifically designed to detect GBS and is highly sensitive. The providers listed on our testing page use the ECM test methodology.
Research shows that, if performed within 5 weeks of delivery, a negative ECM test result was 96% predictive of not carrying GBS at delivery (so around 4% of women acquired GBS between the test and giving birth) and a positive result was 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 you would not be offered intravenous antibiotics in labour against GBS infection in your newborn baby unless known risk factors are present.
Polymerase Chain Reaction (PCR) Test (more info here)
The accuracy of a PCR test is comparable, if not better than the ECM test. However, it is not widely available in the UK at present.