What does my test result mean?
Not all tests for GBS carriage are as reliable as others.
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), and when these samples are cultured using an extra enrichment step to facilitate the growth of the GBS bacteria.
Positive Test Results
A positive test result is highly reliable – it is very unlikely that a test will give a false positive result (ie finding group B Strep when it’s not present). Any positive test result is a strong indication that GBS was present when the samples were taken.
- If you have received a positive test result, tell your health professional.
- If you received the test result by text, screenshot it to keep until written notification arrives from the laboratory.
- A positive GBS test result during pregnancy means that you should be offered intravenous antibiotics ASAP once labour has started and at intervals (4 hourly for penicillin) until your baby is born. These antibiotics dramatically reduce the chance of your newborn baby developing GBS infection.
- If you are allergic to penicillin or any other antibiotic, tell your healthcare professional – alternatives are available.
Read our leaflet Group B Streptococcus (GBS) in pregnancy and newborn babies – you can download it or read it online here.
Negative Test Results
The reliability of a negative result from a test for group B Strep carriage depends both on where the samples were taken from, and what method is used to process those samples. If you were taking or had recently finished taking antibiotics when your samples were taken, this may affect your test results – speak with your midwife or doctor about this.
Find out more about negative results using the different methods below, and read the leaflet Group B Streptococcus (GBS) in pregnancy and newborn babies – download it or read it online here.
Standard Direct Plating Method (more info here)
A negative result from a test for group B Strep carriage cultured using Standard Direct Plating is not necessarily reliable (although a positive result is). The Standard Direct Plating method only finds GBS up to half the times it is present, so around half of those carrying GBS will be falsely told they are not. A positive direct plating result can be trusted, but be wary of a negative one.
If known risk factors for GBS infection developing in a newborn baby are present AND the Standard Direct Plating test was negative, you should be offered intravenous antibiotics during labour against GBS infection in your newborn baby.
Enriched Culture Method (ECM) (more info here)
The ECM test is the ‘gold standard’ for detecting GBS carriage, and is increasingly available within the NHS. Most private suppliers, and all private suppliers listed on our testing page offer the ECM test.
When performed within 5 weeks of delivery, research reported that
- a negative ECM test result was 96% predictive of not carrying GBS at delivery (so 4% of women acquired GBS between taking 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 intravenous antibiotics in labour against GBS infection in your newborn baby are not indicated, unless one or more known risk factors are present.
Polymerase Chain Reaction Method (PCR) (more info here)
The accuracy of a PCR test is comparable if not better than the ECM test. PCR tests are not widely available in the UK at present.