The different types of tests for Group B Strep Carriage.
Make sure you take the right test.
The UK currently does not offer routine antenatal testing for Group B Strep. This is in sharp contrast to many other developed countries who provide GBS screening to all pregnant women. Some women in the UK are offered a Group B Strep test if GBS was detected in a previous pregnancy.
The NHS commonly use two tests to detect Group B Strep carriage; a standard ‘non-selective’ swab test, known as the High Vaginal Swab (HVS) test and a Group B Strep-specific swab test, known as the Enriched Culture Medium (ECM) test.
The High Vaginal Swab (HVS) test is used when a woman has vaginal symptoms needing further investigation and is a general-purpose test. A swab is used to take a sample from high up in the vagina which is then processed on an agar plate in a laboratory. This test is not designed to specifically detect Group B Strep and can often miss GBS bacteria.
A Positive Group B Strep High Vaginal Swab (HVS) test result is highly reliable as there are very few false-positive results. However, A Negative Group B Strep High Vaginal Swab (HVS) test result is not reliable, a high proportion of false-negative results are given finding GBS in only half of those with GBS carriage; meaning 50% are incorrectly told that they are not a GBS carrier.
You can rely on a Positive High Vaginal Swab (HVS) test but be aware of a Negative one.
The Enriched Culture Medium (ECM) test has been specifically designed to detect Group B Strep. It is recommended by the Royal College of Obstetricians & Gynaecologists (RCOG) for women who have carried GBS in a previous pregnancy. It is recognised as the ‘Gold Standard’ Group B Strep test internationally. Some NHS trusts are beginning to offer the ECM test.
Two swabs are used to take samples from the low vagina and rectum which are then processed in a laboratory. When the ECM test is properly performed within 5 weeks of delivery
A negative result is 96% predictive
that a woman will not be carrying Group B Strep at delivery.
A small percentage of women can acquire GBS carriage between the test and giving birth.
A positive result is 87% predictive
that a woman will be carrying Group B Strep at delivery.
Although some women do lose carriage between taking the test and giving birth.
The countries which test all pregnant women for Group B Strep use the Enriched Culture Medium (ECM) test usually at 35-37 weeks of pregnancy.
Health services in the UK currently rely upon a ‘risk factor’ approach to determine which newborns are more likely to be at-risk of developing a GBS infection during birth. The approach looks at factors such as high temperature during labour, labour starting preterm, waters breaking early or a previous GBS baby. A small group of women who carried Group B Strep in a previous pregnancy where their baby did not develop a GBS infection do qualify to be offered an ECM test through the NHS.
Most pregnant women that want to be tested for Group B Strep carriage during their pregnancy buy a private test.
Urine tests offered during pregnancy. An initial dipstick test may signal the presence of infection and that further investigation is needed. But Group B Strep cannot be detected by a urine dipstick test unless cultured in a laboratory from a urine sample. The level of bacteria found and whether the woman shows any symptoms, may indicate a Group B Strep urinary tract infection (UTI) which will need treating with antibiotic tablets. A negative urine test after culture only means that Group B Strep has not been found in the urine. GBS could still be present in the vagina or rectum.
There are currently some clinical research trials in the UK looking at whether a Rapid or ‘bedside’ test or ‘point-of-care’ test, is a good way of determining which women carry Group B Strep. Samples are taken from both the low vagina and rectum using swabs, which are processed in a machine usually on the labour ward.