Group B Streptococcus Lab Test

Group B Streptococcus (GBS) is a normal bacterium which is carried by 20-40% of adults, most commonly in the gut and the vagina.

Most carriers do not know they have it as it is usually without symptoms.

GBS is common in pregnant woman and can be passed from mother to baby during labour, rarely causing any problems. However, for a small number of babies it can be life-threatening, causing blood poisoning, pneumonia and meningitis.

Group B Streptococcus (GBS), is the UK's most common cause of life-threatening infections in newborn babies, and meningitis in babies up to the age of 3 months.

The test helps to detect women carrying GBS, who are more at risk of passing the bacteria onto their baby during labour.

Screening for GBS is not routinely offered on the NHS, but laboratory testing is commonly undertaken at The Doctors Laboratory (www.tdlpathology.com) using the GBS-specific Enriched Culture Medium (ECM) test.

The GBS-specific ECM is the international ‘gold standard’ for detecting GBS carriage and means taking swab samples from the low vagina and rectum which are sent off to the lab for analysis.

These tests are highly reliable and are good predictors of your GBS carriage status for 5 weeks after the swabs have been taken. The ECM test is the test recommended for use by the Royal College of Obstetricians& Gynaecologists for women who carried GBS in their previous pregnancy.  

Thousands of babies are exposed to GBS with no consequence. It is not known why some babies are susceptible to the bacteria and develop infection.

By testing for GBS and the mother receiving treatment (via intravenous antibiotics), the bacteria can be prevented from passing to the baby during labour  and prevent life-threatening infections.

If a woman who carries GBS is given the recommended antibiotics in labour, the baby’s risk is reduced by an estimated 85-90%. 

Any pregnant woman between 35 and 37 weeks of pregnancy.

  GBS can come and go during pregnancy but the ECM test has been found to be a good predictor of your GBS carriage status for the 5 weeks following testing, so the aim is to test in the 5 weeks before you go into labour and give birth. So the 35-37 week ‘window’ for testing is usually thought to be the best time to test for most people, as your GBS carriage status is not likely to change between testing and giving birth, and you should receive the result before your baby arrives.

If you are or have recently taken antibiotics, this may affect the test result so discuss this with your midwife or doctor. If you can, leave it for a couple of weeks after completing the course of antibiotics before testing.

Screening for GBS is not routinely offered on the NHS but if a pregnant woman is offered testing it’s usually the Standard ‘non-selective’ test for group B Strep carriage.

A negative result using this method is not very reliable – it gives a high proportion of falsely negative results. 

GBS will be isolated in only up to 50% of cases where the Mum is carrying GBS. This test is unselective – other bacteria present on the swab may outgrow and ‘swamp’ the GBS sample.

Many health professionals and most pregnant women are unaware of just how high the false-negative rate is for these tests.

• There are two types of GBS infection in newborns,  early and late-onset.

About two thirds of GBS infections in babies are early-onset (developing in the baby’s first 6 days of life) where a significant symptom is the rapid development of breathing problems, associated with blood poisoning.

The remaining third are late-onset, occurring after the baby’s first 6 days and can be responsible for sepsis and meningitis.

Most GBS infections show in the first 2 days of life, they are uncommon after a baby reaches one month old and very rare after age three months. Although intravenous antibiotics in labour are very effective at reducing the chance of a baby developing early-onset GBS infection, currently there are no known ways of preventing late-onset GBS infections. 

GBS is recognised to cause preterm delivery, maternal infections, stillbirths and late miscarriages; preterm babies are known to be at particular risk of GBS infection as their immune systems are not as well developed.

Overall, even with current prevention strategies, approximately 1 in every 1,000 babies born in the UK develops group B Strep infection.

GBS is recognised to cause preterm delivery, maternal infections, stillbirths and late miscarriages; preterm babies are known to be at particular risk of GBS infection as their immune systems are not as well developed.

Overall, even with current prevention strategies, approximately 1 in every 1,000 babies born in the UK develops group B Strep infection.

Two swabs need to be taken (rectal and vaginal). It doesn't matter which swab you use for which, but they need to be labelled correctly. Also, its important that you do not touch the cotton end with your fingers or let it come into contact with other surfaces (for example, clothing, tables or towels).

Each sample must be labelled, indicating whether it's a vaginal or rectal swab, and you must fill in your name, date of birth and the date of taking the sample.

There are full instructions supplied with each test kit.

Your test results will be received 3 days after the lab receives your test samples.

Positive Test Results

If you have received a positive test result, tell your health professional.

You 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.

It’s very important that, if you are allergic to penicillin or any other antibiotic, you tell your healthcare professional.

Negative Test Results

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.       

In the UK, up to two-thirds of GBS disease is early onset (showing within the first week).

Early-onset GBS infection usually presents as sepsis with pneumonia. These “early-onset” infections are usually apparent at or soon after birth, with the typical signs including :-

• Being unresponsive/floppy
• Very fast heartrate
• Very slow heartrate
• Making grunting noises when breathing
• An unusually high temperature
• And unusually low temperature
• Very fast breathing
• Very slow breathing

Call 999 or go to your local A&E if your baby gets any of these symptoms after discharge from hospital. However, most early-onset GBS infections show symptoms within the first 12 hours after birth so will often be identified at the maternity unit.

Late onset GBS infection (age 7-90 days old)

Up to a third of GBS infections in babies are late-onset, occurring after the baby’s first 6 days, usually as meningitis with sepsis. It is uncommon after a baby reaches one month old and very rare after age three months.

Typical signs of late-onset group B Strep infection are similar to those associated with early onset infection and may include signs associated with meningitis such as :-

• Being irritable with high pitched or whimpering cry, or moaning;
• Blank, staring or trance-like expression;
• Floppy, may dislike being handled, be fretful;
• Tense or bulging fontanelle (soft spot on babies’ heads);
• Turns away from bright light;
• Involuntary stiff body or jerking movements; and/or
• Pale, blotchy skin.

More help and information about GBS can be found here.